Topic 2 Structure of Atom

Topic 2

Structure of Atom

Atoms are very small. If you compare an atom with a ping-pong ball, it is equivalent to comparing a ping-pong ball with the earth (as shown in Figure 3-8 ). The mystery that atoms can combine to form molecules is gradually revealed when people study the structure of this small atom.

1. The composition of atoms

Scientific experiments have proved that an atom is composed of a nucleus in the center of the atom and electrons outside the nucleus. 0 The nucleus is composed of protons and neutrons. Each proton carries 1 unit of positive charge, each electron carries 1 unit of negative charge, and neutrons have no charge.

Since the charges carried by the protons in the nucleus are equal to the charges of the electrons outside the nucleus, and the electric properties are opposite, the atoms do not exhibit electric properties. It can be seen that the number of positive charges carried by the nucleus (nuclear charge) is equal to the number of protons in the nucleus and the number of electrons outside the nucleus.

2.The arrangement of electrons outside the nucleus

Compared with atoms, the nucleus is smaller. If the atom is compared to a stadium, the nucleus is only equivalent to an ant in the stadium. Therefore, there is a large space outside the nucleus, and electrons move at high speed in this space.

Scientific research shows that in an atom containing multiple electrons, the electrons outside the nucleus have different motion states. The electrons closer to the nucleus have lower energy, and the farther away from the nucleus, the higher the energy of the electron. The electron layer closest to the nucleus is the first layer, followed by the second layer, and so on as the third, fourth, fifth, sixth, and seventh layers, and the farthest from the nucleus is also called the outermost layer. This layered movement of extranuclear electrons is also called layered arrangement (as shown in Figure 3~10) . It is known that the least number of electrons outside the nucleus of an atom is only one layer, the most is seven layers, and the number of electrons in the outermost layer does not exceed 8 (only one layer, the number of electrons does not exceed 2 ).

The layered arrangement of electrons outside the nucleus can be expressed concisely and conveniently by using the schematic diagram of the atomic structure.

Rare gases such as gas and chlorine are not easy to react with other substances, and their chemical properties are relatively stable. The outermost layer of their atoms has 8 electrons ( 2 electrons for gas). This structure is considered to be a relatively stable structure. The outermost electrons of metal atoms such as sodium, magnesium, and aluminum are generally less than 4 , and they are easy to lose electrons in chemical reactions; the outermost electrons of non-metallic atoms such as chlorine, oxygen, sulfur, and phosphorus are generally more than 4 , in chemical reactions, easy to get electrons; all tend to reach a relatively stable structure.

Take the reaction of metal sodium and chlorine gas as an example, the outermost layer of the sodium atom has 1 electron, and the outermost layer of the chlorine atom has 7 electrons. When sodium reacts with chlorine gas, 1 electron in the outermost layer of the sodium atom is transferred to on the outermost shell of the chlorine atoms, so that both form a relatively stable structure.

 In the above process, the sodium atom is charged with 1 unit of positive charge due to the loss of 1 electron ; the chlorine atom is charged with 1 unit of negative charge due to the gain of 1 electron. Such charged atoms are called ions. Positively charged atoms are called cations, such as sodium ions ( Na+) ; negatively charged atoms are called anions, such as chloride ions ( C ") . The oppositely charged sodium ions interact with chloride ions to form sodium chloride. Visible , ions are also particles that make up matter.

3. Relative Atomic Mass

Atoms have very little mass. For example , the mass of a hydrogen atom is about 1.67 x 10^27 kg, and the mass of an oxygen atom is about 2.657 x 10^26 kg . Because the value of atomic mass is too small, it is inconvenient to write and use, so it is agreed internationally to use relative mass, that is, 1/12 of the mass of a carbon atom is used as the standard, and the mass of other atoms The ratio obtained by comparing the mass with it is used as the relative atomic mass of this atom. According to this standard, the relative atomic mass of hydrogen is about 1, and the relative atomic mass of oxygen is about 16.

The relative masses of protons and neutrons that make up atoms are approximately equal to 1. Compared with protons and neutrons, the mass of electrons is very small, and the mass of the entire atom is mainly concentrated on the nucleus.

In general chemical calculations, the approximate value of relative atomic mass is often used.

You should know after finishing this topic

1.The composition of atoms

2.The extranuclear electrons in an atom are arranged in layers, which can be represented by a schematic diagram of the atomic structure.

3.In addition to atoms and molecules, the particles that make up matter also have ions. Charged atoms are called ions.

4.1/12 of the mass of a carbon atom as a standard, the ratio of the mass of other atoms to it is used as the relative atomic mass of this atom.