ATOMIC STRUCTURE:ELECTRON CONFIGURATION

Atom , of all matter, consists of 3 fundamental particles:• Electrons• Protons• Neutrons Nucle

us

Shell

How do electron organise themselves in an atom?

BOHR MODELIn the Bohr model, electrons are viewed as particles traveling along circular orbits of fixed radius

QUANTUM MECHANICAL MODEL

•Electrons are viewed as waves rather than particles

•These waves are considered to be spread out through a region of space called an orbital.

Quantum mechanic model

An orbital: quantum mechanical equivalent of electron location

Wave function: a mathematical description of a wave

Schrödinger equation: HΨn=EnΨn

H: complex mathematical operatorE: energyΨ: wave function of electron

Further reading can be found in ‘Chemistry for Engineering Students’

Schrödinger equation: describes the energy of electrons

What can we see when we solve Schrödinger equation for an atom?

Ψ: wave function of electron is very complicated

From the wave function, we can get mathematical equation for atomic orbital, to describe this:

Quantum numbers:

Primary quantum number (n)

Secondary quantum number (l)

Magnetic quantum number (ml)

1. Primary quantum number (n) = shell• defines the shell in which a particular orbital is found• must be a positive integer (n = 1, 2, 3, 4, 5,……) When n=1: first shell

n=2: second shell

When number of electron >1 in a shell:• Repulsion between negative charges

Energy difference between orbital in a shell

2. Secondary quantum number (l) = subshell• provides a way to describe energy different

between the orbitals• l : 0, 1, 2, 3, …., n-1

3. Magnetic quantum number (ml):Possible values: from – l to +l

Under normal conditions, atoms can be specified by n and l. Under magnetic field:

Have you ever seen these orbitals?1s 2s 2p 3s 3p 3d 4s 4p 4d 4f etc.

Quantum numbers: n and l

How many electrons can occupy an orbital?

The Pauli exclusion principle states that: no two electrons in an atom may the same set of four quantum numbers: n, l, ml and ms.

THE SPIN QUANTUM NUMBER (MS).When placed in a strong magnetic field, electrons behave like tiny magnets.

Spin up+1/2

Spin down-1/2

How many electron can occupy an orbital?

2 electrons

p

d

f

Maximum occupied electrons

s

Electron configuration

• Fill orbitals with lowest energy – highest energy: the aufbau principle

1H 3Li

1s2 2s1

1s1

• Hund’s rule: within a subshell, electrons occupy orbitals individually whenever possible.

2p6C

1s

2s

2p

Highest energy

lowest energy

Electron configuration

Some examples

8O

15P

79Au

56Ba

The Periodic Table and Electron Configuration

• Electron of the highest energy orbital comes from the same subshell

Periodic trends in Atomic properties

Atomic size: explore the trends in the periodic table• within a group• within a period

Decrease

Atomic size:• within a group: numbers of shell increases

increase in atomic size and radii.

2

Atomic size:• within a period ????

+ -Attraction

+ +Repulsion

+Attraction

Repulsion

Only attraction force

+-

-+ +

Carbon

Hydrogen

Attraction

Repulsion

Carbon

Attraction Repulsion

Shielding: the masking of the nuclear charge by other electrons

Effective nuclear charge= Attraction - repulsion

Atomic size:• within a period:

Increase in atomic number (positively charge nuclear)

increase in the attraction forces (effective nuclear charge) between nucleus and electrons

decrease in atomic size

+ -Attraction

+ +Repulsion

Ionisation( or Ionization) energy

Ionisation energy: the energy required to remove an electron from an atom, forming a cation

First Ionisation energy: the amount of energy needed to induce the reaction

X(gas) X+(gas) + e-

Second Ionisation energy: the amount of energy needed to induce the reaction

X+(gas) X2+(gas) + e-

Ionisation( or Ionization) energy

Electron affinity : the energy required to add an electron to an atom, forming an anion

X(gas) + e- X-(gas) Ionisation energy: always positiveElectron affinity : could be negative or positive

• If X- NOT stable:

Requireenergy

X(gas) + e- X-(gas)

• If X- stable: X(gas) + e- X-(gas)

Release energy

Negative amount of energy

Negative electron affinity

Positive electron affinity

Most electron affinity values are negative