The structure of the atom is basically divided into two regions: the nucleus, which is formed by protons and neutrons, and the electrosphere, formed by electrons and a large void.

We will talk here about the atomic model most studied in high school and which serves to explain most of the physical and chemical phenomena through which matter passes. According to this model, the structure of the atom basically has two distinct regions, which are:

Nucleus : It is a massive, compact and dense region that is in the center of the atom. The atomic nucleus is divisible because it consists of two different particles:

– Protons: are positively charged particles with a relative charge of +1. Its relative mass is also 1.

The number of protons in the nucleus is called the atomic number (Z) and is responsible for the differentiation of one chemical element from another, that is, each chemical element is formed by a set of atoms that have the same atomic number or the same number of protons.

– Neutrons: as the name implies, these are neutral particles, that is, they have no electrical charge. Thus, neutrons decrease the repulsion force between protons in the nucleus (given that charges of the same signal repel each other).

These subatomic particles have a relative mass practically equal to that of protons, that is, 1. But in reality, the mass of the neutron is slightly greater than that of the proton. This is interesting because, if it were the other way around, that is, if protons were slightly heavier than neutrons, all protons would be transformed into neutrons. The result would be that, without protons, atoms would not exist.

For more details on neutrons, read the text The discovery of the third subatomic particle: the neutron .

Some curiosities:

  • The size of the nucleus depends on the amount of neutrons and protons it has. However, it can be said that, on average, the atomic nucleus has a diameter around 10 -14m and 10 -15

Don’t stop now … There’s more after the publicity;)

  • The proton and neutron are particles 100,000 times smaller than the entire atom itself!
  • The mass of the atom is given practically only by the number of protons and neutrons in the nucleus. This is because each proton and each neutron is 1836 times larger than an electron. For this reason, the mass of the electrons becomes insignificant.

* Electrosphere : It is a peripheral region around the atom nucleus where the electrons are rotating around this nucleus.

– Electrons: These were the first subatomic particles discovered (in the 1897s, by JJ Thomson). They are negatively charged particles, whose relative charge is -1. Its charge in Coulombs is equal to 1.6.10-19 C.

Although the electrons are negative, the atom in the ground state is neutral, as it has the same amount of electrons and protons. This means that the negative charges of the electrons cancel out the positive charges of the protons, so the atom is neutral.

When atoms make chemical bonds to form simple and compound substances, so do electrons. There is then a transfer or sharing of electrons between two or more atoms.

The illustrations of the structure of the atom are only models, but do not represent reality. For example, most of the atom is a large void. To give you an idea, think of the hydrogen atom formed by a proton and an electron. If the nucleus of that atom were the size of a tennis ball, its orbiting electron would be three kilometers away! The electrosphere is larger than the nucleus of the atom about 10,000 to 100,000 times.


At 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 in it, 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 added to the number of neutrons is the number of atomic mass. The electrons are outside the nucleus and have small mass.

Particle Characteristics:

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

Electrical loads and masses

Numerous experiments carried out made it possible to establish the properties of the atom’s particles in terms of its electrical charge and mass.

Regarding the electric charge: the proton charge equal to +1 is considered ; the electron charge equal to -1 , and the neutron charge equal to 0 .

As is known, electrical charges of the same intensity and opposite signals are neutralized. That is, the electrical charge of a proton (positive) cancels the electrical charge of an electron (negative). Any atom has the same number of protons and electrons; therefore, it is electrically neutral.

As for mass : taking the value of the mass of the proton as a reference, it is stated that the mass of the neutron is practically equal to the mass of the proton. As the proton has a mass about 2,000 times greater 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 in between. 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 the other.

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

  • Z is the atom’s “identity card” because it indicates which chemical element each atom belongs to.
  • 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 affirm that atoms with different number of protons belong, necessarily, to different chemical elements. Let’s see:

The sodium atom has 11 protons; therefore, its 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 and the number of neutrons. The mass number is represented by the letter A.


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