What really determines how reactive an atom of an element will be?

Question

Why are some elements more reactive than others?

Answer 1

Look at the following features for a metal:
ionisation energy (the smaller the better)
electrode potential (the more negative the better)

For a non-metal look at how positive the electrode potential is. A large electron affinity will also tend to suggest high reactivity.

Answer 2

Electrons. Electrons in orbit around the nucleus of an atom are arranged in shells. Each shell can hold a certain number of electrons before it is filled. Electrons repel each other because they have the same charge (they behave like magnets). However, when an electronic shell is completely filled, it sets up a symmetry which minimizes the interactions of electrons. This stabilizes the shell. Elements are therefore seeking to either fill half filled shells or empty them. This is very easy to do if just a single electron is lost or gained. For example, Sodium is a metal with a single electron in its last shell. It easily disposes of this electron and becomes a positive ion. It is so easy to do that Sodium is never found as an actual metal. It is almost always some sort of salt. Fluorine is 1 electron short of filling its last shell. It grabs an electron from another atom and becomes a negative ion. Again, it is easy to aquire a single electron. Atoms find it harder to fill or empty their shells if more than one electron is involved. Also, as the shells stack up they increase in energy. Gold has many more shells than Copper, so the electrons Gold looses have lots more energy than the ones Copper must loose. This means the electrons in Gold can be only be accepted by very powerful chemicals and explains why Gold is so inert.

Answer 3

The number of protons in the nucleus determines the chemical characteristics of an atom. Neutrons just bind the protons together (and the same atom can have 'extra' neutrons, these are called isotopes).