What is an Atom ?

Typically, only the outermost electron shells matter in chemistry. The inner electron shell notation is often truncated by replacing the longhand orbital description with the symbol for a noble gas in brackets.

Even in the smallest person, like a baby, there are trillions upon trillions of atoms.

This method of notation vastly simplifies the description for large molecules.

Depending on what type of matter you are looking at, there will be a different number of atoms.

For example, the electron configuration for beryllium (Be) is 1s22s2, but it's is written [He]2s2.

Within those same atoms, there will be differing numbers of protons, neutrons, and electrons.

[He] is equivalent to all the electron orbitals in a helium atom. The letters, s, p, d, and f designate the shape of the orbitals and the superscript gives the number of electrons in that orbital. Uranium, as another example, has an electron configuration of 1s22s22p63s23p64s23d104p65s24d105p66s24e145d106p67s25f4, which can be simplified to [RN]7s25f4.

The neutron is used as a comparison to find the relative mass of protons and electrons (so it has a relative mass of 1) and has a physical mass of 1.6749x10-27 kg.

Atoms can last pretty much forever. They can be changed and undergo different chemical reactions, wherein they share their electrons with other atoms.

The neutron's existence was theorized by Rutherford in 1920 and discovered by Chadwick in 1932. Neutrons were found during experiments when atoms were shot at a thin sheet of beryllium.

Electrons are negatively charged particles that are found outside of the nucleus of an atom. They are constantly spinning around the nucleus.

Subatomic particles with no charge were released – the neutron.

Neutrons are uncharged particles found within all atomic nuclei (except for hydrogen-1).

They spin at such a high rate that it can be very difficult for scientists to observe them. Electrons are the smallest particles in an atom.

A neutron's mass is slightly larger than that of a proton. Like protons, neutrons are also made of quarks — one "up" quark (with a positive 2/3 charge) and two "down" quarks (each with a negative 1/3 charge). 

The number of neutrons in a nucleus determines the isotope of that element. For example, hydrogen has three known isotopes: protium, deuterium and tritium. Protium, symbolized as 1H, is just ordinary hydrogen; it has one proton and one electron and no neutrons.

Atoms are the basic building blocks of ordinary matter. Atoms can join together to form molecules, which in turn form most of the objects around you. Atoms are composed of particles called protons, electrons and neutrons.

Deuterium (D or 2H) has one proton, one electron and one neutron. Tritium (T or 3H) has one proton, one electron and two neutrons.

The theory of the atom dates at least as far back as 440 B.C. to Democritus, a Greek scientist and philosopher. Democritus most likely built his theory of atoms upon the work of past philosophers, according to Andrew G. Van Melsen, author of "From Atomos to Atom: The History of the Concept Atom." For example, Parmenides, Democritus' teacher, is known for proposing the principle of identity.

This principle, which states that "all that is, together forms the being," led to other philosophers, including Democritus, to further his work, eventually leading to atomic theory.

Democritus' explanation of the atom begins with a stone. A stone cut in half gives two halves of the same stone.

If the stone were to be continuously cut, at some point there would exist a piece of the stone small enough that it could no longer be cut.

If the stone were to be continuously cut, at some point there would exist a piece of the stone small enough that it could no longer be cut.

The term "atom" comes from the Greek word for indivisible, which Democritus concluded must be the point at which a being (any form of matter) cannot be divided any more.

His explanation included the ideas that atoms exist separately from each other, that there are an infinite amount of atoms, that atoms are able to move, that they can combine together to create matter but do not merge to become a new atom, and that they cannot be divided.