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Atomic Structure · 4 min read

Inside the atom

Illustration for Inside the atom in Atomic Structure
Inside the atom · Atomic Structure

An atom has a small dense nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons in shells.

The atomic number is the number of protons and identifies the element. The mass number is the total number of protons and neutrons. Neutrons = mass number − atomic number.

Isotopes are atoms of the same element with the same atomic number but different mass numbers, for example chlorine-35 and chlorine-37. They have identical chemical properties because chemical behaviour depends on electrons, not neutrons.

Key points

  • •Atomic number = number of protons = number of electrons in a neutral atom.
  • •Mass number = protons + neutrons.
  • •Isotopes differ only in neutron number.