Physics of the structure of matter. Discoveries. Experiments. Calculations

Table of contents:

Physics of the structure of matter. Discoveries. Experiments. Calculations
Physics of the structure of matter. Discoveries. Experiments. Calculations

The physics of the structure of matter was first seriously studied by Joseph J. Thomson. However, many questions remained unanswered. Some time later, E. Rutherford was able to formulate a model of the structure of the atom. In the article we will consider the experience that led him to the discovery. Since the structure of matter is one of the most interesting topics in physics lessons, we will analyze its key aspects. We learn what an atom consists of, learn how to find the number of electrons, protons, neutrons in it. Let's get acquainted with the concept of isotopes and ions.

Discovery of the electron

In 1897, the English scientist Joseph John Thomson (his portrait can be seen below) studied electric current, that is, the directed movement of charges in gases. At that time, physics already knew about the molecular structure of matter. It was known that all bodies are made of matter, which is made of molecules, and the latter are made of atoms.

Joseph John Thomson

Thomson discovered that, under certain conditions, gas atoms emit particles with a negative charge (qel <0). They are called electrons. The atom is neutral, which means that if electrons fly out of it, then positive particles must also be contained there. What is the part of the atom with the "+" sign? How does it interact with a negatively charged electron? What determines the mass of an atom? Another scientist could answer all these questions.

Rutherford's experiment

In 1911, physics already possessed the initial information about the structure of matter. Ernest Rutherford discovered what we today call the atomic nucleus.

Ernest Rutherford

There are matters that have a strange property: they spontaneously emit various particles, both positive and negative. Such substances are called radioactive. Positively charged elements Rutherford called alpha particles (α-particles).

They have a "+" charge equal to two elementary charges (qα=+2e). The weight of the elements is approximately equal to four masses of a hydrogen atom. Rutherford took a radioactive preparation that emits alpha particles and bombarded a thin film of gold (foil) with their stream.

He found that most α-elements barely change their direction when passing through metal atoms. But there are very few who deviate backwards. Why is this happening? Knowing the physics of the structure of matter, we can answer: because insidegold atoms, like any other, there are positive elements that repel alpha particles. But why does this only happen with very few elements? Because the size of the positively charged part of the atom is much smaller than itself. Rutherford came to this conclusion. He called the positively charged part of the atom the nucleus.

The device of the atom

Physics of the structure of matter: Molecules are made up of atoms, which contain a tiny positively charged part (nucleus) surrounded by electrons. The neutrality of the atom is explained by the fact that the total negative charge of the electrons is equal to the positive - the nucleus. qcore + qel=0. Why don't electrons fall on the nucleus, because they are attracted? To answer this question, Rutherford suggested that they rotate like the planets move around the Sun and do not collide with it. It is the movement that allows this system to be stable. Rutherford's model of the atom is called planetary.

If the atom is neutral, and the number of electrons in it must be integer, then the charge of the nucleus is equal to this value with a plus sign. qcores=+ze. z is the number of electrons in a neutral atom. In this case, the total charge is zero. How to find the number of electrons in an atom? You need to use the periodic table of elements. The dimensions of an atom are of the order of 10-10 m. And the nuclei are 100 thousand times smaller - 10-15 m.

Let's imagine that we increased the size of the core to 1 meter. In a solid, the distance between atoms is approximately equal to the size of themselves, which means that the dimensionswill increase to 105, which is 100 km. That is, the atom is practically empty, which is why alpha particles mostly fly through the foil with almost no deflection.

Structure of the nucleus

The physics of the structure of matter is such that the nucleus consists of two kinds of particles. Some of them are positively charged. If we consider an atom that has three electrons, then inside it there are three particles with a positive charge. They are called protons. Other elements do not have an electric charge - neutrons.

The structure of the nucleus

The masses of the proton and neutron are approximately equal. Both particles have a weight much greater than an electron. mproton ≈ 1837mel. The same applies to the mass of the neutron. The conclusion follows from this: the weight of positively and neutrally charged particles is a factor that determines the mass of an atom. Protons and neutrons have a common name - nucleons. The weight of an atom is determined by their number, which is called the mass number of the nucleus. We denoted the number of electrons in an atom by the letter z, but since it is neutral, the number of positive and negative particles must match. Therefore, z is also called the proton or charge number.

If we know the mass and charge number, then we can find the number of neutrons N. N=A - z. How to find out how many nucleons and protons are in the nucleus? It turns out that in the periodic table, next to each element, there is a number that chemists call the relative atomic mass.

Lithium in the periodic table

If we round it up, we get nothing more thanmass number or the number of nucleons in the nucleus (A). The atomic number of an element is the number of protons (z). Knowing A and z, it is easy to find N - the number of neutrons. If the atom is neutral, then the number of electrons and protons is equal.


There are varieties of the nucleus in which the number of protons is the same, but the number of neutrons may differ (meaning the same chemical element). They are called isotopes. In nature, atoms of different kinds are mixed, so chemists measure the average mass. That is why in the periodic table the relative weight of an atom is always a fractional number. Let's figure out what happens to a neutral atom if an electron is removed from it or, conversely, an extra one is placed.


Schematic representation of an ion

Consider a neutral lithium atom. There is a nucleus, two electrons are located on one shell and three on the other. If we take away one of them, we get a positively charged nucleus. qcores =3rd. Electrons compensate only two of the three elementary charges, and we get a positive ion. It is designated as follows: Li+. An ion is an atom in which the number of electrons is less than or greater than the number of protons in the nucleus. In the first case, it is a positive ion. If we add an extra electron, then there will be four of them, and we will get a negative ion (Li-). Such is the physics of the structure of matter. So, a neutral atom differs from an ion in that the electrons in it completely compensate for the charge of the nucleus.

Popular topic