Atoms are the basic unit of matter. They make up molecules and macromolecules, and basically everything you can imagine. Each type of atom, called an element, has a specific structure of sub-atomic particles which gives it certain properties. The structure of each element is organized in the Periodic Table of the Elements.
Basic Structure: Protons, Neutrons, and Electrons
Atoms are made up of three types of particles: Protons have a positive charge, electrons have a negative charge, and neutrons are neutral. Neutrons and protons are basically of the same mass, but electrons are about one two thousandth (1/2000) of that. Protons and neutrons bind together to form the nucleus, and the electrons orbit around it in the electron cloud. The electrons' negative charge is attracted to the protons' positive charge, but the electrons must orbit at a distance because they repel eachother. Most atoms are of neutral charge. This means that the number of protons equals the number of electrons. An atom that has an overall positive or negative charge is called an ion. This is due to the gain or loss of electrons.
Atomic Mass, Weight, and Number
Atomic mass refers to the number of protons and neutrons in the the nucleus of the atom. Atomic weight refers to the weight, which also refers to the number of protons and neutrons. (This is because each proton and neutron weigh about 1 unit each, while electrons, which are 1/2000, are so tiny that they are not even counted.) Atomic number refers to the number of protons. In a neutral atom, this matches the amount of electrons. Any extra electrons or missing electrons are indicated with a - or + sign, respectively. For example, a carbon atom would with an two extra electrons would be written as C2- and a carbon atom which has lost two electrons would be written C2+. This is because with a gain of electrons, the overall charge becomes negative and with a loss of electrons, the overall charge becomes positive. The periodic table of the elements organizes each element according to atomic number. The atomic number also determines each element's properties, because it indirectly indicates the number of electrons in the atom. (see Simple Bonding Properties)
Electrons orbit around the nucleus of the atom. This is called the electron cloud. Withine the electron cloud, there are different energy levels, or electron shells. Higher energy electrons take up outer energy levels, and lower energy electrons take up inner energy levels. Within each energy level there are different orbitals.
In the first energy level, there is only one sphere shaped orbital, labeled the s orbital. In the second energy level, there is an s orbital, and three orbitals shaped like dumbells that extend out of the nucleus at right angles. They are labeled the Px, Py, and Pz, orbitals for their corresponding axis. The third energy level has one s orbital, three p orbitals, and five d orbitals, which are too difficult to diagram. The fourth energy levels has one s orbital, three p orbitals, five d orbitals, and seven f orbitals which are also too difficult to diagram. Known elements have as high has seven energy levels. Levels after the fourth add the same types of orbitals.
Simple Bonding Properties
Bonding between molecules depends on the number of electrons in the outer shell of each atom. These are called valence electrons. Since each orbital in the outer shell can hold only two electrons, the number of unpaired of electrons is called the atoms valence. The valence number is the number of bonds that that atom is capable of forming. Carbon, which has six electrons, has a valence of four because it has four unpaired electrons. The fact that carbon can form the strongest and most bonds explains why it forms the backbone of most molecules, called organic molecules, and is the basis for life. Organic molecules consist of chains of carbons which then are linked to other combinations of atoms that determine the properties of the molecule. Here is a table of common elements and their valences:
|Element    ||# of Electrons (neutral)    ||# of Valence Electrons    ||Valence|