A substance directly composed of (1) atoms: the inside is a huge whole connected by chemical bonds, and there is no small repeating unit (a single atom can be considered as the smallest repeating unit). Common substances directly composed of atoms are: metals, rare gases, and some solid nonmetals such as C, C and Si. Later, it is judged whether a substance is directly composed of atoms. The most intuitive method is to see whether the substance can be directly represented by a single element symbol in the periodic table. What can be directly represented by an element symbol is directly composed of atoms, otherwise, it is not. That is to say, the substance directly composed of atoms must be simple, and the chemical formula has no subscript numbers, only element symbols. (P and S are not directly composed of atoms, because they both have the smallest repeating unit. The real chemical formula of P is P4. P4 is the smallest repeating unit of P, that is, P is composed of countless P4 molecules, so P is composed of molecules. Short for junior high school, the real chemical formula of S is S2, S4, S8, etc. Only junior high school, high school and SiO2. Although it is not a simple substance and has a digital subscript, it is still directly composed of atoms, which is related to its internal structure. The structure of SIO2 _ 2 is a network structure in which SI and O atoms are alternately connected by single bonds with valence of * * *, and develop into the air without restriction. No molecules, no minimum repeating units. I won't explain it here. (2) A substance composed of molecules: there is a minimum repeating unit inside, and the minimum repeating unit is a molecule. Matter is composed of countless of these same minimum repeating units through intermolecular forces, among which the intermolecular forces are very small. That is to say, a substance composed of molecules is not a solid whole. (3) Matter composed of ions: the smallest unit without repetition is itself a whole connected by ionic bonds, but in order to study its structure, we often artificially define its smallest repeating unit, namely the unit cell. It should be noted that each smallest repeating unit is connected by ionic bonds, which are different from the valence bonds of molecular crystals. This is why it is defined as all ions. In junior high school, it is difficult to distinguish it from molecular substances. In fact, there is a very intuitive way to judge. Compounds with metal elements or ammonium ions are composed of ions, and substances without these two things are composed of molecules. (2) If you can judge quickly, you can only remember the main categories. There are no universally applicable formulas and theorems in chemistry. Because of the special structure of some substances, there are bound to be special cases. So just remember the main and common categories. (1) Common substances directly composed of atoms include metals, rare gases and some solid nonmetals such as C, C and Si. The most intuitive way to judge whether a substance is directly composed of atoms is to see whether it can be directly represented by a single element symbol in the periodic table. If it can be directly represented by an element symbol, it is directly composed of atoms. On the contrary, no, it's not. That is to say, the substance directly composed of atoms must be a simple substance, and the chemical formula has no subscript numbers, only element symbols. (Junior high school only) Special case: SiO is not a simple substance, but it is also directly composed of atoms. In addition, most water-insoluble metal oxides consist directly of atoms. (2) Common molecules: As long as the chemical formula contains no metal elements, it can be considered to be composed of molecules containing no NH4+. All gases, all organics, all acids, H2O, I2, Br2, etc. Special case: AlCl3 contains metal elements, but AlCl is composed of Al2Cl6 molecules (two AlCl molecules are polymerized into dimer Al2Cl6). (3) Composed of ions: The general chemical formula contains metal ions, and NH4+ can be considered to be composed of ions. Like all the bases, all of them. Inorganic salts. Special case: AlCl3 can be considered to be composed of molecules, not because of Al3+. Similarly, all water-soluble metal oxides are composed of ions, such as Na2O.