What is an Atom? | Definition, Concept and Parts of an Atom

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  1. Atom
    1. Atom definition
    2. Characteristics of the Atom.
    3. Quarks.
    4. Bohr Model
    5. Properties of atoms
    6. Dalton's atomic theory
    7. Atomic weights
    8. Atoms and the Periodic Table.
    9. It may interest you :


An atom is the smallest unit of particles that can exist as a single substance (chemical element), and that can intervene in a chemical combination.


It consists of a nucleus that represents more than 99.9% of its mass, with a positive electrical charge equal to the number of protons, and electrons, which determine its size.

Its Greek term means"not divisible", proposed by Democritus and Leucippus, who assumed that matter was made up of indivisible and indestructible particles.

The modern concept (modern atomic theory) that we all have today about what an atom is comes from different sectors of the fields of physics and chemistry.

The first ideas on this subject emerged in Ancient Greece, from science and philosophy, which were later fully developed in the chemistry of the 18th and 19th centuries.

From the time of the ancient Greeks to the present day, we have reflected deeply on what matter is made of.

Atom definition

The origin of the word atom comes from the Greek word, which means indivisible. At the time these particles were baptized it was believed that they could not actually be divided.

Today we know that atoms are formed by even smaller particles, distributed in the two parts of the atom, the so-called subatomic particles.

The atom is made up of a nucleus with protons and neutrons and several orbital electrons, the number of which varies according to the chemical element.

In this case, we find that they are the size, the mass, the electrical interactions that are established between electrons and protons or the energy levels.

The atom is also referred to as the fundamental particle, due to its characteristic of not being able to be divided by chemical processes.

From the 16th and 17th centuries, with the development of chemistry, the atomic theory began to advance with certainties that, until then, were impossible to obtain.

Characteristics of the Atom.

Although the atom is a basic unit, it is composed of three substructures:

  1. The protons.
  2. The neutrons.
  3. The electrons.

The nucleus is the central part of the structure of the atom. In the part of the nucleus are the protons (positively charged particles) and the neutrons (non-electrically charged particles).

In the cortex, the outer part of the atom are electrons (particles with negative electrical charge).

Protons, neutrons, and electrons are the subatomic particles that form the structure of the atom. What differentiates one atom from another is the relationship they establish between them.

The negatively charged electrons are the lightest subatomic particles. The positively charged protons weigh about 1,836 times more than electrons.

Neutrons, the only ones without an electrical charge, weigh about the same as protons.
The protons and neutrons are grouped in the central part of the atom formed by the atomic nucleus. For this reason, they are also called nucleons.

Isotopes. The amount of neutrons in a nucleus determines the isotope of each element. Thus, for example, hydrogen has three known isotopes: protium, deuterium, and tritium.


Protons and neutrons are made up of a pair of particles called quarks and gluons.

Protons contain two positively charged ascending quarks (+2/3) and one negatively charged descending quark (-1/3), while neutrons contain one ascending quark and two descending quarks. The gluons are responsible for linking the quarks together.

Bohr Model

This description of the electrons orbiting around the atomic nucleus corresponds to Bohr's simple model.

According to quantum mechanics, each particle has a wave function that occupies all space and electrons are not located in orbits, although the probability of presence is higher at a certain distance from the nucleus.

Properties of atoms

The basic units of chemistry are atoms. During chemical reactions, atoms are preserved as such, neither created nor destroyed, but they are organized differently, creating different bonds between one atom and another.

Atoms group together to form molecules and other types of materials. Each type of molecule is the combination of a certain number of atoms linked together in a specific way.

According to the composition of each atom, the different chemical elements represented in the periodic table of chemical elements are differentiated.

Another important feature of these particles is their mass. Most of the mass is found in the nucleus, that is, in protons and neutrons.

A proton has approximately 1,800 times the mass of an electron. The electrons orbit around the nucleus in a cloud that has a radius of about 10,000 times the size of the nucleus.

The number of protons and electrons defines each of the elements of the periodic table and their number is represented in it as the atomic number.

Dalton's atomic theory

In 1804, John Dalton, a British chemist, based himself on the ideas of the Greeks to present an atomic model that equally affirmed the indivisibility of the atom and its union as the basic structure to form matter.

Additionally, Dalton added that all the atoms of an element are identical and have different weights and unique properties compared to other elements.

For more than twenty centuries, atomism was archived as something of secondary interest among scientists, until the idea was reborn in the first decade of the 19th century by John Dalton.

John Dalton had not intended to formulate a theory on the constitution of matter; he came to it as a result of his research on gases, and his aim was none other than to explain the discoveries made in them.

In his memory Absorption of gases by water and other liquids (1802), he had established his well-known law of partial pressures: the total pressure exerted by a gas mixture is equal to the sum of the partial pressures of each of its components.

Atomic weights

Dalton was the first to publish a table of relative atomic weights. Six elements appear in this table: hydrogen, oxygen, nitrogen, carbon, sulphur, and phosphorus, conventionally attributing the weight of one unit to the hydrogen atom. Dalton provides no indication in this first article of how he had made his calculations.

Atoms and the Periodic Table.

To find the number of protons, electrons, and neutrons in the atom of a given element can be done in the following way:

The atom in the Periodic Table of elements

In the information of all the periodic tables, there is the atomic number and the atomic weight of each element. The atomic number is the number in the upper left corner and the atomic weight is the number at the bottom.
In a nutshell:

  1. Protons = atomic number.
  2. Electrons = atomic number
  3. Neutrons = rounded atomic mass - atomic number.

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