ELECTRONIC STRUCTURE ELECTRONIC STRUCTURE AND THE PERIODIC LAWAND THE PERIODIC LAWDr Sharipah Ruzaina Syed ArisDr Sharipah Ruzaina Syed Aris

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The distribution of electrons among the orbitals of an atom is called the electronic structure or electronic configuration

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To indicate the ground state electron configuration we can:

1) List the subshells that contain electrons and indicate their electron population with a superscript.

2) Represent each orbital with a circle and use arrows to indicate the spin of each electron.

Electron configurations must be consistent with the Pauli principle, aufbau principle, and Hund’s rule

Example: N 1s22s22p3, Na 1s22s22p63s1

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Electron configurations explain the structure of the periodic table

The periodic table is divided into regions of 2, 6, 10, and 14 columns which is the maximum number of electrons in s, p, d, and f sublevels.

Subshells that fill across the periods.

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For the representative elements (A Groups) the electrons with the highest n value or valence shell are normally the only electrons important for chemical properties

For these elements the valence electrons consist of just the s and p subshells encountered crossing the period that contains the element in question

Example: the valence configuration of bromine is Br 4s24p5

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There are few important exceptions to the “expected” electronic figurations of commonly encountered elements

Following the rules for Cr, Cu, Ag, and Au using noble gas notation:

11029

11029

11029

1524

6[Xe]5 6[Xe]5Cu

5[Kr]4 5[Kr]4 Ag

4[Ar]3 4[Ar]3Cu

4[Ar]3 4[Ar]3Cr

alExperiment ExpectedElement

sdsd

sdsd

sdsd

sdsd

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Illustrating Orbital Occupancies

The electron configuration

n l# of electrons in the

sublevel

as s,p,d,f

The orbital diagram (box or circle)

Order for filling energy sublevels with electrons

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dark - filled, spin-paired

light - half-filled

no color-empty

A vertical orbital diagram for the Li ground state

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Condensed ground-state electron configurations in the first three periods.

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A periodic table of partial ground-state electron

configurations

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PERIODIC TRENDS PERIODIC TRENDS IN IN ATOMIC PROPERTIESATOMIC PROPERTIES*size*size*Ionization energy*Ionization energy*electron affinity*electron affinity

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Variation in atomic and ionic radii. Values in picometers (10-12 m)

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The size trends in ions can be summarized:◦Positive ions are always smaller than

the atoms they are formed ◦Negative ions always larger than the

atoms from which they are formedAdding electrons leads to an increase in size of a particle, as illustrated for fluorine. Removing electrons decreases the size of the particle, as shown for lithium and iron.

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Ionization energy (IE) is the energy required to remove an electron from an isolated, gaseous atom

Successive ionizations are possible until no electrons remain

The trends in IE are the opposite of the trends in atomic size

egXgX )()(

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Periodicity of first ionization energy (IE1)

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Variations in first ionization-energies. Elements with the largest ionization energies are in the upper right of the periodic table. Those with the smallest ionization energy are at the lower left.

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Ranking Elements by First Ionization Energy

PLAN:

SOLUTION:

PROBLEM: Using the periodic table only, rank the elements in each of the following sets in order of decreasing IE1:

(a) Kr, He, Ar (b) Sb, Te, Sn (c) K, Ca, Rb (d) I, Xe, Cs

IE decreases as you proceed down in a group; IE increases as you go across a period.

(a) He > Ar > Kr

(b) Te > Sb > Sn

(c) Ca > K > Rb

(d) Xe > I > Cs

Group 8A(18) - IE decreases down a group.

Period 5 elements - IE increases across a period.

Ca is to the right of K; Rb is below K.

I is to the left of Xe; Cs is furtther to the left and down one period.

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The electron affinity (EA) is the potential energy change associated with the addition of an electron to a gaseous atom or ion in its ground state

The addition of one electron to a neutral atom is exothermic for nearly all atoms

The addition of more electrons requires energy

)()( gXegX

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Electron affinities of the main-group elements.

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In general:◦EA increases from left to right in a

period◦EA increases bottom to top in a group

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Trends in three atomic properties.

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Which has the larger second ionization energy?

lithium or berylliumWhy?

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Arrange the elements oxygen, fluorine, and sulfur according to increasing◦ Ionization energy◦ Atomic size