Topic 4 Chemical formula and valence

Topic 4

Chemical formula and valence

1. Chemical formula

We already know that h2o not only represents the substance of water, but also the composition of water. This kind of formula that uses the combination of element symbols and numbers to represent the composition of substances is called chemical formula. In addition to Jue 0 , previously learnedO2 , H2 . CO2 , HCl、Fe2O3,and HgO are all chemical formulas, which represent the composition of oxygen, hydrogen, carbon dioxide, hydrogen chloride, iron oxide, and mercury oxide, respectively.

The composition of each pure substance is fixed, so there is only one chemical formula representing the composition of each substance.

When writing the chemical formula of a compound, in addition to knowing which elements the compound contains and the number ratio of different element atoms, the following points should also be noted:

1.When the atomic number ratio of a constituent element is 1 , 1 is omitted;

2.When writing the chemical formula of an oxide, the element symbol of oxygen is generally written on the right, and the symbol of another element is written on the left, such as C0 2 ;

3.When writing the chemical formula of a compound composed of metal elements and non-metal elements, the symbol of the metal element is generally written on the left, and the symbol of the non-metal element is written on the right, such as NaCl o

The name of a compound composed of two elements is generally pronounced as a chemical, for example, NaCl is pronounced as sodium chloride. Sometimes it is also necessary to read the atomic number of various elements in the chemical formula, for example, CO? is read as carbon dioxide, and Fe 3 O 4 is read as ferric oxide.

2. Valence

The compound has a fixed composition, that is, the elements forming the compound have a fixed atomic number ratio

It can be seen from the above table that when different elements are combined with each other, the ratio of the number of atoms is not all 1 : 1, such as the ratio of the number of atoms combined by H and Cl is 1:1, and HCl is generated ; the atom combined by H and O The number ratio is 2:1, to generate H2O.how do we know the number ratio of different elements combined? In general, the rules can be known through the " valence" of elements. The valence of elements can be positive or negative, and in compounds, the algebraic sum of positive and negative valences is zero. For example, in compounds. Usually -2 valence , H is usually +1 valence, Cl is usually -1 valence, therefore, when hydrogen reacts with oxygen, 2 hydrogen atoms combine with 1 oxygen atom to generate H2O ; when hydrogen reacts with chlorine, It is a hydrogen atom combined with a chlorine atom to produce HCl . In the same way, it can be deduced that Na is +1 valence, and Fe is +3 valence.

In some substances,some charged atomic groups in them, often participate in the reaction as a whole. Such atomic groups are also called roots . The root also has a valence.

3. Calculation of Relative Molecular Mass

The sum of the relative atomic mass of each atom in the chemical formula is the relative molecular mass (symbol M) O can perform the following various calculations according to the chemical formula.

After studying this subject, you should know

1.A formula that expresses the composition of a substance using element symbols is called a chemical formula.

2.A chemical formula represents a substance, the elemental composition of the substance, and the ratio of the number of atoms of the constituent elements.

3.The basis for deriving chemical formulas of substances by applying chemical valences is:

(1) The compound does exist;

(2) The algebraic sum of the positive and negative valences of each element in the compound is 0 .

4.According to the chemical formula, the relative molecular mass of the substance, the mass ratio and mass fraction of the constituent elements of the substance can be calculated.