# The Atom Interior

In the center of an atom is its nucleus, which although small, contains almost the entire mass of the atom. Protons and neutrons are the particles found therein, each with a unitary atomic mass.

The number of protons in the nucleus establishes the atomic number of the chemical element and the number of protons plus the number of neutrons is the atomic mass number. The electrons are outside the nucleus and have small mass.

Particle Characteristics:

• Protons: has positive electric charge and a unit mass.
• Neutrons: has no electric charge, but has unit mass.
• Electrons: has negative electric charge and almost no mass.

## Electric charges and masses

Numerous experiments have made it possible to establish the properties of atom particles in terms of their electric charge and mass.

When to electric charge: the load of the +1 proton; the load of electron equal to -1, and the load of neutron equal to 0.

As is well known, electric charges of the same intensity and opposite signals neutralize each other. That is, the electrical charge of a (positive) proton overrides the electrical charge of a (negative) electron. Any atom has the same number of protons and electrons; therefore, it is electrically neutral.

How much massHaving the proton mass value as a reference, it is stated that the neutron mass is practically equal to the proton mass. Since the proton has a mass about 2,000 times larger than that of an electron, the latter is considered negligible.

## The nucleus of the atom

Now let's take a closer look at the characteristics of the atomic nucleus.

### Atomic number

There are many different atoms between them. For example, the aluminum atom is different from the gold atom. What will be the main difference between the types of atoms?

After studies it was defined that The number of protons is one of the main characteristics that differentiate one atom from another.

This number is called atomic number and is represented by the letter Z.

• Z is the atom's "identity card" because it indicates to which chemical element each atom belongs.
• The set of atoms that have the same atomic number (Z) is called a chemical element.

Therefore, from the concept of chemical element, it is possible to say that atoms with different number of protons belong, necessarily, to different chemical elements. Let's see:

The sodium atom has 11 protons; therefore, their atomic number is equal to 11 (Z = 11), and all atoms with atomic number (Z) equal to 11 belong to the chemical element sodium.

### Mass number

It is the sum of the number of protons with the number of neutrons. The mass number is represented by the letter A. In this expression we have: THE = mass number;

P = number of protons;

no = number of neutrons.

Thus, the difference between mass number and atomic number reveals the number of neutrons. The sum of the number of protons and the number of neutrons, ie the mass number (A), does not correspond to the entire mass of the atom, because there are also electrons. The reason A represents the mass of the atom is that the electron mass is negligible compared to that of protons and neutrons.

Atoms belonging to the same chemical element may have different numbers of neutrons. Silver, for example, is found in nature with distinct neutron numbers: 60 and 62. If we add these numbers to the proton number, which is 47, we have:

• 60 neutrons + 47 protons = 107 as mass number.
• 62 neutrons + 47 protons = 109 as mass number.

Therefore, only the atomic number can identify to which chemical element the atom belongs.