Atoms, Elements and the Periodic Table

Atoms are the building blocks of substances

Every substance is made of atoms. This applies to solids, liquids and gases: the materials may look and behave very differently, but they are all built from atoms.

An atom is the smallest part of an element that can exist. For example, an oxygen atom is the smallest unit that is still oxygen. Atoms contain smaller particles, but an isolated proton or electron is not an atom of oxygen—or an atom of any other element.

An element contains one type of atom

An element is a pure substance made from only one type of atom. Oxygen contains only oxygen atoms, while sodium contains only sodium atoms. Each element has its own kind of atom, so oxygen atoms are different from sodium atoms.

There are about 100 different elements. Chemists organise them in the periodic table, where each element has its own position, name and chemical symbol.

Chemical symbols are a shared shorthand

Each element is represented by a chemical symbol. A symbol represents the element and, when written by itself in this context, can represent one atom of that element. For example:

• O represents oxygen or one oxygen atom;
• Na represents sodium or one sodium atom.

The first letter of a symbol is always a capital. If there is a second letter, it is lower-case. Therefore Na is correct, whereas NA and na are not. Symbols do not always match the opening letters of the English name: potassium has the symbol K.

The first 20 elements

The first 20 elements are numbered in order in the periodic table. You need to connect each name with its symbol.

Notice the patterns, but check every letter carefully. C, Ca and Cl, for example, represent three different elements: carbon, calcium and chlorine.

Finding Groups 1 and 7

A group is a vertical column of the periodic table. Group 1 is the column on the far left, while Group 7 is the column immediately to the left of Group 0.

The Group 1 alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr). Hydrogen is commonly shown above this column because of its position in the table, but it is not an alkali metal.

The Group 7 elements are the halogens: fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). Modern periodic tables also place tennessine (Ts) below astatine.

Some of these are already among the first 20: lithium, sodium and potassium occur in Group 1, while fluorine and chlorine occur in Group 7. This overlap helps connect the numbered list with the table’s columns.

The periodic table supplied in the examination lets you look up an unfamiliar element. Read across its entry to connect the name and symbol, and read vertically to decide whether it belongs to Group 1 or Group 7.