Ionization energy is the minimum amount of energy required to remove an electron from an atom or ion. It increases with each successive ionization as the electron removed is closer to the nucleus. Generally, first ionization energy increases from left to right across a period as atomic radius decreases, making electrons more tightly bound. It decreases from top to bottom in a group as atomic radius increases, meaning electrons are farther from the nucleus. The second ionization energy is always higher than the first as removal of the second electron requires overcoming a stronger force of attraction to the positively charged ion.

1. IONIZATION ENERGY

2. 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.

3. 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.

4. 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.

5. IONIZATION ENERGY may be
expressed in:
 electron volts per atom
(eV/atom),
kilocalories per mole ( kcal/mol),
or
kilojoules per mole (kJ/mol)

6. 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).

7. 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.

8. 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.

9. Farther distance from the
nucleus , which means that
the nuclear attraction is weak.
Thus, it is easier to remove
those electrons.

10. 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.

11. 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
Ionizationenergy(kJ)
Atomic number
Plot of ionization potential versus atomic number
(1eV/atom=96.49 kJ/mol)

12. The relationship between
electron volt and kilojoule is
1eV/atom=96.49 kJ/mol.