Atoms of chemical elements having same atomic mass but a different atomic number are called Isobars. The sum of the number of protons and neutrons together form the atomic mass. Therefore, we can also say the number of nucleons present in the nucleus is equal to the atomic mass of an atom. It will have the same number of nucleons. The number of protons and neutrons alone will vary but the number of nucleons or the sum of protons and neutrons in isobars will always be same. Isobars always have different atomic structure because of the difference in atomic numbers. The number of neutrons makes up the difference in the number of nucleons. Therefore, they are always different chemical elements having same atomic masses. Thus, isobar has different chemical properties. For example : Iron and nickel have atomic number 26 and 28 respectively. However, the mass number is 58. Argon and calcium with atomic number 18 and 20, respectively. However, they have the same atomic mass 40.
Law of Chemical Combination
Given by Lavoisier and Joseph L. Proust as follows:
1. Law of conservation of mass
Mass can neither be created nor destroyed in a chemical reaction.
e.g.
A + B → C + D
Reactants → Products, Mass of reactants = Mass of products
2. Law of constant proportion
In a chemical substance the elements are always present in definite proportions by mass.
e.g.,
in water, the ratio of the mass of hydrogen to the mass of oxygen is always 1 : 8 respectively.
These laws lacked explanation. Hence, John Dalton gave his theory about the matter. He said that the smallest particle of matter is called ‘atom’.
Dalton’s Atomic Theory
· Every matter is made up of very small or tiny particles called atoms.· Atoms are not divisible and cannot be created or destroyed in a chemical reaction.
· All atoms of a given element are same in size, mass and chemical properties.
· Atoms of different elements are different in size, mass and chemical properties.
· Atoms combine in the ratio of a small whole number to form compounds.
· The relative number and kinds of atoms are constant in a given compound.
Atom
Atoms are the smallest particles of an element which can take part in a chemical reaction.
Size of an atom :
Atomic radius is measured in nanometres.
Molecule
It is the smallest particle of an element or a compound which can exist independently.
Molecules of an element constitute the same type of atoms.
Molecules may be mono atomic, diatomic or poly atomic.
Molecules of compounds join together in definite proportions and constitute a different type of atoms.
Atomicity
The number of atoms constituting a Molecule is known as its atomicity.
Ions
The charged particles (atoms) are called ions, they are formed by attaining positive charge or negative charge on it.
Negatively charged ion is called anion (Cl–).
Positively charged ion is called cation (Na+).
Valency
The combining capacity of an element is known as its valency. Valency is used to find out how the atom of an element will combine with the atom of another element to form a chemical compound.
(Every atom wants to become stable, to do so it may lose, gain or share electrons.)
If an atom consists of 1, 2 or 3 electrons in its valence shell then its valency is 1, 2 or 3 respectively,
If an atom consists of 5, 6 or 7 electrons in the outermost shell, then it will gain 3, 2 or 1 electron respectively and its valency will be 3, 2 or 1 respectively.
If an atom has 4 electrons in the outermost shell than it will share this electron and hence its valency will be 4.
If an atom has 8 electrons in the outermost electron and hence its valency will be 0.
Molecular Mass
It is the sum of the atomic masses of all the atoms in a molecule of the substance. It is expressed in atomic mass unit (u).
Mole Concept
Definition of mole: It is defined as one mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.
1 mole = 6.022 x 10^23 in number
Molar mass = mass of 1 mole → is always expressed in grams and is also known as gram atomic mass.
lu of hydrogen has → 1 atom of hydrogen 1g of hydrogen has → 1 mole of hydrogen
= 6.022 x 10^23 atoms of hydrogen