Handout on Chapter 13
The Periodic Table
Trends of the Periodic Table
 Trends of the Group I elements:
Lithium, Sodium, Potassium, Rubidium, Caesium and Francium are the group one elements.
Down the group,
a. The atomic size increases
Down the group:

Each element has one more electron shell than the one above it.

Electron shells are added further from the nucleus.

Increased shielding effect from inner electrons reduces nuclear attraction on the outer
electron.
Because of:

More shells

More shielding
The outer electron is held farther away → atomic radius increases.
b. Melting point (mp) and boiling point (bp) decrease down the group

Group 1 elements are metals held together by metallic bonding.

Each atom contributes one valence electron to the “sea of delocalised electrons.”

As you go down the group:

o
Atomic size increases.
o
The outer electron is farther from the nucleus.
o
The positive ion becomes larger.
This weakens the attraction between the positive metal ions and the delocalised
electrons.
Weaker metallic bonding means:

Less energy is needed to break the bonds.

Therefore, melting and boiling points decrease down the group.
c. Reactivity increases down the group
Group 1 elements react by losing one electron to form a +1 ion.
Down the group:

The outer electron is farther from the nucleus.

Shielding increases.

Nuclear attraction for the outer electron decreases.
So:

The outer electron is lost more easily.

Reactivity increases.
That’s why:

Potassium reacts more violently with water than Sodium.

Caesium is even more reactive.
 Trends of the Group VII Elements:
The group VII elements, the Halogens include Fluorine, Chlorine, Bromine, Iodine.
a. Why atomic size increases down the group
Down the group:

Each element has an extra electron shell.

Shielding increases.

The outer electrons are further from the nucleus.
So, atomic radius increases.
b. Melting point (mp) and boiling point (bp) increase down the group
Halogens exist as diatomic molecules (F₂, Cl₂, Br₂, I₂).

The molecules are held together by weak intermolecular forces (London dispersion
forces).

Down the group:

o
Molecules become larger.
o
They have more electrons.
This strengthens intermolecular forces.
Stronger intermolecular forces require:

More energy to break

Therefore, melting and boiling points increase down the group.
That’s why:

Fluorine and Chlorine are gases,

Bromine is a liquid,

Iodine is a solid.
c. Why reactivity decreases down the group
Halogens react by gaining one electron to form a −1 ion.
Down the group:

Atomic size increases.

Shielding increases.

Attraction between the nucleus and an incoming electron decreases.
Therefore:

It becomes harder to gain an electron.

Reactivity decreases.
This is why:

Chlorine can displace Bromine from solution,

but bromine cannot displace chlorine.
 The table down below summarizes the following information:
 The atomic size decreases across a period (from left to right in the periodic table)
because of the following reasons:
1. Increasing nuclear charge:
o
As you move across a period, each element has one more proton in the nucleus.
o
This stronger positive charge pulls the electrons closer to the nucleus.
2. Electrons added to the same shell:
o
The new electrons go into the same principal energy level (same shell), not a
new, outer shell.
o
This means the number of inner electrons (shielding) doesn’t increase much.
3. Shielding effect stays nearly constant:
o
Since the added electrons are in the same shell, they cannot shield each other
effectively from the nucleus.
o
Therefore, the effective nuclear charge felt by each electron increases.
Result: Electrons are pulled closer to the nucleus → atomic radius decreases across the
period.
In the diagram above, Lithium is larger than Beryllium being in the same period as we
move across the period from left to right.