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The atom is the smallest fraction in which matter can be stably divided; in fact, in Greek the word atom means ” not divisible .”

The first notions of atom arose in the early nineteenth century, with the work of Dalton, who formulated the first atomic theory and described for the first time the existence of spherical particles, tiny and indivisible, integrating all the material and that are equal to each other in each chemical element.

Throughout that century and the beginning of the next the concept was perfected, by the hand of scientists such as Thomson and Rutherford, to the formulation of the Böhr atomic model , proposed by Niels Böhr and according to which electrons revolve around of the nucleus in well-defined levels.

Characteristics of the atom:

  1. Indivisible particle

The atom is an indivisible particle; it is the smallest fraction in which matter can be divided in a stable way; in fact, in Greek the word atom means “not divisible.”

  1. Structure

The atom is constituted by a central nucleus and a crust. The nucleus contains positively charged particles called protons and also particles with no electrical charge known as neutrons.   The cortex is composed of a cloud of electrons, negatively charged, organized as orbitals.

  1. Infinitely light particle

The approximate mass of protons and neutrons of 1,675 x 10 -24 ; that of electrons is even lower: 9.1 x 10 -28 . This makes the atoms extremely light.

  1. Form molecules

The atoms are grouped forming molecules. Each type of molecule is the combination of a certain number of atoms linked together in a specific way, and a molecule can contain atoms of different elements or of the same.

  1. Intalterable

Each atom retains its structural characteristics, beyond being part of different molecules, because during chemical reactions the atoms are not created or destroyed, but they are organized differently creating bonds between one atom and another.

  1. Number of fixed protons

What distinguishes the chemical elements from each other (among many other characteristics) is the number of protons that have their atoms in the nucleus. This number is represented by the letter Z and receives the name of atomic number , which coincides with that of electrons. It usually appears in the periodic table of the elements, located above the chemical symbol, on the left.

  1. Mass of the atom

The mass of an atom is given by the sum of the protons and neutrons of its nucleus (since that of the electrons is infinitely small and therefore negligible). This parameter is called a mass number,  and is represented by the letter A.

It happens that although the number of protons is identical for all the atoms of a given element, the number of neutrons can vary in some of those elements; that happens with carbon or nitrogen, which are elements that have heavier isotopes , such as carbon 14 or nitrogen 15. Their mass numbers in the periodic table are 12 and 14, respectively .

  1. Chemical reactivity

Although protons and neutrons are important in terms of mass, it is the electrons, especially those in the last layer of the electronic cloud, that are responsible for the chemical reactivity of the atoms, which will ultimately allow them to occur. and they degrade an infinite number of chemical compounds all the time.

  1. Trend to stability

The normal tendency of the vast majority of atoms is to join with other atoms, of the same or different types, to form stable groupings, that is, molecules, because in doing so, a minimum energy situation is reached, which is equivalent to say maximum stability. By forming the links, they gain, lose or share electrons. These unions contain energy that, eventually, is released as heat or light.

  1. Comply with octet rule

What justifies the reactivity and link formation is that the atoms comply with the Lewis octet rule, which indicates that the links respond to the need to acquire the electronic configuration that characterizes the noble or “inert” gases, with eight electrons spinning in its outermost layer.


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