Points to Recall

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  1. Atoms are made of protons, neutrons and electrons.

  2. Protons have a positive charge, electrons have a negative charge, neutrons have no charge.

  3. Protons and neutrons are found in the nucleus of an atom. Electrons surround the nucleus.

  4. Protons and neutrons are about the same size. Electrons are much smaller and lighter.

  5. The number of protons determines the atom’s identity, and equals its atomic number.

    1. If it has 17 protons it’s chlorine; if it’s chlorine it has 17 protons.
  6. The number of neutrons determines the atom’s isotope; and contributes, with the protons, to its mass number.

    1. 17 protons plus 18 neutrons make chlorine-35.

      2.
    2. 17 protons plus 20 neutrons make chlorine-37.

      3.
  7. The number of electrons determines the charge.

    1. In an atom, the number of negatively charged electrons around the nucleus equals the number of positively charged protons in the nucleus.

      1. A chlorine atom has 17 protons and 17 electrons.

        2.
    2. In an ion, the number of negatively charged electrons around the nucleus does not equal the number of positively charged protons in the nucleus.

      1. A chloride ion has 17 protons and 18 electrons, and

      2. it is the number of electrons that has changed.
      3. A choride ion is a chlorine atom that has gained an electron.

      4. An electron has a negative charge.

      5. Therefore, a chloride ion has a negative charge, Cl-, and is an anion.

      6. An ion with a positive charge (missing electrons) is a cation.
  8. The numbers of protons and neutrons in the nucleus determine the mass number.

    1. Chorine-35 has a mass number of 35.

      2.
    2. Chorine-37 has a mass number of 37.

      3.

 

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