Laws of Chemical Combination: Antoine Laurent Lavoisier, is known as ‘Father of Modern Chemistry.

Lavoisier put forward the law of conservation of mass, which laid the foundation of chemical sciences. 

Law of Conservation of Mass: Law of Conservation of Mass states that, “mass is neither created nor destroyed in a chemical reaction. 

In other words, the mass of the reactants must be equal to the mass of products.

Law of Constant Proportions or Definite Composition: Law of Constant Proportions or Definite Composition states that, in a pure chemical substance, the elements are always present in definite proportions by mass.

Dalton’s Atomic Theory: 

(i) Every element is composed of extremely small particles called atoms.

(ii) Atoms of a given element are identical, both in mass and properties.

(iiii) Different chemical elements have different kinds of atoms; in particular, their atoms have different masses.

(iv) The atoms neither be created nor be destroyed or transformed into atoms of other elements.

(v) Compounds are formed when atoms of different elements combine with each other in small whole number ratios.

(vi) The relative number and kinds of atoms in a given compound are constant. 

Drawbacks of Dalton’s Atomic Theory:

(i) According to modern theory, an atom is not the ultimate indivisible particle of matter. Today, we know that atoms are divisible, they are themselves made-up of particles (protons, electrons, neutrons,etc.).

(i) In the case of isotopes of an element, the assumption that the atoms of the same element have the same mass does not hold good.

Atom: It is the smallest particle of an element that maintains its chemical identity throughout all chemical and physical changes.

The smallest unit of a substance which can exist independently is called a molecule. 

Atomicity: It is defined as the number of atoms present in a molecule of an element or a compound.

Mono atomic: Molecule having only one atom is called mono atomic,

e.g., He, Ne, Ar.

Diatomic: Molecules made-up of two atoms are called diatomic, e.g., H₂, Cl₂, O₂, N₂. 

Triatomic: Molecules made-up of three atoms, called triatomic.

e.g., O₃, H_₂O, NO₂.

Tetraatomic : Molecules made-up of four atoms, called tetra atomic.

e.g., P₄, NH₃, SO₃

Polyatomic: Molecules made-up of five or more atoms, called polyatomic/

e.g., CH₄.

Polyatomic: Any molecule which is made-up of more than four atoms is called polyatomic,

 e.g., Sg.

Relative Atomic Mass: It is defined as the number of times one atom of an element is heavier than

(1/12)th of the mass of an atom of Carbon – 12.

Relative Atomic Mass (RAM) = Mass of an atom of an element/

¹/12 th mass of C-12

Molecular Mass: The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule of a substance, 

e.g., molecular mass of water is 18 u.

The mole (or mol) is the SI unit of the amount of a substance. One mole is equal to the amount of substance that contains as many elementary units as there are atoms in 12 g of the carbon-12 isotope. 

The elementary units may be atoms, molecules, ions, radicals, electrons, etc., and must be specified. 

This number is called Avogadro’s number (No) or Avogadro’s constant 

[NA = 6.0221367 x 10²³]. Generally, 

Avogadro’s Number is rounded to 6.022 x 10²³.

For better understanding we can compare avogadro number with a dozen as:

One dozen oranges contain 12 oranges, similarly, 1 mole of hydrogen atoms contain 6.022 x 10²³ H atoms.

H₂O = 2 x H + 1 × O

= 2 x 1+1 x 16 = 2+16

= 18 amu or u.

By : 1 mole of a compound has a mass equal to its relative molecular mass expressed in grams.

1 mole = 6.022 × 10²³ number

= Relative mass in grams.

A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance.

A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.

Clusters of atoms that act as an ion are called polyatomic ions. They carry a fixed charge on them.

The chemical formula of a molecular compound is determined by the valency of each element.

In ionic compounds, the charge on each ion is used to determine the chemical formula of the compound.

Scientists use the relative atomic mass scale to compare the masses of different atoms of elements. Atoms of carbon-12 isotopes are assigned a relative atomic mass of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12 atom.

The Avogadro constant 6.022 × 10²³ is defined as the number of atoms in exactly 12 g of carbon-12.

The mole is the amount of substance that contains the same number of particles (atoms/ions/ molecules/formula units, etc.) as there are atoms in exactly 12g of carbon-12. Mass of 1 mole of a substance is called its molar mass.

The relative atomic mass of the atom of an element is the average mass of the atom as compared to 1/12th mass of one carbon-12 atom.

Hint: We know that chemical formulas can also be written using a criss-cross method. In the criss-cross method, the numerical value of the ion charge of the two atoms is crossed over, which becomes the subscript of the other ion. Using this technique, we will write the chemical formula of the given compounds.

Complete step by step answer:

Let’s us discuss about the given compound as,

A.Magnesium chloride

We have to remember that the atomic number of Magnesium is 12 and has a valency of 2. 

It means it has two electrons in the outermost shell for bonding. 

The atomic number of chlorine is 17 and has 7 electrons in the outermost shell. 

It means it just needs one more atom for bonding. 

Hence, we will use atoms of chlorine to bond with one atom of magnesium.

We can apply the criss-cross method for this compound as,

Therefore, the chemical formula of magnesium chloride is MgCl₂

B.Calcium oxide

We have to know that the atomic number of calcium is 20 and has a valency of 2, it means it has 2 two atoms in the outermost shell for bonding. 

The atomic number of Oxygen is 8

8 and has a valency of 2, it has 6 atoms in the outermost shell, it needs 2 more to complete the octet. 

Hence, we need one calcium atom to bond with one oxygen atom.

We can apply the criss-cross method for this compound as,

Therefore, the chemical formula of magnesium chloride is CaO

C. Copper nitrate

We have to know that the atomic number of copper is 29 and has two atoms in the outermost shell for bonding. While a nitrate molecule has only one valence electron. 

We need 2 nitrate molecules to satisfy the valency of 1 copper atom.

We can apply the criss-cross method for this compound as,

Therefore, the chemical formula of magnesium chloride is 

Cu(NO₃)₂

D.Aluminium chloride

We have to know that the atomic number of aluminium is 13 and has a valency of 3 atoms and chlorine atom has a valency of 1. Since it has 7 electrons in the outermost shell. 

Thus, we need 3 chlorine atoms to satisfy the valency of 1 aluminium atom.

We can apply the criss-cross method for this compound as,

Therefore, the chemical formula of magnesium chloride is AlCl_3.

E.Potassium nitrate

We have to remember that the atomic number of potassium is 19 and has a valency of 1 and nitrate also has a valency of 1, since it needs one more atom to complete its octet. Hence, we need only one molecule of nitrate for one atom of potassium.

We can apply the criss-cross method for this compound as,

Therefore, the chemical formula of magnesium chloride is KNO₃.

Note: As we know that the criss-cross method is the most efficient way to write the correct chemical formula of the molecule. It is generally used for finding out the formula of a bonding of a metal with a non-metal to form ionic bonds. Signs of the two ions are dropped, the ion value is crossed which becomes the subscript of the crossed atoms.