The atom has two basic regions; the nucleus and the electron cloud. Changes within the nucleus are the study of radioactivity. These pages focus on changes in the electron cloud, specifically at its outer edge.
One of Niels Bohr's many significant contributions to this discussion is the idea that the electrons occupy orbitals - a tiny bit like planets orbit the sun. Three major differences between planets' and electrons' orbits want to be mentioned. The first is that one can pretty exactly tell where a planet is going to be at any one time. Second, as far as we know, only one planet occupies any given orbit. Third, a planet's orbit describes a physical location.
Over against this, with atoms we cannot tell where an electron will be at any given moment. (Heisenberg is the name associated with this realisation.) We only describe a region in which it has a high probability of being. Second, multiple electrons may occupy any orbit at a given time. Third, the orbits are better understood as differences of energy rather than distances from a physical focal point.
It is useful, at least at first, to imagine the electrons swirling around the nucleus, not in a planet-like orbit, but changing direction frequently, mostly keeping an approximately constant distance from the nucleus most of the time. The total effect is that of a cloud of electron (or electrons) all around the nucleus. The real picture is, of course, a great deal more detailed and difficult than that, but is a useful and roughly accurate model to set out with.
But back to the electrons at the outer edge. They are attracted to the nucleus electrostatically, a result of electromagnetic force. By way of analogy, consider gravity. You are standing on the surface of the Earth. Yuri Gagarin is orbiting in his space capsule high up in the sky. You are noticeably more strongly attracted to the Earth than Yuri is. So it is with electrons and protons. Electrons that are farther from the nucleus are attracted less strongly than closer electrons. Specifically, electrons in the highest energy level are least attracted to - least strongly held by - their nucleus. These electrons are called valence electrons and they are the electrons involved in chemical change, or bond-making and bond-breaking.
There are, in the final analysis, three arrangements by which atoms can make bonds with each other:
Those three arrangements lead to metallic, ionic and covalent bonding respectively.
Related Topics: Ions, Covalent Substances, Comparing Bond Types