Trends in the Periodic Table and Bonding
Periodic Trends in
Ionisation Energies
and Covalent Radii
Covalent Radii of Elements
The size of an atom is measured by it’s covalent radius,
the distance between the nucleus and it’s outer
electrons.
nucleus
covalent
radius
energy
levels
Values for covalent radii can be
found on page 5 of the data book
Looking across a period
Across a period we can see the covalent radius
decreasing.
As we move left to right we are adding a proton to the
nucleus and an electron to the outermost energy level.
So, from lithium to fluorine:
- -
3+
Lithium Atom
-
9+
- Fluorine Atom
-
Looking across a period
The lithium atom
has a smaller
nuclear charge
than neon and so a
larger covalent
radius
Fluorine’s greater
nuclear charge pulls
the outer energy level
in closer.
- - -
3+
radius = 134pm
- -
9+
- - -
- - radius = 71pm
Looking down a group
-
Li
-
- - - - -- - - - - - - -- - Cs
------- ---- - - - - -- - - - - -
single electron
in the
outermost
energy
level isbetween
much
The caesium
atom also
has
many more
electrons
further
from
the nucleus
caesium.
the
single
outer
electron in
and
the nucleus.
causes theeffect
caesium
atom to have
a much larger
This screening
counteracts
the attraction
from
covalent
radius.
the
greater
nuclear charge.
Atomic Size Summary
Decreasing Atomic Size
Across a period from left to right atomic size decreases
This is because of the atom having more electrons & protons and therefore
a greater attraction which pulls the atom closer together hence the smaller
size.
Atomic Size Summary
Increasing Atomic Size
Decreasing Atomic Size
Down a group atomic size increases
This is because of the extra outer energy levels and the screening effect of
the outer electrons.
Ionisation Energy
The ionisation energy is the energy required to remove
one mole of electrons from one mole of atoms in the
gaseous state.
The first ionisation energy of magnesium:
Mg (g)
Mg+ (g)
+
e-
744 kJmol-1
Values for ionisation energies can be
found on page 10 of the data book
Ionisation Energy
The second ionisation energy of magnesium:
Mg+ (g)
Mg2+ (g)
+
e-
1460 kJmol-1
The third ionisation enthalpy shows a massive increase
because it requires an electron to be removed from
magnesium’s second energy level.
Mg2+ (g)
Mg3+ (g)
+
e-
7750 kJmol-1
Looking across a period
From lithium to neon the first ionisation energy
increases. Why?
Li
Be
B
C
N
O
F
Ne
Li (g)
Li+ (g)
+
e-
526 kJmol-1
Ne (g)
Ne+ (g)
+
e-
2090 kJmol-1
An atom of Lithium
The lithium atom has 3 protons inside the nucleus
3+
The outer electron is attracted by a
relatively small nuclear charge
Li (g)
Li+ (g)
+
e-
526 kJmol-1
An atom of Neon
The neon atom has 10 protons inside the nucleus
-
-
-
-
10+
-
-
Each of neon’s eight outer electrons is
attracted by a stronger nuclear charge
Ne (g)
Ne+ (g)
+
e-
2090 kJmol-1
Looking down a group
The first ionisation energy decreases down a group in
the periodic table. Why?
Li (g)
Li+ (g)
+
e-
526 kJmol-1
Cs (g)
Cs+ (g)
+
e-
382 kJmol-1
2.
1. Screening
More Energy
Effect
Levels
As
we saw with
atomicforsize,
the single
electron
in theby
Caesium’s
attraction
its outer
electron
is lowered
outermost
energy
level
is much
fromelectrons.
the nucleus in
the screening
effect
caused
by further
all its other
caesium than in lithium.
-
Li
-
- - - - -- - - - - - - -- - Cs
------- ---- - - - - -- - - - - -
Ionisation Energy Summary
Increasing Ionisation Energy
Across a period from left to right ionisation energy increases
This is due to the increase in atomic charge having a greater pull on the
electrons and therefore more energy is required to remove electrons.
Ionisation Energy Summary
Decreasing Ionisation Energy
Increasing Ionisation Energy
Down a group ionisation energy decreases
This is due to the outer electrons being further away from the nucleus and
so the attraction is weaker and they are more easily removed.