The atomic number of a chemical element indicates the number of protons contained in the atomic nucleus of that element. In the case that it was a neutral atom, then the atomic number will coincide with the number of electrons of said element.
As is known, the atomic nucleus consists of protons and neutrons. Protons have a positive charge and their value coincides with the absolute value of the negative charge of the electron, which is 1.6 x 10-19 coulomb in International System units.
Because they are particles of the atomic nucleus, protons and neutrons are known as nucleons. It is these particles that practically determine the mass of the atom, since the nucleons are about two thousand times more massive than the electrons of the atom. For this reason, the number of nucleons in an atom is called mass number.
However, the atomic number it is the one that determines the chemical affinity of the atomic elements, because in neutral atoms this number coincides with the number of electrons.
In the periodic table, the symbol X of a chemical element is accompanied on the lower left side with the number Z which represents the atomic number of said element, while in the upper left of the chemical symbol of the element the mass number TO.
The following figure shows this notation:
And then, table 1 shows some examples of chemical elements with their respective notation and atomic and mass numbers:
Table 1
| Element | Notation | Atomic number Z | Mass number A |
|---|---|---|---|
| Hydrogen | 1 1 H | 1 | 1 |
| Carbon | 12 6 C | 6 | 12 |
| Oxygen | 16 8 OR | 8 | 16 |
| Uranium | 238 92 OR | 92 | 238 |
The periodic table of chemical elements allows the sequential arrangement and in increasing order of the elements, according to the value of their atomic number.
At least 118 of them are known, starting with hydrogen until reaching element 118 which is the oganeson -before called ununoctium-, whose chemical symbol is Og and its mass number is 294.
The first 10 chemical elements with increasing order in atomic number are much more familiar. So for example we have the best known:
The atomic number indicates the number of protons in the atomic nucleus of a chemical element and each proton, as we have said, has an elemental positive charge of 1,602 x 10-19 C.
If the atom is neutral, then the number of elemental negatively charged electrons is -1.602 x10-19 C must be equal to the atomic number. So, knowing the numerical position of a chemical element in the periodic table, its number of electrons is also known..
In the atom these electrons occupy regions called orbitals, which depend on the energy and the angular momentum of the electron. In turn, both the energy and the angular momentum of the electrons in the atom are quantized. This means that they can only take some discrete values.
Now, the orbitals are populated with electrons from lower to higher energy, following certain rules that determine the electronic configuration.
In this way, the number of electrons in the outermost orbitals or levels of an atom establishes the bonds that it can form with other atoms to form molecules. This number depends on the electron configuration and the atomic number.
The energy of an electron in the atom is determined by its principal quantum number n, which can take integer values 1, 2, 3 ...
The angular momentum of the electron in the atom is determined by the secondary quantum number l (letter ele) and its values depend on n.
For example, for n = 2 the secondary quantum number takes the values from 0 to n-1, that is: 0 and 1. Similarly, for n = 1 which is the lowest energy level, the secondary quantum number l it only takes the value 0.
Secondary quantum numbers 0, 1, 2, 3 are alternately denoted by the letters s, p, d, and f. The s orbitals are spherical and the p orbitals are lobulated.
There are two more quantum numbers:
Finally there is the Pauli exclusion principle, which states that two electrons cannot have all four identical quantum numbers. As a consequence of this principle, an orbital can have a maximum of two electrons with opposite spin.
The electron configuration diagram is used to determine the order of filling of electrons from the lowest energy orbitals to the highest energy ones, once the atomic number of a chemical element is known. Figure 4 shows this diagram:
The idea outlined there is the following: the first number indicates the principal quantum number, then the letter indicates the secondary quantum number, and finally the superscript number indicates the maximum number of electrons possible in that orbital..
As indicated above, carbon has atomic number 6, which means that it has 6 electrons that are distributed as follows: 1s2 2s2 2p2. For its part, silicon has atomic number 14 and its electrons are distributed in this other way: 1s2 2s2 2p6 3s2 3p2.
Both elements are in the same column of the periodic table, because, even though they have different atomic numbers, they have the same electronic configuration in the outermost orbital.
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