Structure of Atoms and Ions

Subatomic Particles

Atoms and ions are made up of protons, neutrons and electrons.

The nucleus

All the significant mass is found in the nucleus which contains protons and neutrons.

• Each element is defined by its atomic number which is equal to the number of protons in each of its atom's nucleus.
• The mass number is the number of protons + the number of neutrons.
  • An isotope is a set of atoms of an element that each have the same number of neutrons as well as the same number of protons in their nuclei.
  • Many elements have several isotopes (different atoms with the same number of protons but differing numbers of neutrons).
  • Relative atomic mass is the average mass of an element's isotopes taking into account their relative abundances.
• Don't muddle relative atomic mass (an average mass of all the isotopes) with mass number which is for one specific isotope.

Electronic Configuration

An atom is electrically neutral because it has the same number of electrons (which are negatively charged) as protons (which are positively charged).

Electrons move very fast around the nucleus at different energy levels which are sometimes called shells.

• The first shell (closest to the nucleus) can accommodate up to 2 electrons before it is full.
• The second shell can accommodate up to 8 electrons before it is full.
• The third shell can accommodate up to 8 electrons and then the 4^th shell starts to fill.
• It is too complicated at GCSE level to fully understand the electronic structure of an atom or ion with more than 20 electrons.
• For any atom in groups 1 - 0/8, the group number is the number of electrons in the outer shell.
• The period number gives the number of shells in an atom.

Ions

In ionic bonding, one or more electrons are transferred from one atom (a metal or hydrogen) to another atom (a non-metal). The particles formed no longer have the same number of electrons as protons and these are called ions.

• The metal atom loses electrons from its outermost shell to form an ion with a positive charge equal to the number of electrons that it lost.
• The non-metal atom gains electrons to form an ion with a negative charge equal to the number of electrons that it gained.
• Most ions from groups 1 - 7 have full outer shells.
• Elements from group 1 (alkali metals) form +1 ions in their compounds.
• Elements from group 2 form +2 ions in their compounds.
• Metallic elements from group 3 form +3 ions in their compounds.
• Metallic elements from group 4 (tin and lead) can form both 2+ ions (that do not have 8 electrons in their outer shells) and 4+ ions in their compounds.
• Non-metal elements from group 5 (nitrogen and phosphorus) form -3 ions in their ionic compounds.
• Non-metal elements from group 6 (oxygen, sulfur and selenium) form -2 ions in their ionic compounds.
• Elements from group 7 (halogens) form -1 ions in their ionic compounds.
• Elements from group 0/8 (noble gases) do not form ions because their atoms have full outer shells or 8 electrons in their outer shells. Therefore they do neither gain nor lose electrons.
• Transition metals form ions whose charges cannot be easily predicted using the periodic table.
• The hydrogen ion, H⁺ is unique in having no electrons. Acids are compounds containing hydrogen ions.