What Are Elements?

Elements are pure substances that cannot be broken down any further by chemical reactions. For example, oxygen (O) and hydrogen (H) are both types of elements. Whereas compounds, such as water (H2O), are made of two or more elements combined.

  • In the case of water, it is made up of two hydrogen atoms and one oxygen atom

An element is made of only one type of atom, which contains electrons, protons and neutrons. The protons and neutrons are located in the nucleus, while electrons occupy electron shells surrounding the nucleus.

Each element has a different number of protons, neutrons, and electrons. These determine the final properties of the element, and are also responsible for its position in the Periodic Table of the Elements.

In the illustration, two scales are shown: one with two hydrogen atoms and the other with an oxygen atom, indicating that oxygen is heavier than hydrogen.

For example, oxygen is heavier than hydrogen because the total number of protons and neutrons in its nucleus is greater than that of hydrogen.

Understanding protons, neutrons and electrons

Protons, neutrons and electrons are the basic building blocks of an atom. To see how they work, let’s look at two simple elements.

A comparison of elements

First, let’s look at hydrogen.

  • Hydrogen is the simplest and lightest element on Earth
  • It is typically found in the form of a colourless gas
  • The most common isotope (structure) of hydrogen has just one electron in the electron shells. In its nucleus, it has just one proton and no neutrons

In contrast, let’s look at the element carbon (C).

  • Carbon is a much heavier element than hydrogen
  • The most common isotope of carbon has 6 electrons in its electron shells. In its nucleus, it has 6 protons and 6 neutrons

How Elements are Presented in the Periodic Table

The first version of the Periodic Table was created by Dmitri Mendeleev, and he arranged the known elements based on their atomic mass. However, elements on the current periodic table are arranged in order of their increasing atomic number.

The Periodic Table is categorised by:

  • Groups – These are columns that run from left to right across the table. They are numbered from 1 to 18 or 1 to 8, depending on the table used
  • Periods – These are rows that run from top to bottom, numbered from 1 to 7.

Atomic number and mass number

The periodic table arranges elements based on their atomic number, which represents the number of protons in the nucleus. In a neutral atom, each element has the same number of protons and electrons. Therefore, the atomic number can be used to determine either the number of electrons or protons.

  • The atomic number is also known as the proton number because it always equals the number of protons in the nucleus

The mass number, which is the sum of an element’s protons and neutrons, is always greater than the atomic number. On the periodic table, the atomic number is usually displayed above the element’s symbol, while the mass number is below it.

Diagram showing the hydrogen (H) element with the atomic symbol 'H'. The atomic number is 1, and the atomic mass is 1.0078 atomic mass units (AMU)

This information is important, as it allows you to determine how many protons, neutrons and electrons each element has.


Hydrogen has an atomic number of 1 and a mass number of approximately 1. So, we can work out that:

  • it has 1 proton and 1 electron
  • it has no neutrons, since the mass number – the atomic number (1 – 1) is 0

Carbon has an atomic number of 6 and a mass number of approximately 12, so, we can calculate that:

  • it has 6 protons and 6 electrons
  • it has 6 neutrons, as the mass number – the atomic number (12 – 6) is 6