Jan 14_Fundamentals of an Atom

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Fundamentals of anAtom

Chemistry 1

By: Ms. Mialo C. Lacaden



Fundamentals of the Atom

An atom consists of a nucleus of protons and neutrons surrounded byelectrons

Each of the elements in the periodictable is classified according to itsatomic number

Protons have a charge of +1, electronshave a charge of -1, and neutronshave no charge

Electrically, neutral atoms have the



Within a given element, atoms with differentnumbers of neutrons are isotopes of that

element and exhibit the same properties

Electrons have such little mass that theyexhibit properties of both particles and waves

Heisenberg's Uncertainty Principle states that itis impossible to know the precise location of an electron

Despite this limitation, there are regionsaround the atom where the electron has a

high probability of being found. Such regionsare referred to as orbitals.



Atomic Orbitals

For isolated atoms (meaning non-bonded),electrons reside in the atomic orbitals of those atoms

Atomic orbitals are classified according to aset of four quantum numbers whichdescribe the energy, shape, and

orientation of the orbital.

1. Principle Quantum Number (n):Indicates how far the orbital is from thenucle.



2. Angular Momentum Quantum Number(l): Describes the shape of the orbital.

The angular momentum number (orsubshell) can be represented either bynumber (any integer from 0 up n -1) or by aletter ( s , p , d , f , g , and then up thealphabet), with 0 = s, 1 = p, 2 = d , and so

on. For example: when n = 1, l can only equal 0; meaningthat shell n = 1 has only an s orbital (l = 0).



when n = 3, l can equal 0, 1, or 2;meaning that shell n = 3 has s , p ,

and d orbitals. s orbitals are spherical, whereas p orbitals are dumbbell-shaped. d

orbitals and beyond are muchharder to visually represent.



3. Magnetic Quantum Number (m): Gives theorientation of the orbital in space; in other words,the value of m describes whether an orbital lies

along the x -, y -, or z -axis on a three-dimensionalgraph, with the nucleus of the atom at the origin.

m can take on any value from -l to l. For ourpurposes, it is only important that this quantum

number tells us that for each value of n there maybe up to

one s –orbital three p –orbitals

five d - orbitals, and so on:



The s orbital (l = 0) has one orbital,since m can only equal 0. That

orbital is spherically symmetricalabout the nucleus.



The p orbital (l = 1) has three orbitals,since m = -1, 0, and 1. These threeorbitals lie along the x -, y -, and z -axes.



The d orbital (l = 2)has five orbitals, since

m = -2, -1, 0, 1, and 2.It is far more difficult todescribe the orientationof d orbitals.



4. Spin Quantum Number (s): Tellswhether a given electron is spin up

(+1/2) or spin down (-1/2).

Pauli Exclusion Principle tells usthat no two electrons of an atom can

have the same set of quantumnumbers, each orbital is limited toholding two electrons at most.



Orbital Energy Diagrams

Note that the relative spacing betweenorbitals becomes smaller for larger n .In fact, as n gets large the spacing

becomes infinitesimally small.

Figure %: Energy diagram of theunoccupied atomic orbitals of hydrogen.

Potential energy is on the y -axis.



Multi-electron atoms

Breaking of degenerancies is due to the factthat the attractive nuclear force any electronfeels is shielded by the other electrons. s -orbitals tend to be closer to the nucleus than

p -orbitals and don't get as much shielding,and hence become lower in energy. Thisprocess of breaking degeneracies within ashell is known as splitting. In general s orbitals become lowest in energy, followed by

p orbitals, d orbitals, and so forth.

Figure %: Splitting of orbital -



Electron Configuration andValence Electrons

Electron Configuration The electrons in an atom fill up its atomic orbitals

according to the Aufbau Principle; "Aufbau," in

German, means "building up.” Electrons always fill orbitals of lower energy

first. 1s is filled before 2s , and 2s before2 p .

If two electrons occupy the same orbital,they must have opposite spin, as requiredby the Pauli Exclusion Principle.

When electrons have to choose between twoor more orbitals of the same energy,

electrons prefer to go into different orbitals.



electrons tend to half-fill orbitals of the same energy before pairing

with existing electrons to fillorbitals. This is known as Hund'sRule



Valency and Valence Electrons The outermost shell of an atom is its

valence shell, and the electrons in the

valence shell are valence electrons.

Valence electrons are the highestenergy electrons in an atom and aretherefore the most reactive.

Inner electrons (those not in thevalence shell) typically don'tparticipate in chemical bonding and

reactions,



Valence electrons can be gained, lost,or shared to form chemical bonds.

For this reason, elements with thesame number of valence electronstend to have similar chemicalproperties, since they tend to gain,lose, or share valence electrons in thesame way.

The Periodic Table was designed withthis feature in mind. Each element

has a number of valence electrons



The Octet Rule

The octet rule states that atoms becomeespecially stable when their valence shellsgain a full complement of valence electrons.For example, Helium (He) and Neon (Ne)

have outer valence shells that are completelyfilled, so neither has a tendency to gain orlose electrons. Therefore, Helium and Neon,two of the so-called Noble gases, exist in freeatomic form and do not usually form

chemical bonds with other atoms.

Most elements, however, do not have a fullouter shell and are too unstable to exist asfree atoms. Instead they seek to fill theirouter electron shells by forming chemicalbonds with other atoms and thereby attain



An element will tend to take the shortestpath to achieving Noble Gasconfiguration, whether that means

gaining or losing one electron.

For example, sodium, which has a singleelectron in its outer 3s orbital, can losethat electron to attain the electronconfiguration of neon. Chlorine, withseven valence electrons, can gain oneelectron to attain the configuration of argon.

Jan 14_Fundamentals of an Atom

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