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# Atoms

Law of constant composition: This states that "A chemical compound (if pure) always contains the same elements combined together in the same proportion by mass regardless of bow the compound is prepared or from where it is obtained".

Law of conservation of mass: This is defined as "In every chemical reaction, the total mass of the products is the same as the total mass of the reactant,".

Law of multiple proportions: This states "as when two elements combine to form two or more different compounds the masses of one element which combine with a fixed mass of the other, bears a simple ratio to one another".

Law of equivalent or reciprocal proportions: This law states "If the two different elements A and B combine with a fixed mass of a third element C. the ratio in which A and B combine with a fixed mass of C would be the same or some simple multiple of the ratio in which they combine with each other".

Gay Lussac's law or gaseous volumes: It states as "In any chemical reaction the volume of all gaseous products and reactants, measured at the same conditions may be expressed in ratios of small whole numbers".

Postulates of Dalton's Atomic Theory:
1 -  All matter is made up of extremely tiny particles called atoms.
2 -  Atoms arc indivisible.
3 - Atoms of the same element are alike.
4 - Atoms of different elements are different.
5 -  Atoms combine in a simple but fixed ratio to form molecules.

Avogadro's law: Equal volumes of all gases contain equal number of molecules under similar conditions of temperature and pressure.

Avogadro's number: It is the number of molecules present in 22.4 liters of a gas at STP. It's value is 6.023 x 10²³. It is denoted by N.

Mole: is defined as the amount of matter which contains 6.023 x 10²³ particles of that matter (particles may be atoms, molecules, ions or electrons) as there are atoms in 12 g of ¹²C.

Atomic number: Atomic number of an element is equal to the number of protons present in the nucleus of an element.
Atomic number =number of protons in one atom of element.

Since the atom as a whole is neutral therefore atomic number is the same as the number of electrons around the nucleus of an atom. Atomic number of an element does not change during chemical reaction, it remains the same.

Mass number: The total number of protons and neutrons present in one atom of an element is known as its mass number,
Mass number = number of protons + number of neutrons.

Isotopes: Isotopes are atoms of the same clement having the same atomic number but different atomic masses. Note that the difference in the masses of isotopes is due to the different number of neutrons in their nuclei. Isotopes have the same chemical properties because they have the same number of electrons. Isotopes have different physical properties because they have different number of neutrons.

Isobars: Nuclei of atoms of different elements which have the same mass number but different atomic number are called isobars. For example 40Ca and 40Ar are isobars. They have entirely different chemical and physical properties. Similarly, 59Co and 59Ni are also isobars.