Language of chemistry

  • 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
  2. Elements
  3. Compounds
  4. Impure matter (Mixture)
  5. Homogeneous mixture
  6. 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:
  1. Made of similar type of atoms
  2. Out of 118 elements, 92 are natural and remaining 26 have been prepared by nuclear reactions
  3. 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:
    1. The smallest unit of an element
    2. Can neither be created nor be destroyed
    3. Takes part in chemical reaction without division
    4. Made up of electron, proton and neutron except hydrogen (neutron absent)
    5. 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)


  • Valency: Combining capacity of an element or a radical with another element or radical to form a compound or molecule
  • Total number of electrons lost, gained, or shared by an atom during chemical combination gives its valency
  • HOW to find the valency of elements?
  1. For group I to IV, Valency = V.E’s number
  2. For group V to VIII, Valency = 8- V.E’s number
  • Valency of a radical = Number of charges present
  • For Transition elements, there are two incomplete outer shells, so the electrons of these two shells participate in bonding and thus show Variable Valencies.
  • “-ous” suffix = lower valency
  • “-ic” suffix = higher valency g. Cuprous (Cu+) = valency 1 , Cupric (Cu++)= valency 2
  • Radicals: Charged atoms or group of atoms having a common charge which act as a single unit during a chemical reaction.
  • Highly reactive & unstable chemical species
  • Don’t occur in free form & make different types of compounds
  • On the basis of Charge:
  1. Electropositive or Basic Radical: K+, Mg++, Al+++,Si++++
  2. Electronegative or Acidic Radical: F,O,NO3,CO3– –
  • On the basis of Construction:
  1. Simple Radical: consists only one atom e.g. Na+, Zn++, B+++,Cl,N
  2. Compound Radical: consists of a group of atoms of different elements e.g. NO3,CO3– –, NH4+
  • Can pass into or out of a compound as a unit without undergoing decomposition
  • Ions: The charged particle formed when an atom gains or loses electron
  • Cations: ion formed when Metal atom loses electron and acquire positive charge. e.g. Ag+, Ca++, B+++, H+(exception)
  • Anions: ion formed when Non-Metal atom gains electron and acquire negative charge. e.g. Br, O– –, N– – –
  • Inert or Noble Gases: Elements which have eight electrons in their valence shell (except He=has 2 electrons) & do not take part in chemical bonding e.g. He, Ne, Ar, Kr, Xe, Rn
  • Fall in the Group O(VIIIA or 18) and are chemically inert because of their stable electronic configuration
  • Duplet State: presence of two electrons in the K-shell of an atom with single shell e.g. He
  • Duplet Rule: the tendency of an atom with single shell to acquire two electrons in its K-shell

e.g. H(1)H(2)

Li (2,1)Li+(2) +1e

  • Octet State: presence of eight electrons in the outermost shell or valence shell of an atom e.g. Ne, Ar, Kr etc.
  • Octet rule: the tendency of an atom to acquire eight electrons in its valence shell by gaining, losing or sharing electrons between the combining atoms during the formation of molecule

e.g. Na(2,8,1)Na+(2,8) +1e


  • Cause of Chemical reaction: losing, gaining, sharing of electrons by an atom to acquire the stable electronic configuration
  • Chemical Bond: The force of attraction by which atoms or group of atoms are combined to form a stable molecule or compound is called chemical bond
  • Types:
  1. Electrovalent or Ionic Bond: the chemical bond formed by the transfer of electron(s) from the valence shell of metal to the valence shell of non-metal
  • The compound formed as a result of electrovalent bonding are called electrovalent compounds. e.g. NaCl, MgCl2,CaO etc.
  • The force of attraction between these opposite charges is called electrostatic force of attraction.
  • Characteristics of Electrovalent Compounds:
  1. They are found in solid state.
  2. They have high boiling and melting point.
  3. They conduct electricity in molten state or aqueous solution.
  4. They are soluble in water but insoluble in organic solvents.
  5. They contain metal atom(s) in their molecule.


  1. Covalent bond: the chemical bond formed by the mutual sharing of electrons between two or more non-metal atoms e.g. CO2, NH3, H2O, CH4
  • Represented by a line (-) in between the bonded atoms.
  • Characteristics of Covalent Compounds:
  1. They are found in solid, liquid and gaseous state.
  2. They have low melting and boiling points in comparison to electrovalent compounds.
  3. They do not conduct electricity.
  4. They are insoluble in water but soluble in organic solvents.
  5. They do not contain metal in them.
  • Molecular formula: indicates the symbolic representation of the molecule of an element or a compound in molecular form
  • Gives the information about the total number of atoms of different elements in a molecule.
  • g. Cl2 represents 1 molecule of chlorine while 2Cl represents 2 atoms of chlorine.
  • NH3 represents 1 molecule of ammonia & 2NH3 represents 2 molecules of ammonia.


  • Information obtained from Molecular formula:
  1. Represents the percentage composition of each element present in the given compound
  2. Represents one molecule of the substance
  3. Tells about the number of atoms of each element present in one molecule of the compound
  4. Helps to know about the molecular weight of the substance

e.g. molecular formula of ammonia is NH3, So its molecular weight=1X14+3X1 = 17 amu

  • How to write Molecular Formula?
  1. The symbol of basic(positive) and acidic(negative) radicals are written side by side
  2. The valency of each radical are written on upper right corner of each
  3. The valency of the radicals are exchanged. The H.C.F is taken to get a simple whole number.
  4. The radicals with exchanged valency are combined.
  5. If a compound radical takes part in the molecular formula, the radical is enclosed in bracket and the valency number is written at the bottom of the formula.


Abbreviation of full name of an element Symbolic representation of a molecule
Represents one atom of an element Represents one molecule of an element or compound
Written according to English or Latin name of an element

e.g. symbol of oxygen is O

Written with the help of symbols and valencies of the elements

e.g. molecular formula of oxygen is O2


The first 20 elements of the Periodic Table in Seconds (Mnemonics):

HyHeLi    BeBoCa    NOFNe

SoMAl     SiPSClAr   PoCal    (where, HyHeLi=Hydrogen, Helium, Lithium

BeBoCa=Beryllium, Boron, Carbon

NOFNe= Nitrogen, Oxygen, Fluorine, Neon

SoMAl=Sodium, Magnesium, Aluminium

SiPSClAr=Silicon, Phosphorus, Sulphur, Chlorine, Argon

PoCal= Potassium, Calcium)

NOTE: The given Mnemonics may /may NOT be the actual symbols the Elements

Compiled by: Sulaksha P. Shrestha

GEMS, Dhapakhel

Language of chemistry