Course 9.2Lesson 2 of 3

Atomic size and metallic character

The atomic size decreases across a period, because the nucleus has more protons and it pulls the shells closer. It increases down a group, because each new period adds a shell. Metallic character is the tendency to lose electrons: it decreases across a period and increases down a group.

about 25 min1 simulation2 practice questions

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

How big is an atom?

The atomic size is the distance from the centre of the nucleus to the outer shell. We measure it in picometres, $\text{pm}$ . One picometre is $10^{-12}\ \text{m}$ .

Across a period

Take period $3$ , from sodium to chlorine. All of these atoms have three shells. But the nucleus has more and more protons: $11$ in sodium and $17$ in chlorine. The electrons are added to the same shell, and the bigger positive charge pulls them closer to the nucleus.

So the atomic size decreases from left to right across a period.

Element	Na	Mg	Al	Si	P	S	Cl
Radius ( $\text{pm}$ )	$186$	$160$	$143$	$118$	$110$	$103$	$99$

Down a group

Take group $1$ : lithium, sodium, potassium. Each one has one more shell than the one above it. The outer electron is farther away from the nucleus.

So the atomic size increases down a group.

Element	Li	Na	K	Rb	Cs
Radius ( $\text{pm}$ )	$152$	$186$	$227$	$248$	$265$

Two bar charts of atomic radius in picometres. Across period 3 the radius decreases: Na 186, Mg 160, Al 143, Si 118, P 110, S 103, Cl 99. Down group 1 the radius increases: Li 152, Na 186, K 227, Rb 248, Cs 265.

You should find that the bars fall across period 3 but rise down group 1.

Metallic character

A metal gives away electrons easily. The metallic character of an element is its tendency to lose electrons.

Across a period, it decreases. The atom is smaller, the outer electrons are held more strongly, and it is harder to remove them. Sodium is a metal. Chlorine is a non-metal.
Down a group, it increases. The outer electron is farther from the nucleus and it is easier to remove. In group $14$ , carbon is a non-metal, silicon is a metalloid, and tin and lead are metals.

An outline of the periodic table with two arrows. A red arrow across a period from left to right says that the size and the metallic character decrease. A blue arrow down a group says that the size and the metallic character increase. The most metallic elements are at the left and the bottom. The most non-metallic elements are at the top right, not counting the noble gases.

So the most metallic elements are at the bottom left of the table. The most non-metallic ones are at the top right, not counting the noble gases.

The metalloids

A thick stair-step line separates metals from non-metals. The elements along it, such as boron, silicon, germanium and arsenic, have properties of both. They are metalloids.

A common mistake

Students say that the atom gets bigger across a period, because it has more electrons. The extra electrons go in the same shell, and the extra protons pull them in. The atom gets smaller.

Practise with new numbers

Try it in Python

This program checks the two trends in the sizes.

Remember these

Atomic size: decreases across a period (more protons pull the same shells in), and increases down a group (a new shell is added).
Metallic character is the tendency to lose electrons. It decreases across a period and increases down a group.
Metalloids lie along the stair-step line.