THE TRUE MYSTERY OF THE WORLD IS THE TRUE MYSTERY OF THE WORLD IS THE VISIBLE, NOT THE INVISIBLE.THE VISIBLE, NOT THE INVISIBLE.

- Oscar Wilde -- Oscar Wilde -

ATOMIC STRUCTUREATOMIC STRUCTURE

"I think I can safely say that nobody understands quantum mechanics."

- Richard Feynman-- Richard Feynman-

QUANTUM OF ENERGYQUANTUM OF ENERGY – THE ENERGY REQUIRED TO MOVE AN – THE ENERGY REQUIRED TO MOVE AN ELECTRON FROM ONE ORBIT OR ENERGY LEVEL TO ANOTHER.ELECTRON FROM ONE ORBIT OR ENERGY LEVEL TO ANOTHER.

PHOTONSPHOTONS OF LIGHT REPRESENT DISCRETE PACKETS OF ENERGY OF LIGHT REPRESENT DISCRETE PACKETS OF ENERGY WITH THE WAVELENGTH OF LIGHT DETERMINING THE AMOUNT OF WITH THE WAVELENGTH OF LIGHT DETERMINING THE AMOUNT OF ENERGYENERGY

E = hc/E = hc/where where h = constant, c = speed of lighth = constant, c = speed of light = wavelength of light= wavelength of light

ERWIN SCHRODINGER USED THESE RELATIONSHIPS TO ERWIN SCHRODINGER USED THESE RELATIONSHIPS TO DESCRIBE THE BEHAVIOR OF ELECTRONS IN ATOMS.DESCRIBE THE BEHAVIOR OF ELECTRONS IN ATOMS.

ATOMIC ORBITALATOMIC ORBITAL – A REGION IN SPACE WITH A HIGH – A REGION IN SPACE WITH A HIGH PROBABILITY OF FINDING AN ELECTRON.PROBABILITY OF FINDING AN ELECTRON.

WHERE DO OUR CURRENT IDEAS WHERE DO OUR CURRENT IDEAS OF ATOMIC STRUCTURE COME OF ATOMIC STRUCTURE COME

FROM?FROM?

de Broglie put forward that all de Broglie put forward that all objects in motion have wave objects in motion have wave nature. The smaller an object is, nature. The smaller an object is, the greater the wave nature. This the greater the wave nature. This means that the electron would means that the electron would behave as much like a wave as a behave as much like a wave as a particle.particle.

Schrodinger came up with a Schrodinger came up with a differential wave equation that differential wave equation that would describe the motion of an would describe the motion of an electron about a nucleus in three electron about a nucleus in three dimensions.dimensions.

Chemists call these wave functions Chemists call these wave functions “orbitals”.“orbitals”.

The math involved is very The math involved is very complex.complex.

However, the results do an However, the results do an excellent job of describing why excellent job of describing why atoms of different elements atoms of different elements behave as they do, and they are behave as they do, and they are very useful in describing how very useful in describing how atoms interact to form molecules.atoms interact to form molecules.

They even do a good job helping to They even do a good job helping to describe the shapes of molecules. describe the shapes of molecules. This is very important in This is very important in biochemistry and medicine.biochemistry and medicine.

From the quantum theory, there are four From the quantum theory, there are four quantum numbers that describe electron quantum numbers that describe electron orbitals.orbitals.

n = principal quantum number - what shell the n = principal quantum number - what shell the electron goes in (n = 1, 2, 3...)electron goes in (n = 1, 2, 3...)

l = angular momentum quantum number - l = angular momentum quantum number - what subshell the electron goes in (0 what subshell the electron goes in (0 << l l << n-1) n-1)

mmll = magnetic quantum number – how many = magnetic quantum number – how many orbitals the subshell is broken into (-l orbitals the subshell is broken into (-l << m mll << l) l)

mmss = spin quantum number (- 1/2 or + 1/2) = spin quantum number (- 1/2 or + 1/2)

Principal Quantum Number(shell)

Number of Subshells Type of Subshell

n = 1 1 1s (1 orbital)

n = 2 2 2s (1 orbital)2p (3 orbitals)

n = 3 3 3s (1 orbital)3p (3 orbitals)3d (5 orbitals)

n = 4 4 4s (1 orbital)4p (3 orbitals)4d(5 orbitals)4f (7 orbitals)

The quantum numbers, in a sense, The quantum numbers, in a sense, tell us how many electrons can go tell us how many electrons can go where.where.

There are some rules that tell us There are some rules that tell us how electrons fill shells.how electrons fill shells.

ELECTRON FILLING RULESELECTRON FILLING RULES

Aufbau (build-up) PrincipalAufbau (build-up) Principal - - Electrons enter and fill lower energy orbitals before higher energy orbitals.

Pauli Exclusion Principal - orbitals can contain a maximum of two electrons which must be of opposite spin.

Hund’s Rule - when there are orbitals of equal energy, electrons will enter the orbitals one-at-a time until all of the orbitals are half filled and only then will pairing occur.

So, let’s take a look at how So, let’s take a look at how electrons go into orbitals as we electrons go into orbitals as we move through the periodic table.move through the periodic table.

Hydrogen has only one electron, Hydrogen has only one electron, so it would go in the first shell. so it would go in the first shell. The first shell has only one The first shell has only one subshell, the 1s.subshell, the 1s.

So, we’d write that as:So, we’d write that as:

Hydrogen, H 1sHydrogen, H 1s11

Helium has an atomic number of Helium has an atomic number of 2, so it has two electrons. This 2, so it has two electrons. This second electron would also go in second electron would also go in the 1s subshell, but it would have the 1s subshell, but it would have opposite spin.opposite spin.

Hydrogen, H 1sHydrogen, H 1s11

Helium, He 1sHelium, He 1s22

This fills up this shell and This fills up this shell and subshell.subshell.

Notice H and He are in the first row Notice H and He are in the first row or first period. Electrons are going or first period. Electrons are going into the first shell.into the first shell.

Lithium has an atomic number of Lithium has an atomic number of 3, so this means that it has 3 3, so this means that it has 3 electrons.electrons.

The first shell is filled, so the third The first shell is filled, so the third electron has to go in the second electron has to go in the second shell. It will go in the 2s subshell, shell. It will go in the 2s subshell, as this has the lowest energy.as this has the lowest energy.

Hydrogen, H 1sHydrogen, H 1s11

Helium, He 1sHelium, He 1s22

Lithium, Li 1sLithium, Li 1s22 2s 2s11

Beryllium has an atomic number of 4, Beryllium has an atomic number of 4, so 4 electrons.so 4 electrons.

Two in the first shell, and two in the 2s Two in the first shell, and two in the 2s subshell.subshell.

Hydrogen, H 1sHydrogen, H 1s11

Helium, He 1sHelium, He 1s22

Lithium, Li 1sLithium, Li 1s22 2s 2s11

Beryllium, Be 1sBeryllium, Be 1s2 2 2s2s22

This fills the 2s subshell. The next This fills the 2s subshell. The next electrons will have to go in the 2p electrons will have to go in the 2p subshell.subshell.

Boron has an atomic number of 5, so it has Boron has an atomic number of 5, so it has 5 electrons.5 electrons.

B 1sB 1s22 2s 2s22 2p 2p11

There are actually 3 2p subshells, all of There are actually 3 2p subshells, all of equal energy, pequal energy, pxx, p, pyy, p, pzz. .

One electron will go into each of these. One electron will go into each of these. After each has an electron, the the After each has an electron, the the additional three electrons will pair up.additional three electrons will pair up.

Right now, don’t be too concerned. Just Right now, don’t be too concerned. Just remember that 6 electrons can go into the remember that 6 electrons can go into the p subshells.p subshells.

So, for the rest of the second row So, for the rest of the second row (period), we have:(period), we have:

Lithium, Li 1sLithium, Li 1s22 2s 2s11

Beryllium, Be 1sBeryllium, Be 1s22 2s 2s22

Boron, B 1sBoron, B 1s22 2s 2s2 2 2p2p11

Carbon, C 1sCarbon, C 1s22 2s 2s2 2 2p2p22

Nitrogen, N 1sNitrogen, N 1s22 2s 2s2 2 2p2p33

Oxygen, O 1sOxygen, O 1s22 2s 2s2 2 2p2p44

Fluorine, F 1sFluorine, F 1s22 2s 2s2 2 2p2p55

Neon, Ne 1sNeon, Ne 1s22 2s 2s2 2 2p2p66

This gives us a filled first shell and a This gives us a filled first shell and a filled second shell.filled second shell.

The order in which the third shell The order in which the third shell would fill would be:would fill would be:

Sodium, Na 1sSodium, Na 1s22 2s 2s2 2 2p2p66 3s 3s11

Magnesium, Mg 1sMagnesium, Mg 1s22 2s 2s2 2 2p2p66 3s 3s22

Aluminum, Al 1sAluminum, Al 1s22 2s 2s2 2 2p2p66 3s 3s2 2

3p3p11

Silicon, Si 1sSilicon, Si 1s22 2s 2s2 2 2p2p66 3s 3s2 2

3p3p22

Phosphorus, P 1sPhosphorus, P 1s22 2s 2s2 2 2p2p66 3s 3s2 2

3p3p33

Sulfur, S 1sSulfur, S 1s22 2s 2s2 2 2p2p66 3s 3s2 2

3p3p44

Chlorine, Cl 1sChlorine, Cl 1s22 2s 2s2 2 2p2p66 3s 3s2 2

3p3p55

Argon, Ar 1sArgon, Ar 1s22 2s 2s2 2 2p2p66 3s 3s2 2

3p3p66

There is one very important reason There is one very important reason we are going through this. we are going through this.

It is the number of electrons in the It is the number of electrons in the outer shell (valence electrons) that outer shell (valence electrons) that determine the properties of the determine the properties of the element.element.

If you look at the periodic table, you If you look at the periodic table, you will see that all of the elements in a will see that all of the elements in a given column have the same given column have the same number of valence electrons.number of valence electrons.

So, they will have similar properties. So, they will have similar properties. A column is called a A column is called a familyfamily..

A row is called a A row is called a periodperiod. The period . The period number tells what shell is being filled.number tells what shell is being filled.

The The octet ruleoctet rule says that the maximum says that the maximum number of electrons that can occur in number of electrons that can occur in any outer shell is 8, except for the first any outer shell is 8, except for the first shell, which is 2. This represents a shell, which is 2. This represents a stable configuration, and elements will stable configuration, and elements will react by losing, gaining or sharing react by losing, gaining or sharing electrons to obtain this configuration.electrons to obtain this configuration.