The Layout of the Periodic Table
The periodic table arranges all known elements in order of increasing atomic number (number of protons).
The arrangement is not random — it groups elements with similar properties together.
Periods — Horizontal Rows
A period is a horizontal row across the periodic table.
- There are 7 periods.
- Period 1 has only 2 elements (hydrogen and helium).
- Period 2 has 8 elements (lithium to neon).
- Period 3 has 8 elements (sodium to argon).
What changes across a period? Moving left → right across a period:
- The atomic number increases by 1 each step.
- The number of protons and electrons increases.
- The number of electron shells stays the same.
- Properties change gradually from metal to non-metal.
Period 3 — sodium to argon
Na (11), Mg (12), Al (13), Si (14), P (15), S (16), Cl (17), Ar (18). All have 3 electron shells. Properties shift from soft metal (Na) to gas (Ar).
Groups — Vertical Columns
A group is a vertical column in the periodic table.
- There are 18 groups (numbered 1–18, or sometimes I–VIII for the main groups).
- Elements in the same group have the same number of outer electrons.
- This gives them similar chemical properties.
| Group | Name | Outer electrons | Typical behaviour |
|---|---|---|---|
| 1 | Alkali metals | 1 | Very reactive metals |
| 2 | Alkaline earth metals | 2 | Reactive metals |
| 7 (17) | Halogens | 7 | Reactive non-metals |
| 0 (18) | Noble gases | 8 (full) | Very unreactive |
The group number = number of outer (valence) electrons for Groups 1–7. These outer electrons determine how an element reacts.
Why Properties Repeat — Periodicity
Mendeleev noticed that properties repeated in a pattern every 8 elements. This is called periodicity.
Elements in the same group behave similarly because they have the same number of outer electrons available for bonding or reactions.
Group 1 — all react with water
Lithium, sodium and potassium all react with cold water to produce a metal hydroxide and hydrogen gas. They have the same number of outer electrons (1), so they react in the same way — though reactivity increases down the group.
Finding an Element's Position
If you know an element's electron configuration you can find its position:
- Number of electron shells → period number
- Number of outer electrons → group number
Position from electron configuration
Applies to the main group (s- and p-block) elements.
Real world: Group 1 metals in your body and food
Sodium (Group 1, Period 3) and potassium (Group 1, Period 4) are both essential to your body. Table salt is NaCl — sodium ions regulate fluid balance and nerve signals. Potassium is in bananas and most vegetables. They behave similarly (same group) but potassium is more reactive than sodium because it is lower in Group 1 — its outer electron is held less tightly.
Real world: Halogens in everyday products
Fluorine (Group 17, Period 2) and chlorine (Group 17, Period 3) are both in the same group. They are both reactive oxidisers. Fluoride is in your toothpaste; chlorine purifies swimming pool water. Their similar reactivity comes from having 7 outer electrons (one short of a full shell) — they both strongly "want" to gain one electron.
Where do you see this in real life? Knowing an element's group and period lets you predict its properties. A period-3 Group 1 element (sodium) will behave like a period-2 Group 1 element (lithium) — reactive with water, forming +1 ions, producing alkaline hydroxides. The periodic table is a prediction tool.
Practice Problems
Sodium has 11 electrons arranged 2, 8, 1. Which period and group is it in?
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Why do fluorine (F) and chlorine (Cl) have similar chemical properties?
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An element is in Period 2 and Group 6. How many electron shells does it have, and how many outer electrons?