Definition of Atom

The smallest particle of a chemical element that can exist.

synonyms:particle, molecule, bit, a little bit, tiny bit, tiny piece, fragment, fraction, grain, granule, crumb,
 morsel, mite, mote, speck, spot, dot

What is an Atom?

The atom is the smallest object that retains the properties of an element.

Atoms are composed of electrons and a nucleus. The nucleus contains protons and neutrons.

All atoms are electrically neutral because every atom has an equal number of electrons and protons.

Nearly all of the atom’s mass is located in the nucleus. The nucleus is tiny compared with the total size of the atom.

Most of the atom’s volume holds the electron cloud, whose mass is tiny.

Atoms are generally spherical, although there are indications that atoms of the very heaviest elements may exist as squashed spherical shapes.

An atom’s chemical behavior is determined by the arrangement of its electrons.

Atoms may combine with one another by chemical bonding to produce molecules. They may also ionize to form ionic compounds.


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

definition of atom


An early model of the atom was developed by the physicist ErnestRutherford in 1912. He was the first to suggest that atoms are like miniature solar systems, except that the attractive force is not caused by gravity, but by opposing electrical charges. In the so-called Rutherford atom, electrons orbit the nucleus in circular paths. Niels Bohr revised Rutherford’s theory in 1913. In the Bohr atom, the negatively charged electrons orbit the nucleus at specific median distances. These distances are represented by spheres, called shells, surrounding the nucleus. Electrons can move from shell to shell. When an electron absorbs enough energy, it moves to a larger, or higher, shell. When it loses a certain amount of energy, it falls to a smaller, or lower, shell.

The total mass of an atom, including the protons, neutrons and electrons, is the atomic mass or atomic weight. Electrons contribute only a tiny part of this mass. For most practical 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.

Atoms having the same number of protons, but different numbers of neutrons, represent the same element but are known as different isotopes of that element. The isotope for an element is specified by the sum of the number of protons and neutrons. Examples of different isotopes of an element are carbon 12(the most common, non-radioactive isotope of carbon) and carbon 14 (a less common, radioactive isotope of carbon).

Atomic particles


Protons and neutrons have approximately the same mass. However, one proton weighs more than 1,800 electrons. Atoms always have an equal number of protons and electrons, and the number of protons and neutrons is usually the same as well. Adding a proton to an atom makes a new element, while adding a neutron makes an isotope, or heavier version, of that atom.


The nucleus was discovered in 1911 by Ernest Rutherford, a physicist from New Zealand, who in 1920 proposed the name proton for the positively charged particles of the atom. Rutherford also theorized that there was also a neutral particle within the nucleus, which James Chadwick, a British physicist and student of Rutherford, was able to confirm in 1932.

Virtually all the mass of the atom resides in the nucleus. The protons and neutrons that make up the nucleus are approximately the same mass (the proton is slightly less) and have the same angular momentum, according to Lawrence Berkeley National Laboratory. 

The nucleus is held together by the “strong force,” one of the four basic forces in nature. This force between the protons and neutrons overcomes the repulsive electrical force that would, according to the rules of electricity, push the protons apart otherwise. Some atomic nuclei are unstable because the binding force varies for different atoms based on the size of the nucleus. These atoms will then decay into other elements, such as carbon-14 decaying into nitrogen-14.


Protons are positively charged particles found within atomic nuclei. Rutherford discovered them in experiments with cathode-ray tubes conducted between 1911 and 1919. Protons are slightly smaller in mass than neutrons with a relative mass of 0.9986 (as compared with the mass of the neutron being 1) or about 1.673×10-27 kg.

The number of protons in an atom defines what element it is. For example, carbon atoms have six protons, hydrogen atoms have one and oxygen atoms have eight. The number of protons in an atom is referred to as the atomic number of that element. The number of protons in an atom also determines the chemical behavior of the element. The Periodic Table of the Elementsarranges elements in order of increasing atomic number.

Three quarks make up each proton — two “up” quarks (each with a 2/3 positive charge) and one “down” quark (with a 1/3 negative charge) — and they are held together by other subatomic particles called gluons, which are massless.


Electrons are tiny compared to protons and neutrons, over 1,800 times smaller than either a proton or a neutron. Electrons have a relative mass of 0.0005439 (as compared with the mass of a neutron being 1) or about 9.109×10-31 kg. 


An atom’s electron configuration is the orbital description of the locations of the electrons in a typical atom. Using the electron configuration and principles of physics, chemists can predict an atom’s properties, such as stability, boiling point and conductivity.

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

For example, the electron configuration for beryllium (Be) is 1s22s2, but it’s is written [He]2s2. [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 1.6749×10-27 kg.

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. Subatomic particles with no charge were released – the neutron.

Neutrons are uncharged particles found within all atomic nuclei (except for hydrogen-1). 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. Deuterium (D or 2H) has one proton, one electron and one neutron. Tritium (T or 3H) has one proton, one electron and two neutrons.

History of the atom

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. 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. However, because most philosophers at the time — especially the very influential Aristotle — believed that all matter was created from earth, air, fire, and water, Democritus’ atomic theory was put aside.

John Dalton, an British chemist, built upon Democritus’ ideas in 1803 when he put forth his own atomic theory, according to the chemistry department at Purdue University. Dalton’s theory included several ideas from Democritus, such as atoms are indivisible and indestructible and that different atoms form together to create all matter. Dalton’s additions to the theory included the ideas that all atoms of a certain element were identical, that atoms of one element will have different weights and properties than atoms of another element, that atoms cannot be created or destroyed, and that matter is formed by atoms combining in simple whole numbers.

Thomson, the British physicist who discovered the electron in 1897, proved that atoms actually can be divided, according to the Chemical Heritage Foundation. He was able to determine the existence of the negatively charged particles by studying properties of electric discharge in cathode-ray tubes. According to Thomson’s 1897 paper, the rays were deflected within the tube, which proved that there is something that was negatively charged within the vacuum tube. In 1899, Thomson published a description of his version of the atom, commonly known as the “plum pudding model,” according to a 2013 article by Giora Hon and Bernard R. Goldstein published in the journal Annalen der Physik. Thomson’s model of the atom included a large number of electrons suspended in something that produced a positive charge giving the atom an overall neutral charge, which resembled a popular British dessert that had raisins suspended in a round cake-like ball.

(source by: LiveScience)

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