Studying the composition of matter, scientists came to the conclusion that all matter consists of molecules and atoms. For a long time, an atom (translated from Greek as "indivisible") was considered the smallest structural unit of matter. However, further studies have shown that the atom has a complex structure and, in turn, includes smaller particles.
What does an atom consist of?
In 1911, the scientist Rutherford suggested that the atom has a central part with a positive charge. So the concept of the atomic nucleus first appeared.
According to Rutherford's scheme, called the planetary model, an atom consists of a nucleus and elementary particles with a negative charge - electrons moving around the nucleus, similar to how the planets orbit around the sun.
In 1932, another scientist, Chadwick, discovered a neutron - a particle that does not have an electric charge.
According to modern concepts, the structure of the atomic nucleus corresponds to the planetary model proposed by Rutherford. The nucleus carries most of the atomic mass. It also has a positive charge. In the atomic nucleus are protons - positively charged particles and neutrons - particles that do not carry a charge. Protons and neutrons are called nucleons. Negatively charged particles - electrons - move in an orbit around the nucleus.
The number of protons in the nucleus is equal to the number of electrons moving in orbit. Therefore, the atom itself is a particle that does not carry a charge. If an atom captures foreign electrons or loses its own, then it becomes positive or negative and is called an ion.
Electrons, protons and neutrons are collectively called subatomic particles.
Atomic charge
The nucleus has a charge number Z. It is determined by the number of protons that make up the atomic nucleus. To know this quantity is simple: it is enough to turn to the periodic system of Mendeleev. The serial number of the element to which the atom belongs is equal to the number of protons in the nucleus. Thus, if the chemical element corresponds to oxygen with serial number 8, then the number of protons will also be eight. Since the number of protons and electrons in an atom coincides, there will also be eight electrons.
The number of neutrons is called the isotopic number and is denoted by the letter N. Their number can differ in the atom of the same chemical element.
The sum of protons and electrons in the nucleus is called the mass number of the atom and is denoted by the letter A. Thus, the formula for calculating the mass number looks like this: A = Z + N.
Isotopes
In the case when the elements have an equal number of protons and electrons, but a different number of neutrons, they are called isotopes of a chemical element. Isotopes can be one or more. They are placed in the same cell of the periodic system.
Isotopes are of great importance in chemistry and physics. For example, a hydrogen isotope - deuterium - in combination with oxygen gives a completely new substance, which is called heavy water. It has a different boiling and freezing point than usual. And the combination of deuterium with another hydrogen isotope, tritium, leads to a thermonuclear fusion reaction and can be used to generate a huge amount of energy.
The mass of the nucleus and subatomic particles
The sizes and masses of atoms and subatomic particles are negligible in human representations. The size of the nuclei is about 10 -12 cm. The mass of the atomic nucleus is measured in physics in the so-called atomic mass units - amu.
For one amu take one twelfth of the mass of the carbon atom. Using the usual units of measurement (kilograms and grams), the mass can be expressed by the following equality: 1 amu = 1,660540 · 10 -24 g. Expressed in this way, it is called the absolute atomic mass.
Despite the fact that the atomic nucleus is the most massive component of the atom, its size relative to the electron cloud surrounding it is extremely small.
Nuclear forces
Atomic nuclei are extremely stable. This means that protons and neutrons are held in the nucleus by some forces. It cannot be electromagnetic forces, since protons are particles of the same name, and it is known that particles with the same charge repel each other. Gravitational forces are too weak to hold the nucleons together. Consequently, particles are held in the nucleus by another interaction - nuclear forces.
Nuclear interaction is considered the most powerful of all existing in nature. Therefore, this type of interaction between the elements of the atomic nucleus is called strong. It is present in many elementary particles, as well as electromagnetic forces.
Features of nuclear forces
- Short action. Nuclear forces, unlike electromagnetic ones, appear only at very small distances comparable to the size of the nucleus.
- Charge independence. This feature is manifested in the fact that nuclear forces act equally on protons and neutrons.
- Saturation. Nucleons of a nucleus interact only with a certain number of other nucleons.
Core binding energy
Closely related to the concept of strong interaction is the binding energy of nuclei. By nuclear bonding energy is meant the amount of energy that is required to divide an atomic nucleus into its nucleons. It equals the energy needed to form a nucleus from individual particles.
To calculate the binding energy of a nucleus, it is necessary to know the mass of subatomic particles. Calculations show that the mass of the nucleus is always less than the sum of the nucleons in its composition. A mass defect is the difference between the mass of a nucleus and the sum of its protons and electrons. Using the Einstein formula on the relationship between mass and energy (E = mc 2 ), it is possible to calculate the energy generated during the formation of the nucleus.
The strength of the binding energy of the nucleus can be judged by the following example: the formation of several grams of helium produces as much energy as the combustion of several tons of coal.
Nuclear reactions
The nuclei of atoms can interact with the nuclei of other atoms. Such interactions are called nuclear reactions. There are two types of reactions.
- Fission reactions. They occur when heavier nuclei, as a result of interaction, decay into lighter ones.
- Synthesis reaction. The opposite process of fission: nuclei collide, thereby forming heavier elements.
All nuclear reactions are accompanied by the release of energy, which is subsequently used in industry, in the military sphere, in energy and so on.
Having become acquainted with the composition of the atomic nucleus, we can draw the following conclusions.
- An atom consists of a nucleus containing protons and neutrons, and electrons around it.
- The mass number of an atom is equal to the sum of the nucleons of its nucleus.
- Nucleons are held together by a strong interaction.
- The huge forces that give the atomic nucleus stability are called the binding energies of the nucleus.