Atom

An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is surrounded by one or more negatively charged electrons. The nucleus is positively charged and contains one or more relatively heavy particles known as protons and neutrons.

Atoms are the basic building blocks of matter. Anything that takes up space and anything with mass is made up of atoms.

What are protons and neutrons?

Protons and neutrons are subatomic particles that make up the center of the atom, or its atomic nucleus.

The mass of a proton or neutron increases when the particle attains extreme speed, for example in a cyclotron or linear accelerator.

The structure of an atom

The total mass of an atom, including the protons, neutrons and electrons, is the atomic mass or atomic weight. The atomic mass or weight is measured in atomic mass units.

Electrons contribute only a tiny part to the mass of the atomic structure, however, they play an important role in the chemical reactions that create molecules. For most purposes, the atomic weight can be thought of as the number of protons plus the number of neutrons. Because the number of neutrons in an atom can vary, there can be several different atomic weights for most elements.

Protons and electrons have equal and opposite charges. Protons have a positive charge and electrons a negative charge. Normally, atoms have equal numbers of protons and electrons, giving them a neutral charge.

An ion is an atom with a different number of electrons than protons and is electrically charged. An ion with extra electrons has a negative charge and is called an anion and an ion deficient in electrons has a positive charge and is called a cation.

Atoms having the same number of protons but different numbers of neutrons represent the same element and are known as isotopes of that element. An isotope for an element is specified by the sum of the number of protons and neutrons. For example, the following are two isotopes of the carbon atom:

The only neutral atom with no neutrons is the hydrogen atom. It has one electron and one proton.