Molecules are "composed" of atoms, which are composed of nuclei (positively charged) and extranuclear electrons (negatively charged), nuclei are composed of protons (positively charged) and neutrons (uncharged), and substances are composed of atoms (metals and some nonmetals) or molecules (usually gases) or ions.
The smallest unit in a substance that can exist independently, is relatively stable and maintains its physical and chemical properties. Molecules are composed of atoms, which are combined into molecules in a certain order and arrangement through a certain force. Taking water molecules as an example, water is continuously divided until the characteristics of bad water are not broken. At this time, the smallest unit is a water molecule composed of two hydrogen atoms and one oxygen atom. Its chemical formula is written as H2O. Water molecules can be subdivided into two hydrogen atoms and one oxygen atom by electrolysis or other methods, but their characteristics are completely different from those of water. Some molecules are composed of only one atom, which are called monoatomic molecules, such as helium and argon, which are both atoms and molecules. A molecule composed of two atoms is called diatomic molecule, such as oxygen molecule (O2), which is composed of two oxygen atoms and is a homonuclear diatomic molecule; Carbon monoxide molecule (CO) consists of one oxygen atom and one carbon atom, which is a heteronuclear diatomic molecule. Molecules composed of more than two atoms are collectively called polyatomic molecules. The number of atoms in a molecule can be several, ten, dozens or even thousands. For example, the carbon dioxide molecule (CO2) consists of one carbon atom and two oxygen atoms. A benzene molecule contains six carbon atoms and six hydrogen atoms (C6H6), and a pig insulin molecule contains hundreds of atoms. The molecular formula is C255H380O78N65S6.
The smallest particle in a substance that can exist independently and maintain its composition and all chemical characteristics. Molecules are made up of atoms bound together by chemical bonds. The interaction between atoms is relatively strong, but the interaction between molecules is quite weak. This force is called van der Waals force, so molecules behave as independent particles to some extent.
Molecules can be composed of the same kind of atoms or different kinds of atoms. The simplest molecules contain only one atom, such as molecules of rare gases. Most nonmetallic molecules are diatomic molecules such as nitrogen and oxygen. Compounds are molecules composed of different elements, with the largest number.
The first chemist who put forward a more accurate concept of molecule was Italian avogadro, who published his molecular theory in181kloc-0/year, and thought: "The atom is the smallest particle that participates in chemical reactions, while the molecule is the smallest particle that simple materials or a compound can exist independently in a free state. Molecules are composed of atoms, simple molecules are composed of atoms of the same element, and composite molecules are composed of atoms of different elements. In chemical changes, various atoms in different substance molecules recombine. "
Since avogadro put forward the concept of molecules, chemists have regarded molecules as particles slightly larger than atoms for a long time. 1920, the German chemist H. staudinger began to doubt the idea that small molecules dominate the world. His basis is that the molecular weight of rubber measured by osmotic pressure method can be as high as 65438+ million. He put forward the concept of macromolecules in his paper, pointing out that natural rubber is not a combination of small molecules, but a long-chain macromolecule with valence bond structure. Polymers also have their own characteristics. For example, unlike small molecules, polymers do not have definite and constant molecular weights, but they use average molecular weights.
With the development of molecular concepts, chemists have a deeper understanding of inorganic molecules. For example, sodium chloride is an infinite structure in which sodium ions and chloride ions are interconnected by ionic bonds. It is difficult to accurately point out how many sodium ions and chloride ions are contained in its molecules, and it is also impossible to determine its molecular weight. This structure also includes molecules such as diamond, graphite, asbestos and mica.
After the appearance of methods to study short-lived molecules, such as picosecond spectroscopy, the lifetime of methyl (CH3) is 10- 13 seconds, which is not only short-lived, but also very lively, because the valence bond of methyl is unsaturated and has the structure of odd electrons. There are also particles such as ch, CN and HO, which are collectively called free radicals. They have only a certain degree of stability and are prone to chemical reactions. This shows that free radicals also have molecular characteristics, so they are classified as molecules. There is another molecule that is unstable in the ground state but stable in the excited state. This molecule is called an excimer.
The science of studying various natural phenomena at the molecular level is called molecular science. Such as zoology, genetics, botany, physiology, etc. We are mastering the properties and structures of different kinds of molecules in various forms, and designing molecules with given properties from the properties and structures of molecules, which is called molecular design.
Protons are the smallest particles in chemical changes. It is the most classic and widely used basic hypothesis of human beings. Atomic hypothesis can be used to accurately explain almost all aspects of physics, such as mechanics, thermodynamics, optics, quantum mechanics, statistical mechanics and so on. , as well as biology (from the physicist's point of view, all biological processes are atomic movements) and chemistry (chemistry can be explained by quantum mechanics, etc. ), also a natural science, may be extended to various disciplines in the future.
The hypothesis of atom is based on the intuitive feeling of human beings-the particle nature of matter. But it can also be magically found in the fluctuation of matter. Perhaps it is because of the hypothesis of atoms that physics has achieved such brilliant results.
The atom can be regarded as a ping-pong ball in a gymnasium as big as the earth (the atomic radius is in the order of 10-10). The method of studying atoms is also like placing table tennis balls with the size of 10 to the 23rd power in this gymnasium, and making these balls keep beating.
The nucleus is composed of protons and neutrons, and the electrons in the outer layer rotate around the nucleus at high speed.
Atoms are the basic units of various elements in nature, and are composed of orbital electrons inside and outside the nucleus (also called bound electrons or orbital electrons). Atoms are very small, with a diameter of only -8 cm of 10 and a small mass. For example, the mass of hydrogen atom is -24g of 1.673 56* 10, and the nuclear mass accounts for more than 99% of the atomic mass. The center of the atom is the nucleus, and its diameter is much smaller than that of the atom.
The nucleus is positively charged, and the bound electrons are negatively charged, with equal charges and opposite signs. Therefore, the atom itself is neutral. Bound electrons move around the nucleus in a certain orbit. When atoms absorb external energy and make orbital electrons move freely without the attraction of atomic nuclei, atoms lose electrons and become ions.
Atoms are the smallest unit of elements and a level of material structure. The word atom comes from Greek, "meaning is inseparable." In the 4th century BC, democritus, an ancient Greek physicist, put forward this concept and regarded it as the smallest unit of matter, but Aristotle and others opposed the atomic view of this matter almost at the same time. They thought that matter was continuous, which was dominant in the Middle Ages. However, with the progress of science and the development of experimental technology, the atomic view of matter was accepted by people after16th century. Famous scholars such as Galileo, Descartes, Newton and others all support this view. The periodic law discovered by Mendeleev, a famous Russian chemist, points out that the correlation between atoms of various chemical elements is a guiding principle when establishing the theory of atomic structure. From the point of view of modern physics, atom is only a level of material structure, which is between molecule and nucleus.
basic concept
Ions are charged particles formed by atoms or atomic groups due to the gain and loss of electrons.
Atoms consist of nuclei and extranuclear electrons. The nucleus is positively charged, while the electrons moving around the nucleus are negatively charged. The nuclear charge of an atom is equal to the number of electrons outside the nucleus, so the atom is electrically neutral. If the energy obtained by the atom from the outside exceeds the binding energy of a shell electron, then the electron can leave the atom and become a free electron. Generally, atoms with fewer electrons in the outermost layer or atoms with larger radius are more likely to lose electrons; On the contrary, it is easier to obtain electrons. When the outermost electron orbital of the atom reaches saturation state (2 shell electrons in the first periodic element and 8 electrons in the second and third periodic elements), the properties are the most stable.
classify
When an atom gains one or more electrons, the number of electrons outside the nucleus is more than the number of nuclear charges, so it is negatively charged, which is called anion.
When an atom loses one or several electrons, the number of electrons outside the nucleus is less than that of nuclear power, so it is positively charged, which is called cation.
(Complex ions refer to complex ions formed by coordination bonds between some molecules, atoms or cations and electrically neutral molecules or anions, such as hydrated ions. The complex ion itself can belong to cation or anion. )
The process of producing ions by electron transfer between atoms of a compound is called ionization, and the energy required or released in the process of ionization is called ionization energy. The greater the ionization energy, the harder it is for atoms to lose electrons.
Ionic compounds, that is, compounds composed of ionic bonds between anions and cations, such as water-soluble acids, bases, salts, etc., when dissolved and ionized in water, under constant conditions, the proportion of ionic state and the proportion of molecular state reach dynamic balance, which is called ion balance.
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