atomic structure

atomic structure

1. All substances are made from atoms. Each atom is made of a nucleus - containing protons and neutrons - surrounded by electrons.

   • The atomic number is the number of protons in an atom. The elements are arranged in the periodic table in ascending order of atomic number.
   • The mass number of an atom is the total of protons plus neutrons. Atoms of the same element with different numbers of neutrons (and hence different mass numbers) are called isotopes of that element.