Introduction to Atoms

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Protons, Neutrons and Electrons

Atoms are incredibly small and are mostly empty space. Each atom consists of a nucleus and a cloud of particles called electrons that fly around the nucleus. The nucleus is a cluster of two kinds of particles, protons and neutrons.

All the particles in an atom are very light. Their mass is measured in atomic mass units. Protons and electrons have an electric charge, whereas neutrons do not have an electric charge.

Particle in atom	Mass	Charge
Proton	1 unit	positive charge (1 +)
Neutron	1 unit	none
Electron	almost nothing	negative charge (1 -)

Atomic Number

The number of protons in an atom can be used to identify the atom.

Every other atom has a different number of protons. For example, since only a sodium atom has 11 protons then any atom with 11 protons would be a sodium atom.

The number of protons in an atom is called its atomic number or proton number. For example, the atomic number of sodium is 11.

Number of Electrons

Every neutral atom has an equal number of protons and electrons.

This means that the total charge on the protons cancels out the total charge on the electrons and the atom has no overall charge. For example, the sodium atom has 11 protons and 11 electrons and there is no overall charge.

Nucleon Number

Protons and neutrons, also called nucleons, are located in the nucleus of the atom. The total number of protons and neutrons in an atom is called its nucleon number. For example, the sodium atom has 11 protons and 12 neutrons. The nucleon number for the sodium atom is 11 + 12 = 23.

Isotopes

Isotopes are atoms of the same element, with different numbers of neutrons.

For example, all carbon atoms have 6 protons but some carbon atoms have more neutrons than others.

Carbon-12 has 6 protons, 6 electrons and 6 neutrons.
Carbon-13 has 6 protons, 6 electrons and 7 neutrons.
Carbon-14 has 6 protons, 6 electrons and 8 neutrons.

Carbon-12, Carbon-13 and Carbon-14 are called isotopes of carbon.

Videos

Introduction to Atoms
The atom, proton, neutron and electron Introduction to Atomic Structure
This video shows how all atoms are made of protons, neutrons (in the nucleus) and electrons (outside the nucleus). The positive charge of the protons and the negative charge of the electrons create an attractive force that holds the atom together. The number of protons in the nucleus determines which element the atom is.
For example, all carbon atoms have 6 protons in their nuclei. Because neutral atoms carry no overall charge, the number of protons and electrons must be equal, thus canceling out their charges.
Electrons are wrapped around the nucleus in shells, with definite rules governing how many electrons each shell can hold. When atoms collide with each other, their outer shells come into contact with each other, so the number of electrons in an atom's outer shell is incredibly important. This number of electrons determines the properties of the atom.