An introduction to chemical formulas and periodic tables:

FORMULAS

A formula is a symbol or a group of symbols that represents a substance. The symbols in a chemical formula identify the elements present in the substance. Molecular Formulas of Compounds

EXAMPLES

Thus NaCl is the formula for sodium chloride (common table salt) and so identifies the elements sodium and chlorine as the constituents of salt.

Salt

H20, tells us that each molecule contains two atoms of hydrogen and one atom of oxygen, or that there are two atoms of Hydrogen and one atom of Oxygen. In other words that there are two atoms of hydrogen per one atom of oxygen (2 atoms H / 1 atom O).

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The formula for aluminum sulfate, Al2(SO4)3, specifies two atoms of Aluminum, Al, for each three sulfate, SO4, groups. Each sulfate group contains one atom of sulfur and four atoms of oxygen. Hence, the formula shows a total of two atoms of aluminum, three atoms of sulfur, and twelve atoms of oxygen.

MOLECULAR FORMULAS

A molecular formula gives the actual numbers of atoms of each element in a molecule. This is called an Empirical Formula. For example, water where the formula shows that there are 2 atoms of hydrogen and one atom of oxygen.

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Since the simplest ratio of hydrogen atoms to oxygen atoms in water is 2 to 1, the empirical formula is also H2O. The molecular formula of benzene, C6H6, identifies the benzene is one carbon atom per one hydrogen atom.

Thus the empirical formula of benzene is CH. Molecular Formulas of Compounds

Many compounds do not contain discrete molecules but instead are composed of particles called ions. Ions are atoms or groups of atoms that are electrically charged. They usually arise in the combination of a metal and a nonmetal.

Thus the composition of compounds such as NaCl, Al2(SO4)3, KOH, and CuSO4 cannot by identified in terms of the nubmer of atoms of each type in a molecule.

MOLECULAR FORMULAS

But only in terms of a formula unit, as such compounds do not contain physically distinct and electrically neutral molecules.

The molecular formula of the most common form of sulfur is S8, showing that each molecule of this element consists of eight atoms.

The molecular formulas for elemental hyrogen and oxygen are H2 and O2, while the formula for neon is Ne.

Note that H2 and 2H do not mean the same thing. Molecular Formulas of Compounds

H2 represents a molecule of hydrogen consisting of two atoms of the element, chemically combined. The expression 2H, on the other hand, is that the two hydrogen atoms are not in combination as a unit but are separate particles.

A formula is a single symbol or a group of symbols that represents a substance. The symbols in a chemical formula identify the elements present in the substance.

Thus NaCl is the formula for sodium chloride (common table salt) and so identifies the elements sodium and chlorine as the constituents of salt.

SUBSCRIPTS

Subscripts are used in formulas to indicate the relative numbers of atoms of each type in the compond. but only if more than one atom of a given element is there.

subscripts

The formula for water, H20, tells us that each molecule contains two atoms of hyrdogen and one atom of oxygen. or that there are two atoms of Hydrogen and one atom of Oxygen.

In other words that there are two atoms of hydrogen per one atom of oxygen (2 atoms H / 1 atom O)

The formula of sodium chloride, NaCl, indicates the presence of equal numbers of atoms of the elements sodium and chlorine (1 atom Na / 1 atom Cl).

Note that in the formulas for water and sodium chloride the subscript 1 is omitted, which is the usual practice.

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The formula for aluminum sulfate, Al2(SO4)3, specifies two atoms of Aluminum, Al, for each three sulfate, SO4, groups.

Each sulfate group contains one atom of sulfur and four atoms of oxygen. Hence, the formula shows a total of two atoms of aluminum, three atoms of sulfur, and twelve atoms of oxygen.