Periodic Trends

Atomic Radius

Atomic radius is controlled by

TWO important factors.

Effective nuclear charge, is the net positive
charge acting on an electron in a multi-electron
atom. The term "effective" is used because the

charged electrons beneath.

Period Trend = ?

Group Trend = ?

+

e-

e- e- e- e-

e-

Electrostatic Repulsion

Electrostatic Attraction

​Outer level electrons are both attracted to the positively charged nucleus and repelled by the negatively charged inner electrons.

electrons repel the

outer (valence)

electrons blocking
the nuclear effect
on these electrons.

The “hold” on the

valence electrons by

the nucleus is

weakened causing the
electrons to become

easier to remove.

Going down the group,

shielding increases
with the addition of
more energy levels.

Cesium is much
more reactive
than lithium!

With very little shielding, the positive nucleus pulls
the fluorine atom in closely, and it is VERY easy to

attract an 8thelectron to its outer energy level.

The most reactive nonmetal is fluorine, F.

Can you and your partner explain this?

1. Which element from group 2 is the most reactive?

a. Sr

b. Mg

c. Ca

d. Ba

2. Which element is more reactive than barium?

a. Ca

b. K

c. Ag

d. Fe

✓

-

1.43 Å

1.43 Å

✓

-

​Atomic Radius: one-half the distance between the nuclei of identical atoms that are bonded together.​

​Atomic radii are usually measured in picometers (pm) or angstroms (Å).
An angstrom is 1 x 10^-10m.​

Period Trend:

Going left to right across a period, the number of
energy levels remains the same.
The number of protons increases in the nucleus, increasing the
positive nuclear charge.

​Atomic radius decreases going across the period.​

The increasing positive charge of the nucleus
attracts electrons toward the nucleus – making
atoms smaller moving across the period left to
right.

Group Trend:

Atomic radius increases down the group.

t

t

t

Moving down a group, each new
period adds another energy level.
Each level is further from the nucleus
and increases the radius of the atom.