prometheus
29th July 2009 - 08:34 AM
Atoms are held together by electromagnetic forces between the positively charged nucleus and orbiting negative electrons. the nucleus is held together against the repulsion of the EM force trying to push the protons apart by the strong nuclear force and electrons are fundamental.
The things that make atoms different are the number of electrons in their outer shell and their overall size (or in a better way, their overall number of electrons, which is the same as the number of protons in the nucleus). For example look at the alkali metals on the left of the periodic table. Sodium (Na) is a metal that has 11 electrons and a single electron in it's outer shell. This makes it very easy for the outer electron to be lost and for the sodium atom to form a positively charged ion, so sodium is pretty reactive. If you go a few steps down from sodium in the periodic table you'll find caesium (Cs) which has the same single electron in the outer shell structure, but is a lot bigger - it has 55 electrons. The single electron is further on average from the positively charged nucleus than in sodium so it's even easier to remove it and form an ion - caesium is very reactive indeed.
Other elements behave in different ways, depending on the configuration of electrons.
Links:
Periodic tableSodiumCaesiumAlkali metals reactions