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Atoms (Greek άτομον) are the basic units that make up all matter. They are sub-microscopic structures and the smallest part of a substance that can take part in chemical reactions. Atoms exist in more than 110 different sizes that we call elements. These elements are arranged into groups in the Periodic Table of Elements based on properties they have in common.
Each atom is made up of a nucleus and a number of orbiting electrons. The nucleus is found at the center of an atom. It is composed of particles called protons, which are positively charged, and neutrons, which have no charge. Therefore the nucleus always has a positive charge. A neutron and a proton are jointly called a nucleon.
The electrons that orbit the nucleus have a negative charge. When atoms have the same number of electrons and protons, they have a neutral charge. When atoms have more protons than electron they have a positive charge. By contrast, if there are more electrons than protons, the atom is negatively charged. Charged atoms are called ions.
