An atom is the most basic unit of an element. It is comprised of three different subatomic particles- protons, neutrons and electrons. Protons are positively charges particles. The quantity of protons an atom of an element contains determines the atomic number of the element, and therefore its placement on the periodic table. Neutrons are neutral particles, meaning they have no charge. Different samples of an element may differ in the number of neutrons its atom contains. This does not change the chemical reactivity of an element. Protons weigh relatively the same as neutrons. These two subatomic particles are found in the nucleus and their sum accounts for the mass number, or relative weight of the atom. When atoms have the same number of protons and differing amounts of neutrons, we call them isotopes. The molar mass, the weight of a mole of that element’s atoms, seen on the periodic table is the weighted average of an element’s isotopes. To calculate this, you must multiply the mass numbers of all the stable isotopes by their proper percent abundance (in decimal form) and add all of them up. Electrons are negatively charged particles that weigh significantly less than protons and neutrons, and therefore do not affect the mass number. When an atom is neutral, it contains the same number of protons and electrons.
Electrons constantly move around the nucleus, their exact position impossible to calculate. Instead, we determine their probable locations based on electron configurations. The first level of arrangement is an energy level. Different energy levels are able to contain different numbers of electrons. Each level is though to be further away from the nucleus. Within these energy levels are