Atomic Structure

Atomic Structure

  • An atom is the smallest particle of a chemical element that can exist and retains the properties of that element.
  • Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons.
  • The nucleus, at the centre of the atom, contains protons and neutrons. It is very dense and positively charged because of the protons.
  • The electrons orbit the nucleus in shells or energy levels. They are negatively charged and occupy most of the atom’s volume.
  • The atomic number represents the number of protons (which equals the number of electrons in an uncharged atom) and determines the type of atom or element.
  • The mass number is the total number of protons and neutrons in an atom.

Charges and Masses of Subatomic Particles

  • Protons have a relative charge of +1 and a relative mass of 1.
  • Neutrons have no charge (neutral) and have a relative mass of 1.
  • Electrons have a relative charge of -1, but their relative mass is negligible (almost zero).

Isotopes

  • Isotopes are atoms of the same element with the same number of protons and electrons, but different numbers of neutrons.
  • Different isotopes of an element have the same atomic number but different mass numbers.
  • Isotopes have similar chemical properties (because they have the same electronic structure) but differing physical properties (due to differences in mass).

Electronic Configuration

  • The electron shell or energy level closest to the nucleus is filled first, with up to 2 electrons. Subsequent shells can hold up to 8 electrons.
  • The arrangement of electrons within the shells or energy levels is called the electron configuration, and it determines how an atom will interact with other atoms.
  • The outermost shell or outer electron shell is known as the valence shell, and its electrons are the valence electrons. These are the electrons involved in chemical reactions.

Ions

  • Ions are atoms or groups of atoms that have lost or gained electrons, and so carry a charge.
  • An atom that has lost an electron and become positively charged is a cation.
  • An atom that has gained an electron and become negatively charged is an anion.
  • The charge on an ion is equal to the number of electrons lost (which is positive) or gained (which is negative).