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  1. Introduction to ions
  2. Recall particles and charge.
  3. Why atoms form ions.
  4. How atoms form ions.
  5. Predicting common ions.
  1. Use information in the Periodic Table to find the number of sub-atomic particles in elements.
    How many electrons are in an atom of the following species?
    i) P
    ii) Ru
    iii) Rb
    1. Identify atoms and their charge when given the number of sub-atomic particles.
      Identify the element and the charge on the particle described below.
      1. i) A particle with 50 protons and 52 electrons.
        ii) A particle with 13 protons and 10 electrons.
      2. i) A particle with 16 protons and 18 electrons.
        ii) A particle with 3 protons and 2 electrons.
    2. Find out the number of sub-atomic particles when given ions and their charge.
      Find the number of protons and electrons in the following species:
      1. i) Cr3+^{3+}
        ii) F^-
      2. i) Sb3+^{3+}
        ii) Zn+^+
    Topic Notes
    In this lesson, we will learn:
    • To understand the reason chemical atoms form ions and the particle that causes ionization.
    • To calculate the charge of ions from electron and proton numbers
    • To apply knowledge of ion charges to find number of electrons in ions
    • To predict an element's stable ions based on the Periodic table


    • Atoms are comprised of protons, neutrons and electrons. In a neutral atom, the number of electrons is equal to the number of protons.

    • Many elements and substances engage in chemical reactions to obtain a full outer shell of electrons, which involves metal atoms losing electrons to empty their outer shell ('dropping down a shell' in the process) and non-metal atoms gaining electrons to complete a full outer shell.

    • These changes in number of electrons changes the overall charge of an atom, if the number of protons and electrons isn't equal in an atom then it becomes a charged atom.


    • Chemical reactions only involve outer-shell electrons. They virtually never change the nucleus of an atom.

    • To find the charge of an atom or ion, subtract the number of electrons from the number of protons in the particle.

    • The electron shell configuration (and therefore its position in the Periodic Table) of an element is strongly related to the stable ions it is able to form – atoms gain or lose a number of electrons to fill their outer shell, which dictates their charge!

    Ion formation normally occurs when electrons are transferred from one atom, usually a metal, and donated to another atom, usually a non-metal – this is how ionic compounds are formed.