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Atoms are the smallest stable units of matter. They maintain all the properties of a chemical element. They are organized and classified according to their atomic numbers, chemical properties, and electronic charge in the periodic table.

Atoms are made up of smaller parts called subatomic particles, which include protons, neutrons, and electrons. These micro-units combine and form molecules that interact with each other. 

Atoms of the same element are identical; what differentiates them is the way they combine to form chemical compounds. This means that hydrogen atoms all over the Universe are identical to those in the human body, food, or materials used in industry.

What characteristics does an atom have?

In addition to their basic characteristic of being the smallest particle of matter, atoms also:

  • They are very light particles, weighing very little. 
  • They retain their original properties when a chemical reaction takes place. This means that they are neither created nor destroyed; they are just organized in different ways to create new bonds between different atoms.
  • They are organized or grouped to form molecules, and these can consist of the same or different chemical elements. When grouped together, they reach a state of minimum energy and maximum stability, gaining, losing, or sharing electrons. Eventually, the stored energy is released as heat or light.
  • Atoms obey Lewis’s octet rule, proposed by chemical physicist Gilbert Newton Lewis. This rule states that chemical bonds acquire the electronic configuration of noble gases, with eight electrons located at their last energy level, which makes them very stable and unreactive. 

What are the parts of an atom?

Every atom consists of a complex structure divided into:

  1. Nucleus: the part of the atom that contains the protons (positively charge) and neutrons (neutral charge). 99% of the mass of an atom is concentrated in the nucleus.
  2. Electron cloud: the part that surrounds the nucleus where electrons are located (negatively charged particles); represented by the shape of the atomic orbitals.

Though it is believed that atoms are indivisible particles, they contain the following subatomic particles:

  • Protons: subatomic particles with a positive electric charge that determine the atomic number of the element.
  • Neutrons: subatomic particles with a neutral electric charge — that is, equal to zero — which makes them easy to penetrate and difficult to manipulate. 
  • Electrons: subatomic particles with a negative electric charge; they represent less than 0.06% of the total mass of the atom and orbit around the nucleus. 

How do atoms behave according to their parts?

Protons and electrons are attracted by the electromagnetic interaction, while protons and neutrons are attracted to each other by the nuclear force, the exclusive force of the particles that make up the atom’s nucleus.

Normally, an atom’s charge is neutral since it has as many protons as electrons, allowing the positive charges to cancel out the negative charges. 

What are the properties of the atom?

  • Every atom has mass which mainly comes from the protons and neutrons of the nucleus. In chemistry, the unit used to denote mass is the mol, which weighs as many grams as the atomic mass of an element. 
  • Every atom has a size, though it is not delimited, and it is determined by the electron cloud. Its dimensions are so small that they cannot be observed by optical measuring instruments. 
  • Every atom has energy levels. An electron in an atom has potential energy that is inversely proportional to its distance from the nucleus, which means that it increases in energy according to that distance. The unit for expressing atomic energy is the electron volt.
  • Every atom establishes electrical interactions between protons and electrons in its nucleus.

What atomic theories have there been over time?

Interest in and study of the atom dates back to Ancient Greece, but it was not until the nineteenth century that the first theories began to develop. The main ones are: 

  • The theory by English chemist John Dalton, which established that matter was made up of indivisible, equal elementary particles.
  • The atomic model by English scientist J.J. Thomson, where he suggests the existence of electrons and refutes the theory of his predecessor regarding the indivisibility of the atom. His model is known as the plum pudding model, and it explains that atoms are masses with positive and negative charges. 
  • The theory of the nuclear atom developed by New Zealand scientist Ernest Rutherford, who discovered that most of the mass of an atom is located in its nucleus, with negative charges orbiting around it. 
  • The atomic model by Danish physicist Niels Henrik David Bohr, proposing that an atom’s electrons are in orbit at some distance from the nucleus and that they can jump from one orbit to another with just the right amount of energy.
  • Quantum mechanical model of the atom, developed by physicists Werner Heisenberg, Louis de Broglie, and Erwin Schrödinger. It states that electrons behave like standing waves orbiting in an electron cloud. 
  • Finally, in 1932, scientist James Chadwick discovered the neutron, thus completing the model of the atom known today.

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