What is an atom

The atom is the simplest particle that makes up matter. At present it is known that the atom is made up of electrons, protons and neutrons, which in turn are made up of smaller particles, quarks.

Atoms of the same class form the elements of the periodic table. Thus, hydrogen atoms are characterized by having one electron and one proton, while the oxygen atom has 8 electrons, 8 protons and 8 neutrons.

Atoms can join to form molecules. For example, water is made up of hydrogen and oxygen atoms that combine to form the water molecule H2Or, that is, two hydrogen atoms for one oxygen atom.

If this combination changes, we are in the presence of another substance, such as hydrogen peroxide, which is made up of two hydrogen atoms and two oxygen atoms.

Oxygen and hydrogen atoms make up the water molecule.

One oxygen atom and two hydrogen atoms come together to form a water molecule.

The first notion of the atom arose in ancient Greece (5th century BC) with Leucippus and Democritus. The word “atom” appears at this time, to indicate something that was impossible to divide.

The idea of ​​the atom reappeared until the beginning of the 19th century, thanks to the English meteorologist John Dalton. In the last two centuries, technological advances have made it possible to deepen our knowledge of the atom, to discover its components and structure.

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Structure and parts of the atom

image of the atom showing its parts and structure

The atom is structured as follows:

  • Nucleus: concentrates the protons and neutrons in the center of the atom. It has a positive charge given by the protons and represents 99.9% of the total mass of the atom. However, the nucleus occupies a tiny part of the atom.
  • electronic cloud: outer region of the atom where the electrons are found. It occupies most of the atom.

The atom is composed of the following elements:


Particle without electrical charge that is in the nucleus. The number of neutrons changes between isotopes of the same atom, that is, atoms with the same number of protons but different numbers of neutrons. For example, nitrogen (N) has 7 protons in its nucleus, but it can have two isotopes: one with 7 neutrons and one with 8 neutrons.

The neutron was discovered in 1932 by James Chadwick and the only element that does not have neutrons is hydrogen.


Particle with a positive electrical charge, with the value of +1, found in the nucleus. The number of protons in an atom determines the atomic number Z, the number with which it is ordered on the periodic table. For example, hydrogen has one proton and its atomic number is 1, oxygen has 8 protons and its atomic number is 8.

The proton was discovered in 1919 by Ernest Rutherford.


The electron is the negatively charged particle that orbits the atomic nucleus. Electrons determine the chemical reactivity between atoms and their electromagnetic properties.

The discovery of the electron in 1897 allowed JJ Thomson to postulate his atomic model.

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See also Proton, Neutron, Electron.

history of the atom

atomic models through time

The atom has been represented with different atomic models through the times.

The Greek philosophers Leucippus and Democritus raised in the fifth century BC. from C. that the materials could be divided into smaller portions, up to a point where it would no longer be possible to continue dividing. That would be the atom, which in Greek means “indivisible.”

The first atomic theory was proposed by John Dalton in 1805. In his theory, Dalton presented atoms as simple particles, which, when united, formed the materials we see and know.

When the electron was discovered at the end of the 19th century, JJ Thomson proposed a model where the atom was represented by a positively charged mass with inclusions of electrons. This model became known as the pudding model with raisins.

In 1911 Rutherford established a new atomic model similar to the solar system, where the atom was represented by a central nucleus that was surrounded by electrons.

In 1913, Niels Bohr criticized Rutherford’s atomic model, and created the planetary atomic model. In it, the electrons revolve in predetermined orbits around the central nucleus, just as the planets orbit the Sun.

Only in 1919 was the proton discovered and a decade later the neutron. Later, with the construction of particle accelerators, other subatomic particles have been detected, such as quarks.

The most recent quantum atomic model establishes that the atom is structured as a central nucleus where protons and neutrons are grouped, while the electrons are in a cloud of probabilities around the nucleus.

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The first photo of heavy atoms, uranium and thorium, was taken in 1970 using a high-resolution electron microscope.

See also Atomic mass, Atomic nucleus, chemical element, Molecule, Levels of organization of matter.


Crewe, AV, Wall, J., Langmore, J. (1970) Visibility of single atoms. Science 168: 1338-1340

Rosenberg, JL, Epstein, LM, Krieger, PJ (2007). Schaum’s outline of Theory and Problems of College Chemistry. 9th ed. McGraw-Hill. NY

Sundaresan, MK (2001) Handbook of particle physics. CRC Press, Washington, D.C.