The Electronic Structure of Atoms. Electromagnetic Radiation Early atomic scientists studied the...

The Electronic The Electronic Structure of Structure of

AtomsAtoms

Electromagnetic Electromagnetic RadiationRadiation

Early atomic scientists studied Early atomic scientists studied the interaction of matter with the interaction of matter with electromagnetic radiationelectromagnetic radiation. .

Electromagnetic radiation, or Electromagnetic radiation, or radiant energy, includes visible light, radiant energy, includes visible light, infrared, micro and radio waves, and infrared, micro and radio waves, and X-rays and ultraviolet light.X-rays and ultraviolet light.


Electromagnetic radiation Electromagnetic radiation travels in waves. travels in waves.

The waves of radiant energy The waves of radiant energy have three important characteristics:have three important characteristics:

1. Wavelength - 1. Wavelength - λλ - (lambda) - (lambda)

2. Frequency – 2. Frequency – νν – (nu) – (nu)

3. Speed – c – the speed of light3. Speed – c – the speed of light

WavelengthWavelength

Wavelength, Wavelength, λλ, , is the distance is the distance between two between two adjacent peaks or adjacent peaks or troughs in a wave.troughs in a wave.

The units may The units may range from range from picometers to picometers to kilometers kilometers depending upon the depending upon the energy of the wave.energy of the wave.

FrequencyFrequency

Frequency, Frequency, νν, is the number , is the number of waves (or of waves (or cycles) that pass cycles) that pass a given point in a given point in space per second.space per second.

The units are The units are cycles/s, scycles/s, s-1-1 or or hertz (Hz).hertz (Hz).

The Speed of LightThe Speed of Light

All electromagnetic radiation All electromagnetic radiation travels at the same speed. The travels at the same speed. The speed of light ( c ) is:speed of light ( c ) is:

c = 2.9979 x 10c = 2.9979 x 1088 m/s m/s

Wavelength and Wavelength and FrequencyFrequency

Wavelength Wavelength and frequency are and frequency are inversely related. inversely related. That is, waves That is, waves with a low with a low frequency have a frequency have a long wavelength. long wavelength. Waves with a high Waves with a high frequency have frequency have short wavelengths.short wavelengths.

Properties of Light - Properties of Light - AmplitudeAmplitude

DiffractionDiffraction

Waves of electromagnetic radiation Waves of electromagnetic radiation are bent or are bent or diffracteddiffracted with they a passed with they a passed through an obstacle or a slit with a size through an obstacle or a slit with a size comparable to their wavelength.comparable to their wavelength.


The relationship between The relationship between wavelength and frequency is:wavelength and frequency is:

λνλν= c= c

Matter and EnergyMatter and Energy

By 1900, physicists thought that the By 1900, physicists thought that the nature of energy and matter was well nature of energy and matter was well understood and distinct. understood and distinct.

Matter, a collection of particles, have Matter, a collection of particles, have mass and a defined position in space. mass and a defined position in space. Radiant energy, as waves, is massless and Radiant energy, as waves, is massless and delocalized.delocalized.

It was also believed that particles of It was also believed that particles of matter could absorb or emit any energy, matter could absorb or emit any energy, without restriction.without restriction.

Planck & Black Body Planck & Black Body RadiationRadiation

Max Planck (1858-1947) studied Max Planck (1858-1947) studied the radiation emitted by objects the radiation emitted by objects heated until they glowed. He found heated until they glowed. He found that the energy emitted was not that the energy emitted was not continuous, but instead was released continuous, but instead was released in multiples of in multiples of hhνν..

∆∆E = nhE = nhννwhere n=integerwhere n=integer

νν = frequency = frequencyh = 6.626 x 10h = 6.626 x 10-34 -34 J-s (Planck’s constant)J-s (Planck’s constant)

Planck & Black Body Planck & Black Body RadiationRadiation

∆∆E = nhE = nhνν

Planck’s work showed that when Planck’s work showed that when matter and energy interact, the energy matter and energy interact, the energy is is quantizedquantized, and can occur only in , and can occur only in discrete units or bundles with energy discrete units or bundles with energy of of hhνν. Each packet or bundle of . Each packet or bundle of energy is called a energy is called a quantumquantum. A fraction . A fraction of a quantum is never emitted.of a quantum is never emitted.

Einstein – Photoelectric Einstein – Photoelectric EffectEffect

Albert Einstein (1879-1955) won a Albert Einstein (1879-1955) won a Nobel Prize for his explanation of the Nobel Prize for his explanation of the photoelectric effectphotoelectric effect. .

When light of sufficient energy When light of sufficient energy strikes the surface of a metal, electrons strikes the surface of a metal, electrons are emitted from the metal surface. are emitted from the metal surface. Each metal has a characteristic Each metal has a characteristic minimum frequency, minimum frequency, ννo o , called the , called the threshold frequencythreshold frequency, needed for , needed for electrons to be emitted.electrons to be emitted.

The Photoelectric EffectThe Photoelectric Effect

ObservationsObservations

1. No electrons are emitted if the 1. No electrons are emitted if the frequency of light used is less than frequency of light used is less than ννoo, , regardless of the intensity of the light.regardless of the intensity of the light.

2. For light with a frequency≥ 2. For light with a frequency≥ ννo o , electrons , electrons are emitted. The number of electrons are emitted. The number of electrons increases with the intensity of the light.increases with the intensity of the light.

3. For light with a frequency > 3. For light with a frequency > ννoo , the , the electrons are emitted with greater electrons are emitted with greater kinetic energy.kinetic energy.

ExplanationExplanation

Einstein proposed that light is Einstein proposed that light is quantized, consisting of a stream of quantized, consisting of a stream of “particles” called “particles” called photonsphotons. .

If the photon has sufficient energy, it If the photon has sufficient energy, it can “knock off” an electron from the can “knock off” an electron from the metal surface. If the energy of the metal surface. If the energy of the photon is greater than that needed to photon is greater than that needed to eject an electron, the excess energy is eject an electron, the excess energy is transferred to the electron as kinetic transferred to the electron as kinetic energy.energy.

The Photoelectric EffectThe Photoelectric Effect

EEphotonphoton= h= hνν = hc/ = hc/λλ

If incident radiation with a frequency If incident radiation with a frequency ννii is is used:used:

KEKEelectronelectron = h = hννii -h -hννoo = ½ mv = ½ mv22

The kinetic energy of the electron The kinetic energy of the electron equals the energy of the incident radiation equals the energy of the incident radiation less the minimum energy needed to eject an less the minimum energy needed to eject an electron.electron.

Particle-Wave DualityParticle-Wave Duality

Einstein’s work suggested that Einstein’s work suggested that the incident photon behaved like a the incident photon behaved like a particle. If it “hits” the metal particle. If it “hits” the metal surface with sufficient energy (hsurface with sufficient energy (hννii), ), the excess energy of the photon is the excess energy of the photon is transferred to the ejected electron.transferred to the ejected electron.

In the atomic scale, waves of In the atomic scale, waves of radiant energy have particle-like radiant energy have particle-like properties.properties.


Einstein also combined his Einstein also combined his equations:equations:

E=mcE=mc22

with with

EEphotonphoton= hc/= hc/λλto obtain:to obtain:

m= m=

m=m=

E c2

= hc/hc/λλc2

h λc


The apparent mass of radiant The apparent mass of radiant energy can be calculated. Although a energy can be calculated. Although a wave lacks any mass at rest, at times, wave lacks any mass at rest, at times, it behaves as if it has mass.it behaves as if it has mass.

Einstein’s equation was Einstein’s equation was confirmed by experiments done by confirmed by experiments done by Arthur Compton in 1922. Collisions Arthur Compton in 1922. Collisions between X-rays and electrons between X-rays and electrons confirmed the “mass” of the radiation.confirmed the “mass” of the radiation.

Louis de BroglieLouis de Broglie

Einstein showed that waves can Einstein showed that waves can behave like particles. In 1923, Louis behave like particles. In 1923, Louis de Broglie (1892-1987) proposed de Broglie (1892-1987) proposed that moving electrons have wave-like that moving electrons have wave-like properties.properties.


Using Einstein’s equation:Using Einstein’s equation:

m=h/m=h/λλvvwhere v is the velocity of the particle, where v is the velocity of the particle,

de Broglie rearranged the equation de Broglie rearranged the equation to calculate the wavelength to calculate the wavelength associated with any moving object.associated with any moving object.


λλ=h/mv=h/mv

de Broglie’s equation was tested de Broglie’s equation was tested using a stream of electrons directed at using a stream of electrons directed at a crystal. A diffraction pattern, due to a crystal. A diffraction pattern, due to the interaction of waves, resulted. The the interaction of waves, resulted. The experiment showed that electrons have experiment showed that electrons have wave-like properties.wave-like properties.

Wave-Like Nature of the Wave-Like Nature of the ElectronElectron


It is important to note that the It is important to note that the wavelike properties of moving particles wavelike properties of moving particles are insignificant in our everyday world. are insignificant in our everyday world. A moving object such as a car or a A moving object such as a car or a tennis ball has an incredibly small tennis ball has an incredibly small wavelength associated with it.wavelength associated with it.

It is on the atomic scale that the It is on the atomic scale that the dual nature of particles and light dual nature of particles and light become significant.become significant.

Emission Spectrum of Emission Spectrum of HydrogenHydrogen

When atoms When atoms are given extra are given extra energy, or energy, or excitedexcited, they , they give off the give off the excess energy excess energy as light as they as light as they return to their return to their original energy, original energy, or or ground stateground state. .

Hg He

H2


Scientists expected atoms to be able Scientists expected atoms to be able to absorb and emit a continuous range of to absorb and emit a continuous range of energies, so that a continuous spectrum energies, so that a continuous spectrum of wavelengths would be emitted.of wavelengths would be emitted.


A continuous spectrum in the visible A continuous spectrum in the visible range, would look like a rainbow, range, would look like a rainbow, with all colors visible.with all colors visible.

Instead, hydrogen, and other excited Instead, hydrogen, and other excited atoms emit only specific atoms emit only specific wavelengths of light as they return wavelengths of light as they return to the ground state. A to the ground state. A line spectrumline spectrum results.results.


Instead, only a few wavelengths of light Instead, only a few wavelengths of light are emitted, creating a are emitted, creating a line spectrumline spectrum. . The spectrum of hydrogen contains four The spectrum of hydrogen contains four very sharp lines in the visible range.very sharp lines in the visible range.


The discrete lines in the spectrum The discrete lines in the spectrum indicate that the energy of the atom indicate that the energy of the atom is is quantizedquantized. Only specific energies . Only specific energies exist in the excited atom, so only exist in the excited atom, so only specific wavelengths of radiation are specific wavelengths of radiation are emitted.emitted.

The Bohr Atomic ModelThe Bohr Atomic Model

In 1913, Niels Bohr (1885-1962) In 1913, Niels Bohr (1885-1962) proposed that the electron of hydrogen proposed that the electron of hydrogen circles the nucleus in circles the nucleus in allowed orbitsallowed orbits. .

That is, the electron is in its That is, the electron is in its ground state in an orbit closest to the ground state in an orbit closest to the nucleus. As the atom becomes excited, nucleus. As the atom becomes excited, the electron is promoted to an orbit the electron is promoted to an orbit further away from the nucleus.further away from the nucleus.


Classical physics Classical physics dictates that an dictates that an electron in a circular electron in a circular orbit must constantly orbit must constantly lose energy and emit lose energy and emit radiation. radiation.

Bohr proposed a Bohr proposed a quantum modelquantum model, as , as the spectrum showed the spectrum showed that only certain that only certain energies are energies are absorbed or emitted.absorbed or emitted.


Bohr’s model of the hydrogen Bohr’s model of the hydrogen atom was consistent with the atom was consistent with the emission spectrum, and explained emission spectrum, and explained the distinct lines observed.the distinct lines observed.


Bohr also developed an equation, Bohr also developed an equation, using the spectrum of hydrogen, that using the spectrum of hydrogen, that calculates the energy levels an calculates the energy levels an electron may have in the hydrogen electron may have in the hydrogen atom:atom:

E=-2.178 x 10E=-2.178 x 10-18-18J(ZJ(Z22/n/n22))

Where Z = atomic numberWhere Z = atomic number

n = an integern = an integer


The Bohr model didn’t work for The Bohr model didn’t work for atoms other than hydrogen. Though atoms other than hydrogen. Though limited, Bohr’s approach did attempt limited, Bohr’s approach did attempt to explain the quantized energy to explain the quantized energy levels of electrons.levels of electrons.

Later developments showed that Later developments showed that any attempt to define the path of the any attempt to define the path of the electron is incorrect.electron is incorrect.

The Quantum The Quantum Mechanical ModelMechanical ModelThe quantum mechanical atomic The quantum mechanical atomic

model was developed based on the model was developed based on the theories of Werner Heisenberg theories of Werner Heisenberg (1901-1976), Louis de Broglie (1892-(1901-1976), Louis de Broglie (1892-1987) and Erwin Schrödinger (1887-1987) and Erwin Schrödinger (1887-1961).1961).

They focused on the wave-like They focused on the wave-like nature of the moving electron.nature of the moving electron.

The Quantum The Quantum Mechanical ModelMechanical Model

The electron in an The electron in an atom was viewed atom was viewed as a as a standing standing wavewave. For an . For an energy level to energy level to exist, the wave exist, the wave must reinforce must reinforce itself via itself via constructive constructive interferenceinterference..

The Quantum The Quantum Mechanical ModelMechanical ModelSchrödinger developed complex Schrödinger developed complex

equations called equations called wave functions wave functions ( ( ΨΨ). ). The wave functions can be used to The wave functions can be used to calculate the energy of electrons, calculate the energy of electrons, not only in hydrogen, but in other not only in hydrogen, but in other atoms. atoms.

The Quantum The Quantum Mechanical ModelMechanical ModelThe wave functions also The wave functions also

describe various volumes or describe various volumes or spaces where electrons of a spaces where electrons of a specific energy are likely to be specific energy are likely to be found. These spaces are called found. These spaces are called orbitalsorbitals..

The Quantum The Quantum Mechanical ModelMechanical ModelOrbitals are not orbitsOrbitals are not orbits. .

The wave functions provide no The wave functions provide no information about the path of the information about the path of the electron. Instead, it provides the electron. Instead, it provides the space in which there is a high space in which there is a high probability (90%) of finding an probability (90%) of finding an electron with a specific energy.electron with a specific energy.

The Heisenberg The Heisenberg Uncertainty PrincipleUncertainty PrincipleWerner Heisenberg showed that, Werner Heisenberg showed that,

due to the wave nature of the electron, due to the wave nature of the electron,

It is impossible toIt is impossible to knowknow both the both the precise position and the momentum of precise position and the momentum of the electron at the same time.the electron at the same time.

This is known as the Heisenberg This is known as the Heisenberg Uncertainty Principle.Uncertainty Principle.

The Heisenberg The Heisenberg Uncertainty PrincipleUncertainty PrincipleIt is impossible toIt is impossible to knowknow both the both the

precise position and the momentum of precise position and the momentum of the electron at the same time.the electron at the same time.

In mathematical terms, the principle is:In mathematical terms, the principle is:

((ΔΔx) (x) (ΔΔmv) ≥ (h/4mv) ≥ (h/4ππ))

The Heisenberg The Heisenberg Uncertainty PrincipleUncertainty PrincipleIt is impossible toIt is impossible to knowknow both the both the

precise position and the momentum of precise position and the momentum of the electron at the same time.the electron at the same time.

The Heisenberg The Heisenberg Uncertainty PrincipleUncertainty Principle

((ΔΔx) (x) (ΔΔmv) ≥ (h/4mv) ≥ (h/4ππ))

There is a limit to how well we can There is a limit to how well we can determine position (x), if mass and determine position (x), if mass and velocity are known precisely.velocity are known precisely.

For large particles, the uncertainty For large particles, the uncertainty is insignificant. However, on the atomic is insignificant. However, on the atomic scale, scale, we cannot know the exact motion we cannot know the exact motion of an electronof an electron..

The Heisenberg The Heisenberg Uncertainty PrincipleUncertainty Principle

((ΔΔx) (x) (ΔΔmv) ≥ (h/4mv) ≥ (h/4ππ))

For an electron in a hydrogen For an electron in a hydrogen atom, the uncertainty in the position atom, the uncertainty in the position of the electron is similar in size to of the electron is similar in size to the entire hydrogen atom. Thus the the entire hydrogen atom. Thus the location of the electron cannot be location of the electron cannot be determined.determined.

OrbitalsOrbitals

The Schrödinger equation is used The Schrödinger equation is used to describe the space in which it is to describe the space in which it is likely to find an electron with a specific likely to find an electron with a specific energy. energy.

The equation provides us with a The equation provides us with a probability distributionprobability distribution, or an , or an electron electron density mapdensity map. It is important to . It is important to remember that the resulting shape does remember that the resulting shape does not give us any information about the not give us any information about the path of the electrons.path of the electrons.

OrbitalsOrbitals

The orbital of The orbital of lowest energy is lowest energy is the 1s orbital. the 1s orbital. The probability The probability distribution distribution shows electron shows electron density in all density in all directions, directions, creating a creating a spherical shape.spherical shape.

OrbitalsOrbitals

OrbitalsOrbitals

The first energy level of hydrogen The first energy level of hydrogen (n=1) consists of a 1s orbital.(n=1) consists of a 1s orbital.

The second energy level of The second energy level of hydrogen (n=2) consists of a 2s orbital hydrogen (n=2) consists of a 2s orbital and 2p orbitals.and 2p orbitals.

The third energy level of hydrogen The third energy level of hydrogen (n=3) consists of a 3s orbital, 3p (n=3) consists of a 3s orbital, 3p orbitals, and 3d orbitals.orbitals, and 3d orbitals.

OrbitalsOrbitals

As the value As the value of n increases, of n increases, the orbitals, on the orbitals, on average, average, become larger, become larger, with more with more electron density electron density farther from the farther from the nucleus.nucleus.

OrbitalsOrbitals

The “white The “white rings” in the rings” in the drawings are drawings are nodesnodes. This is . This is the region where the region where the wave function the wave function goes from a goes from a positive value to positive value to a negative value.a negative value.

OrbitalsOrbitalsp orbitals are “dumbbell” p orbitals are “dumbbell”

shaped, with two lobes. In one lobe, shaped, with two lobes. In one lobe, the wave function is positive, in the the wave function is positive, in the other lobe, it is negative.other lobe, it is negative.

OrbitalsOrbitalsp orbitals come in sets of three, called a p orbitals come in sets of three, called a

subshellsubshell. The three orbitals are designated . The three orbitals are designated as pas pxx, p, pyy and p and pzz, because the electron density , because the electron density lies primarily along either the x, y or z axis.lies primarily along either the x, y or z axis.

OrbitalsOrbitalsAll three orbitals have the exact All three orbitals have the exact

same energy. Orbitals with the same energy. Orbitals with the same energy are called same energy are called degenerate.degenerate.

OrbitalsOrbitalsThe n=3 The n=3 level level contains contains s, p and d s, p and d orbitals. orbitals. The d The d orbitals orbitals are are shown.shown.

OrbitalsOrbitals

The The n=4 level n=4 level contains s, contains s, p, d and f p, d and f orbitals. orbitals. The f The f orbitals are orbitals are shown.shown.

Quantum NumbersQuantum Numbers

In addition to n, the principal quantum number, there are three additional quantum numbers which describe the type of orbital ( l ) , the spatial orientation of the orbital (ml ) , and the spin of the electron (ms ) .

Energy LevelsEnergy LevelsIn any atom or ion In any atom or ion with only 1 with only 1 electronelectron, the , the principal quantum principal quantum number, n, number, n, determines the determines the energy of the energy of the electron. For n=2, electron. For n=2, the 2s and 2p the 2s and 2p orbitals all have the orbitals all have the same energy.same energy.

Energy LevelsEnergy Levels

Likewise, Likewise, the 3s, 3p and the 3s, 3p and 3d orbitals are 3d orbitals are all degenerate, all degenerate, with the same with the same energy.energy.


In a multi-electron atom, there In a multi-electron atom, there is interaction between electrons. As is interaction between electrons. As a result of this interaction, the a result of this interaction, the various subshells of a principal various subshells of a principal quantum level will vary in energy.quantum level will vary in energy.


The energy The energy diagram for the diagram for the first three first three quantum levels quantum levels shows the shows the splitting of splitting of energies. energies.


For a given For a given value of n, the value of n, the energies of the energies of the subshells is as subshells is as follows:follows:

ns<np<nd<nfns<np<nd<nf


The subshells The subshells have different have different energies due to energies due to the the penetrating penetrating abilityability for each for each type of orbital.type of orbital.

Electrons in a Electrons in a 2s orbital can get 2s orbital can get nearer to the nearer to the nucleus than those nucleus than those in a 2p orbital. in a 2p orbital.

Energy Energy LevelsLevels

The electrons The electrons in the 3s orbital in the 3s orbital (top diagram) have (top diagram) have higher probability higher probability to be found near to be found near the nucleus, and the nucleus, and thus greater thus greater penetrating ability penetrating ability than those in 3p or than those in 3p or 3d orbitals.3d orbitals.

Multi-electron AtomsMulti-electron Atoms

Orbitals of Orbitals of any type can be any type can be empty, or have 1 empty, or have 1 or two electrons. or two electrons.

Experimental Experimental data indicate that data indicate that if two electrons if two electrons are in the same are in the same orbital, they will orbital, they will spin in opposite spin in opposite directions.directions.


Electron configurations are a Electron configurations are a way of noting which subshells of an way of noting which subshells of an atom contain electrons. atom contain electrons. Although Although much of the periodic table was much of the periodic table was developed before the concept of developed before the concept of electron configurations, it turns out electron configurations, it turns out that the position of an element on that the position of an element on the periodic table is directly related the periodic table is directly related to its electron configuration.to its electron configuration.

Magnetic Properties of Atoms

Rotating electrons create a magnetic field. If electrons are unpaired, the atom will be attracted to a magnetic field and be paramagnetic.

If all of the electrons in an atom are paired, the atom will be weakly repelled by a magnetic field, and be diamagnetic.

Magnetic Properties of Atoms

Hund’s Rule states that electrons will occupy degenerate orbitals singly with parallel spins.

The Pauli Exclusion Principle states that two electrons occupying the same orbital will have opposite spins.

Representation of Orbitals

Orbitals are usually represented by a horizontal line, with the electrons appearing as arrows. Arrows up and down indicate electrons of opposite spins.

Multi-electron Atoms

The electron configuration for N is: 1s22s22p3

N atoms have 3 unpaired electrons: __ __ __

Multi-electron AtomsThe electron configurations for Cr and Cu differ from that expected based on their positions in the periodic table.

Cr and Cu

The electron configuration for Cr is: [Ar] 4s13d5 with 6 unpaired electrons

The electron configuration for Cu is: [Ar] 4s13d10 with the unpaired electron

in the 4s orbital

Valence Electrons

The electrons in the highest occupied quantum level are called valence electrons. These are the electrons that are involved in bonding.

Elements in the same family or group all have the same number of valence electrons and thus exhibit similar chemical behavior.

Valence Electrons

All of the group IA elements have a single valence electron (ns1). Group IIA elements have two valence electrons, etc. The noble gases all have eight valence electrons(ns2np6).

When the main group elements form ions, they usually lose or gain enough electrons to attain the same electron configuration as a noble gas.

Periodic TrendsPeriodic Trends

Many of the properties of atoms Many of the properties of atoms show clear trends in going across a show clear trends in going across a period (from left to right) or down a period (from left to right) or down a group.group.

In going across a period, each In going across a period, each atom gains a proton in the nucleus atom gains a proton in the nucleus as well as a valence electron.as well as a valence electron.


The increase of positive charge The increase of positive charge in the nucleus isn’t completely in the nucleus isn’t completely cancelled out by the addition of the cancelled out by the addition of the electron. electron.

Electrons added to the valence Electrons added to the valence shell don’t shield each other very shell don’t shield each other very much. As a result, in going across a much. As a result, in going across a period, the period, the effective nuclear chargeeffective nuclear charge (Z(Zeffeff) increases.) increases.

Effective Nuclear ChargeEffective Nuclear Charge

The The effective nuclear chargeeffective nuclear charge (Z(Zeffeff) equals the atomic number (Z) ) equals the atomic number (Z) minus the shielding factor (σ).minus the shielding factor (σ).

ZZeffeff= Z-σ= Z-σ


ZZeffeff= Z-σ= Z-σ


Electrons in Electrons in the valence shell the valence shell are partially are partially shielded from shielded from the nucleus by the nucleus by corecore electrons. electrons.


Electrons in Electrons in pp or or dd orbitals don’t orbitals don’t get too close to the get too close to the nucleus, so they are nucleus, so they are less shielding than less shielding than electrons in electrons in ss orbitals. As a result, orbitals. As a result, effective nuclear effective nuclear charge increases charge increases across a period.across a period.


In going down a group or family, a In going down a group or family, a full quantum level of electrons, along full quantum level of electrons, along with an equal number of protons, is with an equal number of protons, is added. added.

As n increases, the valence As n increases, the valence electrons are, on average, farther from electrons are, on average, farther from the nucleus, and experience less nuclear the nucleus, and experience less nuclear pull due to the shielding by the “core” pull due to the shielding by the “core” electrons. As a result, Zelectrons. As a result, Zeffeff decreases decreases slightly going down a group.slightly going down a group.

Trends- Atomic RadiiTrends- Atomic Radii

Atomic radii are obtained in a variety of Atomic radii are obtained in a variety of ways:ways:

1. For metallic elements, the radius is half 1. For metallic elements, the radius is half the internuclear distance in the crystal, the internuclear distance in the crystal, which is obtained from X-ray data.which is obtained from X-ray data.

2. For diatomic molecules, the radius is 2. For diatomic molecules, the radius is half the bond length.half the bond length.

3. For other elements, estimates of the 3. For other elements, estimates of the radii are made.radii are made.

Trends- Atomic RadiiTrends- Atomic Radii

Atomic radii follow trends directly Atomic radii follow trends directly related to the effective nuclear charge. related to the effective nuclear charge. As ZAs Zeffeff increases across a period, the increases across a period, the electrons are pulled closer to the nucleus, electrons are pulled closer to the nucleus, and atomic radii decrease.and atomic radii decrease.

As ZAs Zeffeff decreases down a group, the decreases down a group, the valence electrons experience less nuclear valence electrons experience less nuclear attraction, and the radius increases.attraction, and the radius increases.

Trends- Trends- Atomic Atomic RadiiRadii

Atomic size Atomic size roughly halves roughly halves across a period, across a period, and doubles and doubles going down a going down a group.group.

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