Key Concepts: The Atom

Every substance in the world is made up of tiny particles called atoms.
An atom is the smallest unit of a chemical element that can exist, maintaining the properties of that element.
The central part of an atom is the nucleus. It contains protons which are positively charged, and neutrons which have no charge.
Moving around the nucleus are electrons. They are in energy levels (shells) and carry a negative charge.
Protons and neutrons have roughly the same mass, but electrons are much lighter.
The number of protons in an atom defines the atomic number of that element.
The mass number of an atom is the total number of protons and neutrons.
The chemical properties of an atom are determined by the number and arrangement of its electrons.
In an atom, the total number of electrons is equal to the total number of protons in the nucleus. This means that overall, an atom is electrically neutral.
The model of atom has developed over the years, from the solid sphere model, to the plum pudding model, to the nuclear model, to finally, the quantum model which is the current understanding.
Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number, but different mass numbers.
The Relative Atomic Mass (Ar) refers to the weighted average of the atomic masses of the different isotopes in a sample of the element. This takes account of both mass and abundance of isotopes present.
Atoms in elements can combine in various ways to create compounds. This involves a chemical change, and new substances with different properties result.
A molecule is the smallest part of a chemical compound that can exist, maintaining the properties of that compound. It consists of two or more atoms bonded together.
The Periodic Table is a systematic arrangement of elements, based on their properties, atomic number and the arrangement of electrons in outer shells, and it is highly useful for predicting chemical behaviour.