IONIZATION ENERGY

What is IONIZATION ENERGY?

It is the certain amount of

energy that is necessary to

knock off electrons from an

atom to form a positive ion.

The minimum amount of

energy required to dislodge

the least firmly attached

electron from an atom in the

gaseous state is called the

FIRST IONIZATION ENERGY.

SECOND IONIZATION ENERGY is

required to remove the second electron

from the gaseous atom; and so forth.

The SECOND IONIZATION ENERGY is

always HIGHER than the FIRST

IONIZATION ENERGY, since the electron

being removed is from an inner energy

level which has stronger electrostatic force

with the nucleus.

IONIZATION ENERGY may be

expressed in:

electron volts per atom

(eV/atom),

kilocalories per mole ( kcal/mol),

or

kilojoules per mole (kJ/mol)

Example:

The FIRST IONIZATION ENERGY for Sodium

(Na), for example, is 495 kJ/mol. This means that

495 kJ of energy must be supplied to remove the

first or least tightly held outermost electron from

each atom in 1 mole of sodium.

The SECOND IONIZATION ENERGY is 4560

kJ/mol. TAKE NOTE that the SECOND

IONIZATION ENERGY is significantly higher than

the FIRST IONIZATION ENERGY because the

next electron comes from the second period which

has a stronger electrostatic force with the nucleus

(nuclear charge).

The FIRST IONIZATION ENERGY for the atoms

going across a period from left to right generally

INCREASES.

Because the smaller the atom, the tightly its electrons

are held to the positively charged nucleus and the

more they are difficult to remove.

Note that the atomic radius decreases from left to

right (horizontally).

Atom such as that of group 1A, is larger than atom

such that of noble gases because the atomic radius

decreases from left to right.

Li has a larger atom compared to Ar, so Li needs

lower amount of energy to knock off electrons than Ar

which needs higher amount of energy because of its

smaller size and its electron are closely held to the

nucleus thus strong nuclear attraction than Li which

has a weak nuclear attraction.

FIRST IONIZATION ENERGY ENERGY for the atoms

from top to bottom generally DECREASES.

Because the bigger the atom, the loosely its electrons

are held to the positively charged nucleus and the

more they are easier to remove.

Note that atomic radius increases from top to bottom

(vertically).

Atom such that of noble gases like Helium, Neon,

Argon, Radon has a decreasing order of ionization

energy.

Helium which has a smaller atom, needs high amount

of energy to knock off because the electron is nearer

from the nucleus thus, strong nuclear attraction;

compared to Radon, w/c needs low amount of energy

because electrons has a farther distance from the

nucleus thus, weak nuclear attraction.

Farther distance from the

nucleus , which means that

the nuclear attraction is weak.

Thus, it is easier to remove

those electrons.

However, groups 3A and 6A have lower

ionization energies than 2A and 5A.

For example, the ionization energy of

Boron 3A (801 kJ/mol) is less than

Beryllium 2A (900 kJ/mol).

The electron removed from Boron 3A

comes from 2p and in Beryllium comes

from 2s, because 2p has a farther distance

from the nucleus thus less electrostatic

force with the nucleus than 2s that is nearer

from the nucleus that needs higher amount

of energy to knock off the electron.

Helium

Beryllium

Carbon Oxygen

Neon

Magnesium Silicon

Sulfur

Argon

Calcium

0

500

1000

1500

2000

2500

2 4 6 8 10 12 14 16 18 20

Ion

izati

on

en

erg

y (

kJ)

Atomic number

Plot of ionization potential versus atomic number (1eV/atom=96.49 kJ/mol)

The relationship between

electron volt and kilojoule is

1eV/atom=96.49 kJ/mol.