Electron Configurations and Periodicity

Stern and Gerlach (1921) first observed electron spin magnetism.

Wolfgang Pauli (1925) - The Pauli Exclusion Principle governs the arrangement of

electrons in an atom. It summarizes experimental observations and states that "no

two electrons in an atom can have the same four quantum numbers".

An electron configuration of an atom is a particular distribution of electrons among

available subshells.

Example: Li

B

C

Hund's rule - for degenerate orbitals, the lowest energy is attained when the

number of electrons with the same spin is maximized. Therefore the electrons will

occupy singly to the maximum extent possible with spins parallel.

N

O

F

Based on, in part, the fact that electrons repel one another, occupying different

orbitals, electrons remain far apart minimizing electron-electron repulsion.

Ne

Inner shell = core electrons

Outer shell = valence electrons

Aufbau Principle, also known as the building up principle is a scheme used to

reproduce the electron configuration of the ground states of atoms by successfully

filling subshells with electrons in a specific order (the building up order).

Z = atomic number = number of electrons in the atom

Writing Electron Configurations Using the Periodic Table

Use the building up principle

If you write the configurations of an atom with its subshells arranged by shells, the

orbitals will usually be in order by energy, with the subshells used to describe

chemical reactions at the right.

For example, the configuration of Br would be written [Ar] 3d10

4s24p

5, however,

you will see the orbitals in a configuration written in the building up order, which

is an empirical order that has been found to reproduce most of the atomic

configurations. The configuration of Br would be written [Ar]4s23d

104p

5

Unpaired electrons exhibit magnetism

Paramagnetic substances are substances that are weakly attracted to a magnetic

field.

Diamagnetic substances are substances that are not attracted to a magnetic field.

Electron configurations of atoms display periodic variation with an increase of

atomic number. these variations can be seen in their chemical and physical

properties.

3 Physical Properties of an Atom

1. Atomic radius

2. Ionization energy

3. Electron affinity

Atomic Radius - determined from the distance between two bonded atoms

General trends in atomic radius by two factors;

1. The principle quantum number, n of the orbital

2. Effective nuclear charge - the positive charge that an electron experiences

from the nucleus, equal to the nuclear charge but reduced by any shielding or

screening from any intervening electron distribution.

2. Ionization Energy - the minimum energy required to remove an electron from

the ground state of the isolated gaseous atom.

3. Electron Affinity is the energy change for the process of adding an electron to a

neutral atom in the gaseous state to form a negative ion.