An atom is the basic unit of chemical substances. All matter is composed of atoms. Dalton, the first scientist to define the atom on the basis of scientific evidence, thought that atoms were indestructible and not able to be divided. In the 1890s through the 1930s, with the discovery of electrons by Thomson, protons by Rutherford, and neutrons by Chadwick, it was clear that atoms were composed of these smaller subatomic particles.
Protons have a mass of about one atomic mass unit (which is 1/6.02 x 10^23 of a gram), and an electrical charge of +1. Electrons have a mass that is about 1/2000 of an atomic mass unit and an electrical charge of -1. Neutrons have a mass of about one atomic mass unit and no electrical charge.
The arrangement of these subatomic particles was discovered by a remarkable experiment conducted by Rutherford. He shot radioactive alpha particles (having a positive electrical charge) at a very thin sheet of gold foil and found that most of the particles were not deflected, and those that were deflected came back at angles that almost bounced back toward the source. Neither of these results was expected. He concluded that atoms are mostly empty space with a very small, dense positively charged nucleus (the protons) in the center.
Later, with contributions from Bohr and other quantum mechanical scientists, the electrons were found to orbit around the nucleus at different energy levels. Neutrons were discovered later.
Thus, an atom has a nucleus consisting of protons and neutrons, with electrons spinning around the nucleus at various energy levels. There are different kinds of atoms, called elements, which differ in the number of protons in their nucleus. Since atoms are electrically neutral, the number of electrons outside the nucleus equals the number of protons.
Thus an atom of carbon has 6 protons and 6 electrons. The most common form of carbon also has 6 neutrons, for an atomic mass of 12 atomic mass units. Nitrogen has 7 protons, 7 electrons, and usually 7 neutrons, for an atomic mass of 14 atomic mass units. (Electrons have almost no mass, compared to protons and neutrons.)
Since we know that opposite electrical charges attract and like electrical charges repel each other, how do protons, with their positive charge, stay in the nucleus, and how do electrons, with their negative charges, stay in their energy levels and not crash into the positively-charged nucleus? In the nucleus, there are forces called the strong and weak forces that are stronger than electrical repulsive forces. The neutrons also play an important role in buffering electrical repulsion. Electrons are so small that they behave more like waves than like particles, so they go around the nucleus in patterns that resemble three dimensional waves.
Atoms can combine with other atoms to form molecules and compounds, through their outermost electrons (valence electrons). Each energy level can have a specific number of electrons, and when an atom has more electrons than an energy level can hold, the next electrons are put into the next energy level. Atoms with a completely filled outermost energy level are the most stable, and do not react to form compounds. These are the noble gases, on the farthest right side of the periodic table.
All atoms react by losing, gaining, or sharing electrons, to have the same electron arrangement as the noble gases. Atoms that lose electrons become positively charged ions, called cations. Atoms that gain electrons become negatively charged ions, called anions. Cations are attracted to anions and form ionic bonds, or ionic compounds. Ionic compounds are also called salts.
Atoms can also share electrons, by pairing unpaired electrons from the outermost energy level of each atom. The shared electrons count toward the number needed to fill the outermost energy level. These bonds are covalent bonds and the compounds are called molecules.
Different atoms (elements) can be thought to be like the letters of the alphabet. When they combine, they form compounds, the words of chemistry. Chemical reactions, where chemical compounds interact and exchange atoms, could be thought of as sentences.