Describe the structure of an atom.

An atom is composed of a nucleus containing protons and neutrons, surrounded by electrons in energy levels.

An atom, the smallest unit of a chemical element, is made up of three types of subatomic particles: protons, neutrons, and electrons. The nucleus, located at the centre of the atom, contains the protons and neutrons. This is the heaviest part of the atom, as protons and neutrons are much more massive than electrons. The number of protons in the nucleus determines the atomic number and identifies the element.

Surrounding the nucleus are the electrons, which are arranged in energy levels, also known as shells or orbitals. These energy levels can be visualised as concentric circles around the nucleus, with each level having a maximum capacity of electrons it can hold. The first energy level can hold up to 2 electrons, the second can hold up to 8, and so on. The arrangement of electrons in these energy levels determines the chemical behaviour of the atom.

The electrons in the outermost energy level, known as the valence electrons, are particularly important as they are involved in chemical reactions. Atoms strive to have a full outer energy level, which usually means having 8 electrons in it. This is known as the octet rule and is a key principle in understanding chemical bonding.

In a neutral atom, the number of protons (positively charged) is equal to the number of electrons (negatively charged), resulting in an overall charge of zero. However, atoms can lose or gain electrons to become ions with a positive or negative charge.

The atomic mass of an atom is the sum of the number of protons and neutrons in the nucleus. It's worth noting that the mass of an electron is so small that it doesn't significantly contribute to the overall atomic mass.

In summary, the structure of an atom is a central nucleus of protons and neutrons, surrounded by electrons in energy levels. The arrangement of these particles determines the atom's properties and behaviour.