- Chemistry: Branch of Science which deals with the study of matter, its composition and properties
- Matter: anything that occupies space and has mass e.g. stone, book, water
Classification of Matter:
1. Pure matter
a.Elements
b.Compounds
2. Impure matter (Mixture)
a. Homogeneous mixture
b. Heterogeneous mixture
- Pure matter: composed of its unit particles of similar kinds e.g. a 24 carat gold ring consists of the smallest particles of gold, similar in nature and properties
- Impure matter: composed of unit particles of different nature i.e. mixture
Mixture:
- substance composed of two or more elements or compounds in any proportion by weight
- Each of the substance retains its identity and specific properties e.g. air, salt solution
- Components can be separated by easy mechanical means e.g. evaporation, distillation, condensation.
Elements: the simplest pure form of a substance made of similar type of atoms which cannot be spilt up into other simpler substances by any chemical method. e.g. Helium, Calcium, Gold etc.
- Characteristics:
- Made of similar type of atoms
- Out of 118 elements, 92 are natural and remaining 26 have been prepared by nuclear reactions
- The basic unit and the building block of all complex substances
Atom (Greek: Indivisible): the smallest particle of an element which takes part chemical reaction without its division
- Characteristics:
- The smallest unit of an element
- Can neither be created nor be destroyed
- Takes part in chemical reaction without division
- Made up of electron, proton and neutron except hydrogen (neutron absent)
- Most do not exist independently except the atoms of inert gases e.g. He, Ne, Ar etc.
Structure of Atom: (Neil Bohr: 1913)
- Diameter of sphere: 10-10m
- Smaller particles of an atom are called subatomic/elementary/fundamental particles.
| S.N | Fundamental Particle | Symbol | Location | Mass (a.m.u) | Charge(e.s.u) |
| 1 | Electron | e_ | shell | 1/1837 | -1 |
| 2 | Proton | p+ | nucleus | 1 | +1 |
| 3 | Neutron | n0 | nucleus | 1 | 0 |
- 1 a.m.u.(atomic mass unit)= 1.66×10-24g
- 1 coulomb= 6.25X1018s.u= charge of 6.25X1018 electrons
- An atom is electrically neutral as the number of positively charged protons = number of negatively charged electrons
- Nucleus is positively charged as it has only positively charged protons and neutral neutrons
Compounds: Substance formed by chemical combination of atoms of two or more elements in a definite( fixed) proportion by weight e.g.H2O is a compound having hydrogen: oxygen as 1:8 (by weight)
- Formation of a compound is a chemical change. So the properties of a compound differ from its constituent elements. e.g. NaCl, CH4
Molecules: The smallest particle of an element or compound capable of independent existence.
- Molecule of element (Homonuclear molecule): made up of one or more atoms of same type. e.g. H2, N2, O3, Ar etc.
- Molecule of compound (Heteronuclear molecule): made up of dissimilar atoms e.g. NH3, H2O, HNO3
Symbol: Abbreviation of full name of an element which is represented by just the first letter or the first and any other letter of the name of the element
- If it has 2 letters, the first letter is written in capital and the second letter in the small
| Element | Symbol | Element | Symbol |
| Oxygen | O | Helium | He |
| Phosphorus | P | Zinc | Zn |
- Symbols of certain elements are used from the Latin or German name
| English Name | Latin Name | Symbols |
| Antimony | Stibnum | Sb |
| Copper | Cuprum | Cu |
| Gold | Aurum | Au |
| Iron | Ferrum | Fe |
| Lead | Plumbum | Pb |
| Mercury | Hydrargyrum | Hg |
| Potassium | Kalium | K |
| Silver | Argentum | Ag |
| Sodium | Natrium | Na |
| Tin | Stannum | Sn |
| Tungsten | Wolfan (German Name) | W |
Atomic Number (Z): total number of protons present in the nucleus of an atom or the number of electrons present in a neutral atom.
- Z= p =e (in a neutral atom)
Atomic Mass (A) : sum of number of protons and the number of neutrons present in the nucleus of an atom
- A= p+n
- Mass of electron is negligible as compared to the mass of proton and neutron. So, mass of electron is not considered to calculate A
Electronic Configuration: The arrangement of electrons in the various shells of an atom of the element
- The number of shells in an atom ranges from 1 to 7 i.e. K,L,M,N,O & P
- K-Shell: nearest to the nucleus & has the minimum energy level
- Q-Shell: farthest from the nucleus & has the maximum energy level
Bohr-Bury Scheme i.e. 2n2 rule: It is used to determine the maximum number of electrons in shell upto the fourth shell (N-shell) only.
| SHELL | n | Maximum number of electrons i.e. 2n2 |
| K-shell | 1 | 2X12= 2 |
| L-shell | 2 | 2X22= 8 |
| M-shell | 3 | 2X32= 18 |
| N-shell | 4 | 2X42= 32 |
- This rule is not applicable for 5th, 6th & 7th shells where the maximum number of electrons is 32, 18 & 8 respectively.
- The maximum number of electrons is not more than 8 & 18 in the outermost shell and second last shell respectively.
- It is not necessary to fill the electrons according to the 2n2 A new shell can be started when there are 8 electrons in the second last shell.
Subshell or orbital wise electronic configuration:
- 2n2 rule cannot explain electronic configuration of all elements.
- Subshell: the area/region in a shell where there is maximum probability of finding electrons
- The subshell concept explains the electronic configuration of all the elements
- Number of subshell in any shell (K-N shell) is equal to the number of main shell i.e.
K-shell (1st shell) =>‘s’ orbital
L-shell (2nd shell) =>‘s’ & ‘p’ orbitals
M-shell (3rd shell) =>‘s’, ‘p’ &‘d’ orbitals
N-shell (4th shell) =>‘s’, ‘p’,‘d’ & ‘f’ orbitals
| Subshell | Maximum number of electrons |
| s (sharp) | 2 |
| p (principal) | 6 (i.e. 2+4) |
| d (diffused) | 10 (i.e. 6+4) |
| f (fundamental) | 14 (i.e. 10+4) |
| Shell |
K |
L |
M |
N |
||||||
| Sub-shell | s | s | p | s | p | d | s | p | d | f |
| No. of electrons | 2 | 2 | 6 | 2 | 6 | 10 | 2 | 6 | 10 | 14 |
Aufbau Principle [Aufbau(German= Build up)]
It states, “Electrons are filled in various subshells in order of their increasing energy i.e. from lower energy level to the higher energy level.” i.e. 1s, 2s2p, 3s3p, 4s, 3d, 4p….
- Valence Shell: The outermost shell of an atom from where loss or gain of electrons takes place
- Valence Electrons: The total number of electrons which are present in the valence shell
e.g. In Na: 2,8,1 So V.E of Na=1 . Likewise 1s2, 2s22p6, 3s1 (VALENCE SHELL)
(CORE)
Author: Sulaksha Purna Shrestha