The Atom

All matter in the universe is made out of atoms. An atom is the smallest particle of an element that has the properties of that element. Atoms contain three types of subatomic particles: protons, neutrons, and electrons, with the exception of hydrogen, whose one isotope does not contain any neutrons. A proton is a positively charged subatomic particle in the nucleus of an atom. A neutron is a subatomic particle that has a neutral charge in the nucleus of an atom. The mass of the atom is concentrated at the atomic nucleus, which is a positively charged center of an atom containing protons and neutrons. The mass of one proton is equal to the mass of one neutron, or 1 atomic mass unit (amu). An electron is a negatively charged subatomic particle. Electrons move freely around the nucleus of the atom and contribute a negligible, but existent, amount of mass to the overall mass of the atom. Since electrons are in constant motion around the nucleus, they can move from one atom to another atom. The negative charge of one electron is exactly opposite and equal to the positive charge of one proton.

An element is a substance composed of atoms that have the same number of protons. The number of protons in each atom of an element is called the atomic number. There are 118 known elements, but only 92 of them are found on Earth naturally; the remaining 26 elements have been produced in laboratories and are unstable. Each element has a unique atomic number and is identified by a one- or two-letter chemical symbol. The element with the atomic number 2 is helium, which consists of 2 protons and has the chemical symbol He. Carbon has 6 protons, an atomic number of 6, and the chemical symbol C. Sodium has 11 protons, an atomic number of 11, and the chemical symbol Na. The mass of an element is known as its atomic weight, which is the average mass of all the isotopes of an element, based on the relative abundance of each isotope. An isotope is one of two or more atoms of an element that have the same number of protons but different numbers of neutrons. The approximate atomic weight of helium, for example, is 4.002602. The approximate atomic weight of hydrogen is 1.00794, and it is the lightest element on the periodic table. Atomic weight is reported in units of grams per mole (g/mol). A mole is the amount of a substance that contains as many particles as 12 grams of pure carbon-12 and is equal to $6.023\times10^{23}$ particles, called Avogadro's number. Carbon-12 is a naturally occurring isotope of carbon, and it is used as the standard for measuring atomic weights of other elements. A mole simply refers to the number of particles, the same way a dozen can refer to the number of doughnuts or eggs. For atoms with a high atomic weight, 1 mole of atoms will have a greater mass than atoms with a low molecular weight, just like one dozen doughnuts might weigh less than one dozen eggs.