Electron configuration (3.1.1.3)

What is an orbital?

An orbital is a 3D region of space within an atom in which an electron is most likely to be located.

What are the different orbitals and what are their shapes?

Sub-shell	Shape	Orbitals
s	sphere	one orbital per energy level
p	dumbell	three orbitals per energy level, from 2
d	various	five orbitals per energy level, from 3
f	various	seven orbitals per energy level, from 4

There are 2 electrons in each orbital.

What are the rules that are followed when orbitals are filled up?

1) Each orbital can hold a maximum of 2 electrons. When electrons are paired up they have an opposite spin - Pauli’s exclusion principle

2) Electrons always fill up the lowest energy orbital available - Aufbau principle

3) Electrons will occupy different orbitals of the same energy before pairing up in the same orbital - Hund’s rule

How can electron configurations be displayed?

Configuration

The energy level is show with the sub-shell and how many electrons are in each. E.g. Carbon with 6 electrons: 1s² 2s² 2p²

Shorthand can also be used where the previous noble gas can be used in place of the first energy levels. E.g. K with 9 electrons: [Ar] 4s¹

Box notation

Boxes are drawn, one for s orbitals, 3 for p and 5 for d. Electrons are then drawn as arrows facing up and down as they have opposite spin. Fill up each orbital of a sub-shell with one before filling up one with 2.

How do orbitals fill up and what are the exceptions to this rule and why?

Electrons fill out the lowest energy orbitals first. Thus, 4s comes before 3d and then 4p follows

Exceptions

	Chromium	Copper
Should be	… 4s2 3d4	… 4s2 3d9
Actually is	… 4s1 3d5	… 4s1 3d10
Reason	Minimises electron - electron repulsion. There are small differences in energy between 4s and 3d so the electrons are more stable.	Minimises electron - electron repulsion. There are small differences in energy between 4s and 3d so the electrons are more stable.

How do ions form?

Electrons are added or taken away from the orbitals. N.B. while the 4s orbital is filled before the 3d orbital, when an ion is form, electrons are taken from the 4s orbital before the 3d orbital.

What are the first and second ionisation energies?

First 

The energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1 + ions.

X_(g) → X⁺_(g) + e^-

Second 

The energy required to remove one mole of electrons from one mole of gaseous ions of an element to form one mole of gaseous 2+ ions.

X⁺_(g) → X²⁺_(g) + e^-

What are the points that need to be mentioned when talking about the trend in ionisation energies?

Nuclear charge

Atomic radius

Shielding

Attraction

What is the trend in first ionisation energy across a period and the exceptions?

General trend

The general trend is that the first ionsation energy increases across the period as there is an increasing nuclear charge with little/no increase in shielding. Thus the atomic radius decreases and thus the attraction between the outer electron and nucleus increases.

Exceptions

Between Be and B and Mg and Al the ionisation energy decreases as the p-electron is held further away from the nucleus so there is less attraction and less energy needed to remove it.

Between N and O and P and S the ionisation energy decreases because from 3p³ to 3p⁴ electrons have to pair, thus electron - electron repulsion makes it easier to remove an electron. This is known as spin pair repulsion.

What is the trend in first ionisation energy down a group?

General trend 

The first ionisation energy decreases down a group as although the nuclear charge increases, the atomic radius also increases as more shells of electrons are added. The shielding also increases as there are more shells of electrons. Thus the attraction between the nucleus and outer electron decreases.

What is the trend in successive ionisation energies of an element?

The successive ionisation energies of an element increases. This is because as electrons are removed the attraction between the outer electron and nucleus increases as the electron being removed is closer to the nucleus

When one electron is removed a positive ion is formed, this ion increases the attraction on the remaining electrons so more energy is required to remove the outer electron

What does a jump in energy between successive ionisation energies show?

It shows a new shell