STRUCTURE OF AN ATOM AND THE PERIODIC TABLE . _Structure of an atom What is an atom ? An atom is the smallest particle of an element which can take part in a chemical reaction. – An atom has a diameter of 10–⁸ cm. ( 0.00000001) . – An atom is made up of three sub atomic particle; a) Protons b) Neutrons c) Electrons – The protons and neutrons are found inside the nucleus of an atom while electrons are found in the region around the nucleus ( in the shells or energy levels ) . – Protons are positively charged (+ve) while electrons are negatively charged (-ve) . The neutrons have no charge ( neutral particles ) . Why is an atom said to be electrically neutral ? In any given atom , the number of protons ( in the nucleus) equal to the number of electrons( in the shells ) p=e. Where p= protons e= electrons Why are protons and neutrons referred to as nucleons ? This is because both the protons and neutrons are found in the nucleus of an atom of an element.