Atom

Atoms are those elements which can not be broken further. They are smallest particles that cannot be further divided. Atoms are made of protons, electrons and neutrons.

Atomic weight is weight of nucleus i.e. weight of protons and neutrons. Atomic weight of an element is that number which show how many times it is heavier than 1/16 part weight of oxygen or 1/12parts of carbon.

Atomic number is the number of protons present in the nucleus.

Symbol	Name	Atomic number
H	Hydrogen	1
O	Oxygen	8
C	Carbon	6
N	Nitrogen	7
Li	Lithium	3
He	Helium	2
B	Boron	5
F	Florine	11
Cl	Chlorine	17

Isotope

Isotopes are elements of different atomic weight but same atomic number. Isotope exists due to presence of different number of neutrons in the nucleus. Isotopes of chlorine is mixture of chlorine 35 & 37 in 3:1 ratio. Hydrogen has three isotopes. Isotope of carbon C-14 is radioactive having half-life of 5,730 years is used by archeologist in radiocarbon dating of ancient artifacts.

Element (Atomic Number)	Isotope	Proton(s)	Neutron(s)
Hydrogen(1)	Protium (1H)	1	–
	Deuterium (2H)	1	1
	Tritium (3H)	1	2
Chlorine (17)	35Cl	17	18
	37Cl	17	20
Carbon (6)	12C	6	6
	13C	6	7
	14C	6	8

Use of isotopes

1. In medicine
2. In determination of reactions path way.  Sometimes it is difficult to determine the pathway of reactions in plants or animals. Isotopes help in the pathway determination.
3. In finding age of old articles.

Isobar

Atoms of same atomic weight but different atomic number are called isobars.

Atomic model
J.J. Thompson gave plum pudding model for atom. Electrons are studded like that of plum in pudding
Neil Bohr said electrons are moving in shell. He discovered nuclear concept.

Rutherford model
Rutherford bombarded He atom with Alpha particles placing gold foil. He discovered electrons are moving in orbital as planets are moving around sun.

Atomic shell

The electrons revolve around the nucleus (protons and neutrons) in orbits or shells.  These electron shells have been named as K, L, M,… where each shell can hold 2n² number of electrons. Each shell is composed of one or more sub-shells. Each sub-shell can contain one or more orbital.

Shell number (n)	Shell name	Subshell name	Maximum number of electrons in subshell	Maximum number of electrons in shell (2n2)
1	K	1s	2	2
2	L	2s	2	8
		2p	6
3	M	3s	2	18
		3p	6
		3d	10
4	N	4s	2	32
		4p	6
		4d	10
		4f	14

The electrons are filled according to Aufbau principle (a German noun that means ‘construction’) which states that electron occupy the subshell in the order of their energy levels. Thus 1s is filled before 2s. The complete order is given below.

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, …

The outermost shell is called the valence shell and the electrons in this shell determines the behaviour of the atom in chemical reactions. The elements whose valence shell is complete (Noble gases) are non-reactive in nature. The elements having single electron in valence shell (alkali metals) and those having one less than complete (Halogens) are most reactive during chemical reactions because they can easily give or take electron(s) to complete their shell.