Electrons and protons, along with neutrons, are parts of an atom. Protons have a mass of about 1 atomic mass unit (1/602,000,000,000,000,000,000,000 of a gram) and an electrical charge of +1. Neutrons also have about the same mass as a proton, but have no charge. Protons and neutrons are located in the very dense center of an atom, called the nucleus. Electrons are much smaller, with a mass of about 1/1860 of an atomic mass unit, and they have an electrical charge of -1. Electrons spin around outside the nucleus in different energy levels.
Elements of the periodic table are different because they have different numbers of protons, which identifies the element. For a neutral atom, the number of electrons equals the number of protons. The number of neutrons can differ for the same element, giving rise to isotopes. Thus, for example, one isotope of the element carbon has six protons, six neutrons, and six electrons. Another isotope may have six protons, seven neutrons and six electrons.
Usually, negatively-charged objects are attracted to positively-charged objects, and like-charged objects repel each other. These forces are called electrostatic forces. Why don't electrons, with a negative charge, dive into the positively charged nucleus? And why don't nuclei, with positive and neutral charges, fly apart?
There are forces within the nucleus, called the strong force and weak force that are stronger than electrostatic repulsion. The neutrons are important in keeping the nucleus together with these forces. Protons and neutrons are composed of smaller subatomic particles. Nuclear chemistry is the result of unstable nuclei that release particles and energy from the protons and neutrons.
Electrons obey the laws of quantum mechanics, because they are so small, they behave more like waves than like particles. Thus, quantum mechanics has the electrons in discreet energy levels, each higher level further away from the nucleus. These energy levels are like tracks around the nucleus. Upon being energized with light, heat, or electricity, electrons can jump to higher energy levels, but not in between. The evidence for this is that when the electrons come back down to their original energy level they give off light energy in discreet bands of color, rather than the whole rainbow of colors.
Each energy level has a limit for the number of electrons that can be in it. As we go across the periodic table, the number of protons increases as well as the number of electrons. When an energy level is filled, electrons are then put into the next higher energy level. For chemistry, the outermost levels of electrons are the most important ones. These are called valence electrons. The most stable elements are the ones that have their energy level filled; thus, other elements will gain, lose, or share electrons in order to fill an energy level.
When atoms lose or gain valence electrons, there is a net positive or negative charge; these are called ions. Ions with positive charge(s) are called cations, and ions with negative charge(s) are called anions. When the positive ions unite with negative ions, ionic compounds, or salts, are formed. (A compound is a substance with two or more elements combined with chemical bonds in a fixed ratio.) Also adjacent atoms can share valence electrons in order to fill an energy level, forming covalent compounds.
Electricity is the result of valence electrons moving through a substance. Metal atoms easily give up their valence electrons, so they are very good conductors of electricity.