An atom is defined as the smallest particle into which an element can be divided without losing the chemical properties that characterize it.
It is composed by the atomic nucleus, a positively charged center where most of the mass is concentrated, and by the electrons, a certain number of negatively charged particles that make up the cortex.
An atom is defined as the smallest particle into which an element can be divided without losing the chemical properties that characterize it
The atomic nucleus
It is composed by protons and neutrons, thus denominated nucleons, with a positive charge equal to the negative charge of the electrons, in such a way that the total electric charge of the atom is neutral (protons have positive charge and neutrons have no electric charge).
The particles that constitute the atom, along with their mass and charge, are:
- Electron: Mass (kg) = 9.1 x 10-31, Charge (C) = 1.602 x 10-19 (-)
- Proton: Mass (kg) = 1.673 x 10-27, Charge (C) = 1.602 x 10-19 (+)
- Neutron: Mass (kg) = 1.696 x 10-27, Charge (C) = 0
Atomic number (Z)
The number of protons present in the atomic nucleus, which characterizes a chemical element and determines its position on the periodic table, which also coincides with the number of electrons.
Mass number (A)
The total number of nucleons (protons plus neutrons) in the atomic nucleus (A = Z + N, where N= number of neutrons).
Every atom is defined by its atomic number (Z) and its mass number (A). Isotopes are atoms with the same atomic number but different mass number. For instance, in the case of Hydrogen, with only one proton in the nucleus, there are two isotopes: Deuterium, with one proton and one neutron, and Tritium, with two neutrons and one proton. Uranium, whose mass number is 238, has two isotopes with mass numbers 233 and 235, respectively.
Every atom is defined by its atomic number (Z) and its mass number (A)
The number of known elements (those with the same atomic number) are presented in a “Periodic Table of the Elements”, wherein elements are ordered according to their increasing atomic number. They are set up on 18 vertical columns known as groups. Each group contains the elements with similar properties due to the common electronic structure at their most external level.
The energies that keep protons and nuclei together inside the nucleus vary. They are greater in intermediate elements and lower in lighter elements with less Z, and in heavier elements with greater Z. This entails a greater stability in intermediate elements. Thus, the division (fission) of a Uranium nucleus into two or three intermediate elements, or the union (fusion) of very light nuclei such as hydrogen and its isotopes would give way to a more stable configuration.