Which is larger, a sodium atom or a chlorine atom?

Both are in period 3 and have three occupied shells. Chlorine has more protons in its nucleus. A stronger pull draws the outer shell inwards. Sodium. Across a period the atom shrinks

An element in period 3, group 17 — 7 in the older numbering. What can you say about it?

Three occupied shells, seven valence electrons. One short of a full shell, so it takes an electron in. It sits to the right of the staircase line. Chlorine: a reactive non-metal forming an ion of charge 1−

Why is silicon used in chips while copper is used in cables?

Copper is a metal and always conducts well. Silicon is a metalloid with middling conductivity. Conductivity that can be controlled makes a switch possible. Because a switch needs controllable conductivity and a wire needs constant conductivity

The Periodic Table · Grade 9

What changes as you move across a single row of the periodic table?

Everything, and quickly. A row opens with soft metals that give electrons away easily, passes through metalloids in the middle, and reaches brittle non-metals that take electrons in. It ends at a noble gas that does not react. Across that row the proton count rises, so the atom gets smaller, not larger.

Learning objectives

Sort elements into metals, non-metals and metalloids using both their position and their properties
Describe how atomic size and reactivity change across a period and explain why
Locate the staircase line that separates the metals from the non-metals
Predict three properties of an unfamiliar element from where it sits in the table

What one row passes through

Period 3, for instance, opens with sodium — a soft metal that reacts vigorously with water. Then magnesium, harder and less energetic, and then aluminium.

In the middle sits silicon, a metalloid: shiny like a metal, brittle like a non-metal, and of middling electrical conductivity — the property the entire chip industry rests on.

Towards the end come phosphorus, sulfur and chlorine, non-metals that take electrons in, and the row closes with argon, which does not react. End to end in eight boxes.

Why the atom shrinks

Across a period electrons are added, but all of them go into the same shell. At the same time a proton is added at every step, so the nuclear charge grows.

A more highly charged nucleus pulls that same outer shell in harder, and it is drawn inwards. The result is counter-intuitive: adding electrons along a row makes the atom smaller.

Going down a group the picture is reversed. There a whole new shell is added at every step, so the atom grows. Both directions are governed by one question: was a shell added, or only a proton?

The staircase line

A stepped line on the right-hand side of the table separates the metals to its left from the non-metals to its right. The elements the line itself touches are the metalloids.

A metal is shiny, conducts heat and electricity, can be hammered into shape, and gives electrons away. A non-metal usually does not conduct, is brittle as a solid, and takes electrons in. A metalloid sits between the two on each of those properties.

Combining group and period lets you predict three things about an element you have never studied: whether it is metallic, how many electrons will move, and whether it is especially reactive or barely at all.

Worked examples

Which is larger, a sodium atom or a chlorine atom?
1. Both are in period 3 and have three occupied shells
2. Chlorine has more protons in its nucleus
3. A stronger pull draws the outer shell inwards
Answer: Sodium. Across a period the atom shrinks
An element in period 3, group 17 — 7 in the older numbering. What can you say about it?
1. Three occupied shells, seven valence electrons
2. One short of a full shell, so it takes an electron in
3. It sits to the right of the staircase line
Answer: Chlorine: a reactive non-metal forming an ion of charge 1−
Why is silicon used in chips while copper is used in cables?
1. Copper is a metal and always conducts well
2. Silicon is a metalloid with middling conductivity
3. Conductivity that can be controlled makes a switch possible
Answer: Because a switch needs controllable conductivity and a wire needs constant conductivity

Common mistakes

Assuming the atom grows across a period because electrons are added: They are added to the same shell while the nuclear charge rises. The pull strengthens and the atom contracts.
Using the same rule going down as going across: Down a group a whole shell is added and the atom grows; across a period only a proton is added and it shrinks. It is not the same rule.
Classing a metalloid as a weak metal: It is not a poor metal but a category of its own, with properties between the two. Middling conductivity is what makes it useful, not a shortcoming.

What to remember

Across a period: metals, metalloids, non-metals, noble gas.
Across, the atom shrinks; down, it grows.
The staircase line separates metals from non-metals.
Group and period together are enough to predict an unfamiliar element.