[explanation] 1. The number written on the fractional line in a score.
2. The smallest particle in a substance that can maintain all the chemical properties of the original substance and exist independently. Molecules are made up of atoms. In chemical changes, molecules can be subdivided, but atoms cannot be subdivided.
Molecules are the smallest particles that exist independently and maintain the chemical properties of substances.
Molecules have a certain size and mass; There is a certain interval between molecules; Molecules are constantly moving; There is a certain force between molecules; Molecules can make up matter, and molecules can be divided into smaller particles in chemical changes: atoms.
The same molecule has the same properties, but different molecules have different properties. The smallest molecule is the isotope of hydrogen molecule, which is a hydrogen molecule without neutrons and has a mass of 1. The relative molecular mass of macromolecules can be as high as several million. Molecules with relative molecular mass exceeding several thousand are called polymers. Molecules are tiny units that make up matter. They are the smallest particles that can exist independently and maintain all the original chemical properties of substances. Molecules are generally composed of smaller particle atoms. According to the number of atoms, molecules can be divided into monoatomic molecules, diatomic molecules and polyatomic molecules. According to the electrical structure, it can be divided into polar molecules and nonpolar molecules. Molecules of different substances have different microstructures and shapes. The ideal model of molecules is to regard them as spheres with a diameter of 10- 10 m and a molecular weight of about 10-26 kg.
Molecular molecule
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 connected to each other through 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 difficult 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.
molecular movement
Molecules can exist in gas, liquid or solid state.
In addition to translational motion, there are molecular rotation and various types of vibrations of atoms in molecules. The amplitude of internal vibration and rotation of molecules is much smaller than that of translation and rotation of molecules in gas and liquid, and this internal movement of molecules will not destroy the inherent characteristics of molecules. Generally speaking, the molecular structure is the structure when these atoms are in an equilibrium position. The internal motion of molecules determines the properties of molecular spectrum, so we can use molecular spectrum to study the internal motion of molecules.
In molecules with the same molecular configuration and conformation, different molecules can be formed if the arrangement order and mode of atoms are different. For example, C2H6O molecules can be arranged into ethanol molecules or dimethyl ether molecules, and its structural formula is shown in Figure 2, which reflects the arrangement order of atoms in the molecules. The components of the constituent molecules are the same, but the arrangement order is different, forming two or more molecules. This phenomenon is called isomerism, and these molecules with the same composition but different structures are called isomers.
Generally, the structural formula of a molecule only reflects the connection order of atoms in the molecule, and the values of bond length and bond angle that determine the shape of the molecule need to be determined by experiments. Reflecting the arrangement order and distribution of atoms in a molecule in space is called molecular configuration. The length and angle of chemical bonds between atoms in a molecule are called three-dimensional configuration parameters.
For some molecules, when the configuration is determined, the shape and size of the molecules are also determined, such as water molecules, methane molecules, benzene molecules and so on. Under certain configuration conditions, the shapes of some molecules will change with the relative positions of atoms. For example, ethane (C2H6) molecules can have two different shapes: crossing (A in Figure 3) and overlapping (B in Figure 3) under the same connection order, the same bond length and the bond angle data of pure rotation spectrum of diatomic molecules. This situation is called molecular conformation. Molecules with different conformations have different energies and symmetries. For ethane molecules, the cross conformation is relatively stable at room temperature.
Molecular constant
In a certain state, the shape and size, structure and properties of molecules are certain.
By studying the experimental data of molecular mechanical properties, thermal properties, electrical properties and molecular spectrum, we can get the average moving speed, collision frequency, molecular diameter (calculated by the diameter of a sphere), ionization potential (that is, the energy difference between the lowest energy state of a neutral molecule and the lowest energy state of an ion), dissociation energy (that is, the energy difference between the lowest energy state of a molecule and the ground state of an atom), the distance between nuclei (that is, the bond length), the force constant of molecular vibration, dipole moment and so on. These data, collectively called molecular constants, are important data to describe molecular structure and physical properties. See the pure spin spectrum of diatomic molecules for specific values.
Molecular mass atoms are combined into molecules by chemical bonds, and molecules have a certain mass. The ratio of molecular mass to atomic weight of112 of 12C is called molecular weight. The usual carbon elements are composed of 12C, 13C and 14C, so the atomic weight of carbon is 12.438+0 1. The atomic weight of hydrogen is 1.088, the atomic weight of oxygen is 15.999, and the molecular weight of ethanol (C2H6O) is 2×12.01+6×1.088. 0.0 12 kg of 12C contains 6.022 52× 1023 atoms, which is called 1 mole (or 1 gram atom); Similarly, 46.069 grams of ethanol contains the same number of ethanol molecules, which is called 1 mol (or 1 mol) ethanol.
Molecules with a molecular weight greater than 10 000 are usually called polymers, although this limitation is not absolute. When the molecular weight reaches a certain level, molecules will have some unique properties. Polymers are very important in industry and biochemistry, such as plastics, rubber, paints, wood, protein, nucleic acids and polysaccharides.
The molecular weight of molecules can be determined by experiments. There are many methods to determine molecular weight, among which mass spectrometry is the most superior, and the accuracy of modern high-resolution mass spectrometry can be higher than one ten thousandth of mass. Others, such as gas method, X-ray diffraction method and solution osmotic pressure method, can measure the molecular weight of gas molecules, crystals and polymers.
Molecular lifetime
Under the action of light, heat, electricity and other forms of energy, molecules in the ground state may change their structure and form excited (or excited) molecules. Excited molecules often exist for a short time, and some molecules have a life span of only microseconds or less, so they are also called stimulated excimer molecules. The lifetime, structure and other molecular constants of some excimer molecules were studied by flash photolysis and molecular spectroscopy.
From the research of radio astronomy and molecular spectroscopy, we know that there are many molecules among stars, such as OH, CN, SiO, CS, HCN, SO, ch, N2H, NS, HCO and so on. These molecules are extremely unstable on earth, but they can exist stably in interstellar space, because they are with extremely thin molecules in the sky and can exist for a long time without interference from other molecules.
supplement
Molecules are the smallest particles that exist independently and maintain the chemical properties of substances.
Molecules have a certain size and mass; There is a certain interval between molecules; Molecules are constantly moving; There is a certain force between molecules.
The same molecule has the same properties, but different molecules have different properties. The smallest molecule is hydrogen, and its relative molecular mass is 2. The relative molecular mass of macromolecules can be as high as several million. Molecules with relative molecular mass exceeding several thousand are called polymers.
Add one more thing.
A drop of water consists of100000000000 (210) molecules, which are invisible under the optical microscope.
The distance between molecules in solid is small, the distance between molecules in liquid is larger than that in solid, and the distance between molecules in gas is the largest.
Molecules are the smallest particles that can exist alone and maintain the chemical properties of pure substances.
A molecule is made up of multiple atoms connected by * * * electrons in valence bonds. It can be composed of atoms of the same chemical element, such as oxygen O2; It can also be a different element, such as water molecule H2O. In the abstract, a single atom can also be regarded as a molecule (monoatomic molecule), but in fact, "molecule" usually refers to a compound with multiple atoms.
The number of atoms in an element molecule is called the number of atoms in that element.
Among the gas elements, the atomic number of hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2) and chlorine (Cl2) is 2. The rare gas (such as argon ar) is 1. Among the solid elements, the atomic number of yellow phosphorus (P4) is 4 and that of sulfur (S8) is 8. Therefore, argon (Ar) is monoatomic, oxygen (O2) is diatomic and ozone (O3) is triatomic.
Substances made up of molecules are called molecular compounds.
Most molecules are too small to be seen by the naked eye, but there are exceptions, such as DNA, a polymer compound.
chemical equation
One of the characteristics of molecules is that the proportion of elements that make up compounds is always an integer. For example, the ratio of hydrogen to oxygen in pure water is always 2: 1, and the ratio of carbon, hydrogen and oxygen in ethanol is always 2:6: 1. The experimental formula of molecules can be formed by using the proportions and chemical symbols of various elements. However, it is impossible to determine the category of molecules only by the experimental formula-for example, the experimental formula of ethylene is the same as that of propylene (both are CH2), although the quantity or mass of the two molecules are different.
chemical formula
In order to reflect the true number of atoms in a molecule, we must use chemical formulas. For example, the chemical formulas of ethylene and propylene are C2H4 and C3H6, respectively.
However, the same chemical formula does not mean that two molecules are the same substance, because the arrangement and combination of atoms in the molecule, that is, the structure of the molecule, is also an element that determines the properties of the molecule. Molecules with the same atoms but different arrangements are called isomers. Isomers have the same chemical formula, but have different characteristics due to different structures. Stereoisomers are special isomers. They can have very similar physical and chemical properties, but at the same time they have very different biochemical properties.
Through the calculation of the laws of quantum mechanics, molecules have a fixed equilibrium geometric state-the length of bonds and the angle between them. Pure matter is composed of molecules with the same geometric structure. The chemical formula and structure of a molecule are two factors that determine its characteristics, especially its chemical activity.