Chemical formula:
The formula for expressing the composition and atomic number of pure matter with element symbols is called chemical formula.
Pure substances have a certain composition, which can be expressed by corresponding chemical formulas (the composition of each pure substance is fixed, so there is only one chemical formula to express the composition of each substance). Some chemical formulas can also represent the molecular composition of this substance. This chemical formula is also called molecular formula. The chemical formula only represents pure substances, and the mixture has no chemical formula.
Molecular formula: The molecular formula is a chemical composition formula that expresses the molecular composition and relative molecular mass of pure substances (simple substances and compounds) with elemental symbols. Some substances are indeed composed of molecules, in which atoms are connected by valence bonds and molecules are connected by van der Waals force or hydrogen bonds. These substances all have molecular formulas.
For example, oxygen molecules use o? Hydrogen chloride molecules are represented by HCl. The molecular formula not only represents the composition of a substance, but more importantly, it can represent a molecule of a substance and its components and composition (the number, molecular weight and weight ratio of each element atom in the molecule). So the molecular formula has a broader meaning than the simplest one.
Structural formula:
Structural formula is a chemical composition formula that uses element symbols and short lines to indicate the arrangement and combination of atoms in the molecule of a compound (or simple substance). This is a simple way to describe the molecular structure.
Type of experiment:
Experimental formula, also known as empirical formula in physics, is a formula derived from experiments or guesses, not directly from "first principles", which can predict the observation results. For example, Rydberg formula for predicting the wavelength of hydrogen atomic spectral lines.
2. The difference between the expression content:
Chemical formula:
A chemical formula can represent 1 molecule of a substance, as well as the types of elements and the number of atoms that make up the molecule. If you want to represent several molecules of a substance, you can add a coefficient before the chemical formula. Point out the number of molecules of a substance. For example, two oxygen molecules can be represented by 2O2, and ferroferric oxide can be represented by Fe3O4.
Molecular formula:
The molecular formula can indicate the name of the substance, the relative molecular mass, the number of atoms of elements contained in a molecule and the mass ratio of elements.
The molecular formula that can be used is generally molecular crystal, and molecules are formed by van der Waals force (intermolecular force) interaction.
Generally, molecular crystals have molecular formulas, while ionic crystals, atomic crystals and metals have no molecular formulas.
Structural formula:
Due to the isomerism in organic compounds, a molecular formula may represent two or more substances with different structures. In this case, knowing the molecular formula of a substance, we can often use the special properties of the substance to determine its structural formula through qualitative or quantitative experiments.
Different structural formulas and the same chemical formula are not necessarily the same substance, and their properties are often different. For example, isomers of various organic compounds have the same chemical formula, but different structural formulas and show different properties.
Not to mention different kinds of substances with the same chemical formula, such as dimethyl ether and ethanol, the molecular formula is C2H6O, but the structure is different.
Type of experiment:
Experimental formula, also known as the simplest formula or experimental formula, is a kind of chemical formula. The simplest relation of atomic number ratio of each element in a compound molecule is expressed by element symbols. Because the experimental formula is generally obtained by elemental analysis in analytical chemistry, especially by combustion method in organic chemistry, it is called experimental formula.
Many compounds, such as ionic compounds, usually do not exist in the form of molecules, but are actually expressed by experimental formulas. For example, NaCl only has the experimental formula of sodium chloride, that is to say, Na? With Cl? The ratio is 1: 1.
Different organic compounds often have the same experimental formula. For example, the experimental molecular formula of acetylene and benzene is CH, but the molecular formula of acetylene is C2H2 and the molecular formula of benzene is C6H6.
3. The difference between writing:
Chemical formula:
Writing of simple chemical formula:
Write the symbols of the elements that make up a simple substance first, and then write the number of atoms that make up a simple substance molecule with numbers in the lower right corner of the symbol. Rare gases are directly composed of atoms, and their chemical formulas are usually represented by elemental symbols.
The structure of metal and solid nonmetallic elements is very complex, and it is customary to use element symbols to represent their chemical formulas.
Writing of chemical formula of compound:
First, write down all the element symbols that make up the compound in positive and negative order, and then write down the number of atoms of the element in each compound molecule with numbers in the lower right corner of each element symbol.
A certain order usually means: for compounds composed of oxygen and another element, the oxygen symbol should be written on the right, for compounds composed of hydrogen and another element, the hydrogen symbol should be written on the left, and for compounds composed of metal elements, hydrogen elements and nonmetal elements, the nonmetal symbol should be written on the right.
The chemical formula of a compound directly composed of ions is usually expressed by the simplest integer ratio of its ions.
note:
When the atomic number ratio of a constituent element is 1, 1 is omitted, and the symbol of oxygen is generally written on the right, while the symbol of another element is written on the left, such as CO2.
For compounds composed of metallic elements and nonmetallic elements, when writing their chemical formulas, the symbols of metallic elements are generally written on the left, and the symbols of nonmetallic elements are written on the right, such as NaCl. The algebraic sum of positive and negative valence is zero.
Molecular formula:
The molecular formula is different from the simplest molecular formula. For a compound, its molecular formula is an integer multiple of the simplest formula, or its relative molecular mass is an integer multiple of the simplest formula. Only when the relative molecular mass is the same as the simplest formula, the simplest formula is the same as the molecular formula, and then the simplest formula is the molecular formula.
When the molecular formula is the same, it may not be a substance, but it may be an isomer.
For example, the molecular formula of oxygen is O? It shows that 1 oxygen molecule consists of 2 oxygen atoms, and its molecular weight is 3 1.9988.
Another example is that the molecular formula of acetic acid is C? h？ o？ The molecular weight of 1 acetic acid is 60.05, which is composed of 2 carbon atoms, 4 hydrogen atoms and 2 oxygen atoms.
The molecular formula of water molecule is H? O, which means that 1 water molecule consists of 2 hydrogen atoms and 1 oxygen atoms. (Figure: molecular formula and structural formula)
The molecular formula of hydrogen chloride molecule is HCl, which means that 1 hydrogen chloride molecule consists of 1 hydrogen atom and 1 chlorine atom.
Structural formula:
The structural formulas "-",= "and ≦" respectively represent 1, 2 and 3 pairs of * * * electrons, and "→" represents 1 pair of coordination electrons. On the left of the arrow symbol is the party providing lone pair, and on the right is the party accepting empty orbit electrons.
Type of experiment:
inorganic matter
Represents the composition of a compound that does not normally exist in the form of a single real molecule. Such as ionic compounds such as anhydrous calcium chloride, potassium sulfate, sodium chloride, sodium hydroxide, etc. Usually represented by the simplest chemical formulas CaCl2, K2SO4, NaCl and NaOH, respectively.
Substances (atomic crystals) which are composed of crystals with valence bonds between atoms are often expressed by the simplest molecular formula, such as diamond represented by C and silicon carbide represented by SiC.
The chemical formula is expressed in the form of a single molecule, and the simplest formula is used when it is difficult. For example, the chemical formula of red phosphorus is directly expressed as p.
Abbreviation of the same simple substance or compound with the same element composition ratio.
For example, white phosphorus P4 can be simply expressed as P (it can also be used to express the composition of the element mixture of white phosphorus, red phosphorus and other uncertain allotropes), and P4O 10 is abbreviated as P2O5, which is called phosphorus pentoxide. Sulfur vapor contains molecules such as S2, S4 and S8, which are collectively denoted as S.
organic compound
In organic matter, there are many kinds of carbon bonds, and different compounds can have the same simplest formula, because the simplest formula is only expressed as the simplest integer ratio of atoms in the constituent molecules.
For example, the simplest form of benzene and acetylene is CH, and the simplest form of monoolefin (general formula CnH2n) is CH2. In addition, the molecular formula of isomers is the same, so is the simplest molecular formula.
It should be noted that if and only if the simplest formula and the molecular formula are the same, the simplest formula represents the real molecule of the substance (representing the real composition and molecular weight of the molecule).
Although the simplest formula and empirical formula are generally regarded as synonyms, they are customarily expressed with different emphases. The simplest formula is the chemical formula of a substance whose elements have been determined, in which the atomic number ratio of each element is generally a coprime integer, but the experimental formula is not limited to this.