Chapter 3
atoms and molecules
️ Laws of Chemical Combination :-
The chemical reaction between any two or more substances is based on certain principles. These principles are called the laws of chemical combination.
️ Laws of Chemical Combination :-
- law of conservation of mass
- law of constant proportions
Law of Conservation of Mass :-
According to this law, "the rise or destruction of mass is not possible". During any chemical reaction, the sum of the masses of the substances will be equal to the sum of the masses of the products of that reaction.
Law of constant proportion :-
According to this law, a pure chemical compound will always be formed from the same elements from which it is formed, and the ratio of the masses of these elements will always be the same, no matter where the compound is obtained from or by whatever method it is made. have gone
Dalton's Atomic Theory :-
Dalton's atomic theory, based on the law of chemical combination, proves the 'law of conservation of mass' and 'law of constant proportion'.
Important parts of Dalton's atomic theory :-
- All matter is made up of atoms.
- Atoms are the smallest indivisible particles that are neither created nor destroyed in a chemical reaction. (This passage proves the law of conservation of mass)
- All atoms of a given element have the same mass and chemical properties.
- Atoms of different elements have different mass and chemical properties.
- Atoms of different elements combine with each other in the ratio of smaller whole numbers to form compounds. (This fraction proves the law of constant proportion)
- The relative number of atoms in any compound is fixed in a way.
️ Atomic :-
According to modern atomic theory, atom is the smallest part of any element that is used in a chemical reaction without changing its chemical and physical properties.
Atoms are the smallest parts of an element which cannot be seen even with a powerful microscope.
️ Measurement of atomic radius :-
Atomic radius is measured in nanometers.
- 10-⁹ m = 1nm
- 1m = 10⁹ nm
️ Atomic Mass :-
The mass of an atom of any element is called its atomic mass.
In the year 1961, IUPAC adopted the "unit of atomic mass" or "u" as a measure of the mass of atoms.
️ Unit of atomic mass :-
The mass of a unit of atomic mass is equal to the mass of 1/12th of that of a C¹² isotope.
How do atoms exist :-
Atoms of most of the elements are never found in free state because they are highly reactive. Only the atoms of inert gases are found in free state.
️ Molecule :-
A molecule is formed due to the formation of chemical bonds between two or more atoms.
Molecule is the smallest unit of any substance (except elements). Which can live freely and it can show all the properties of that substance like, H₂0 molecule shows all the properties of water.
The formation of any molecule can be due to the presence of chemical bonds between atoms of the same type or different types of atoms.
️ Atomicity :-
The number of atoms present in a molecule is called atomicity.
️ Chemical Formula :-
The chemical formula of a compound is the symbolic representation of its constituents.
Features of chemical formula :-
The valencies or charges of the components of a chemical formula must be equal.
In the structure of the chemical formula of a compound of a metal and a non-metal, the metal is written first and the non-metal after it.
In case of a polyatomic ion in the chemical formula, this ion is placed in a bracket. Then the connector or charge is placed under the bracket.
️ Rules for writing chemical formula :-
- First of all the symbols of the atoms of the elements are written.
- Now under these symbols their valencies are written.
- Now let us cross the valencies of the combined atoms.
- As a result the first atom acquires the valency of the second atom and the second atom acquires the valency of the first atom.
- The chemical formula is formed by crossing the valencies.
️ Molecular mass :-
The sum of the masses of the atoms present in any one molecule is called molecular mass. Like the atomic mass, its unit is also the mass unit of the atom.
️ Formula unit mass :-
The formula unit mass of a substance is the sum of the atomic masses of all its constituent atoms.
Difference between formula mass and molecular mass :-
The only difference between formula mass and molecular mass is that here we use the formula unit mass of the substance whose constituent ions are.
️ Ion :-
An ion is an atom or group of atoms on which some charge (positive ion or negative) must be present.
- Positively charged ions – Na+, K+ , Ca2+, Al3+
- Negative ions– CI− , S2− , OH− , SO2−
❇️ Value- Concept :-
The number of quantities in one mole of a species (atom, molecule, ion or particle) is equal to its atomic or molecular mass in grams. There is a fixed number of particles in one mole of a substance whose value is 6.022×10²³.
️ Molar mass :-
Molar mass is the sum of the masses of one mole of the particles of any substance.
