Elements are arranged in order of increasing atomic number.

Definitions:

• The atomic number (Z) of an element is the number of protons in the nucleus of one atom of that element E.G; Z=2 for helium.
• A group is therefore a vertical column of elements.
• A period is a horizontal row of elements on the periodic table.
• The mass number (A) of an atom is the number of protons and neutrons in the nucleus.
• The neutron number is the number of neutrons in the nucleus of an atom.
• A nuclide is an atom whose proton number is written as X, where X is the chemical symbol.

par Claude Foumtum