Still swamped, but National Chemistry Week must go on. Here's a post from the archives about one of the basic concepts of chemistry, what defines an element.
As far as chemists are concerned, the world is made up of atoms and various assemblies and modifications thereof. Those atoms and modifications of atoms are, in turn, made up of protons, neutrons, and electrons. Protons have a +1 charge and a mass of 1.0073 amu . Neutrons have zero charge and a mass of 1.0087 amu. And electrons have a -1 charge and a mass of 5.49 x 10-4 amu. Various combinations of these three will give you atoms, radicals, and ions . Protons and neutrons hang out together in the nucleus of your atom (or radical or ion), while electrons can be thought of as zipping around the nucleus .
An element is defined by the number of protons in the nucleus. The element oxygen has 8 protons in the nuclei of its atoms. Any atom (or radical or ion) that has exactly 8 protons is an oxygen atom, and all oxygen atoms (or radicals or ions) have exactly 8 protons. It doesn't matter how many electrons there are zipping around the nucleus; that determines the net charge. It doesn't matter how many neutrons there are in the nucleus; that determines the atomic mass (and which isotope of oxygen you have). The number of protons in the nucleus is all that counts when you're determining the element you're dealing with.
Lots of compounds (like water) are made up of more than one element (here, hydrogen atoms and oxygen atoms in a ratio of 2:1). Elements, however, have molecules that are made up of a single kind of atom -- elemental hydrogen is H2, while elemental oxygen comes in two forms, O2 and O3 (ozone). Most textbooks will define an element as a substance that can't be broken down into simpler substances. (This means that chemists must view protons, neutrons, and electrons not as substances, but as the building blocks from which substances are made.)
 The abbreviations "amu" stands for atomic mass unit. 1 amu = 1.66056 x 10 -27 kg.
 Ions are nuclei (or multinuclear assemblies) where the total number of protons does not equal the total number of electrons — meaning they have a net-positive or net-negative charge. For example, Cl- has one more electron zipping around the Cl nucleus than there are protons in that nucleus.
A radical is a nucleus (or a multinuclear assembly) with an unpaired electron that’s “looking for action” (i.e., is generally highly reactive). For example Cl. has the same number of protons and electrons (i.e., a neutral charge), but one of its 17 electrons is not paired, and thus the radical is “looking” for an opportunity to react with something else that will provide an electron to pair with.
Not to get too anthropomorphic or anything …
 Strictly speaking, you really shouldn't think of electrons as having a well-defined location until you go looking for them with a "measurement event". But as far as anyone can tell, they probably don't stray too far from the positive charge concentrated in the nucleus.