Atoms are constructed of the 3 fundamental particled, protons, neutrons and electrons:

PROTONS: Relative charge 1+. Relative mass of 1. Appear in the nucleus.

NEUTRONS: Realtive charge 0. Relative mass of 1. Appear in the nucleus.

ELECTRONS: Relative charge 1-. Relative mass of 0. Appear in energy levels, or more technically they appear in electron orbitals surrounding the nucleus.

Chemical properties of an atom depend on the electronic arrangement (configuration). Physical properties of an atom depend on the number of protons in the nucleus (ATOMIC NUMBER). The number of protons and neutrons in the nucleus if the MASS NUMBER. An atom with a differing number of neutrons but the same number of protons is called an ISOTOPE.