A typical model of the atom is called the Bohr Model, in honor of Niels Bohr who proposed the structure in 1913. The Bohr atom consists of a central nucleus composed of neutrons and protons, which is surrounded by electrons which “orbit” around the nucleus.

Protons carry a positive charge of one and have a mass of about 1 atomic mass unit or amu (1 amu =1.7x10^-27 kg, a very, very small number). Neutrons are electrically “neutral” and also have a mass of about 1 amu. In contrast electron carry a negative charge and have mass of only 0.00055 amu. The number of protons in a nucleus determines the element of the atom. For example, the number of protons in uranium is 92 and the number in neon is 10. The proton number is often referred to as Z.

Atoms with different numbers of protons are called elements, and are arranged in the periodic table with increasing Z.

Atoms in nature are electrically neutral so the number of electrons orbiting the nucleus equals the number of protons in the nucleus.

Neutrons make up the remaining mass of the nucleus and provide a means to “glue” the protons in place. Without neutrons, the nucleus would split apart because the positive protons would repel each other. Elements can have nucleii with different numbers of neutrons in them. For example hydrogen, which normally only has one proton in the nucleus, can have a neutron added to its nucleus to from deuterium, ir have two neutrons added to create tritium, which is radioactive. Atoms of the same element which vary in neutron number are called isotopes. Some elements have many stable isotopes (tin has 10) while others have only one or two. We express isotopes with the nomenclature Neon-20 or ²⁰Ne₁₀, with twenty representing the total number of neutrons and protons in the atom, often referred to as A, and 10 representing the number of protons (Z).