Published by: Nuru

Published Date: 20 Nov 2021

Matter in Chemistry for Grade 8 Science



Anything that occupies space and has weight is called matter.


An element is the pure kind of matter which can neither be broken down into any thing simpler by ordinary chemical means nor built up from any simpler form of matter. 

There are about 118 elements known to us at present. An element can be defined as the pure form of substance which is made up of similar kinds of atoms. Elements are the building units of the universe. Example; Copper, Silver, Gold, Hydrogen, Oxygen, Nitrogen, Iron, etc.


Compounds are those substances which are formed by the chemical combination of two or more elements in a fixed proportion by weight. For example; water(H2O) is a compound because water is formed by the combination of two elements Hydrogen and Oxygen in a fixed ratio 1:8 by weight. The properties of a compound are different from those of elements from which it is formed.


Atom is the smallest particle of an element that can take part in chemical reaction, An atom is compound of a very tiny particles called electron, proton, and neutron. These particles are called sub-atomic particles.


  1. It is negatively charged particle.
  2. It is represented by e−.
  3. Mass of an electron = 1/1837 times the mass of a hydrogen atom.
  4. They revolve around the nucleus in their own fixed path called orbit or shell or energy level.


  1. It is posotively charged particle.
  2. Mass of a proton = mass of a hydrogen atom.
  3. They are located in the nucleus of an atom.
  4. A proton is denoted by p+.


It is an electrically neutral particle i.e. it carries no charge.
Mass of a neutron = mass of hydrogen atom.
They are located in the nucleus of an atom.
A neutron is denoted by n 0.

An atom of each element is electrically neutral because number of proton is equal to the number of electrons and the net charge is zero.

Sub-atomic particle Symbol Mass Charge Location
Proton p+ 1 amu + Nucleus or Central part of an atom
Electorn e− 1/1837 amu - Shell or Orbit or Energy level
Neutron n0 1 amu 0 Nucleus or Central part of an atom

Atomic Mass Unit (amu)

Mass of sub-atomic particles is very small which cannot be expressed in gram, kilogram, milligram, etc. The atomic mass unit is the unit of measuring extremely small masses. The mass of electron, proton, and neutron is expressed in terms of atomic mass unit (amu). The mass of a hydrogen atom is taken as equal to 1 amu. Mass of a proton is also equal to 1 amu.

1 gm = 6*10^23 amu
1 p+ = 1 n0 = 1837 e− = 1 amu

Electric charge

Electrons and protons have a special property called electric charge. Electrons carry negative charge and protons carry positive charge. However, an atom is electrically neutral because the number of protons is equal to the number of electrons and the net charge in an atom is zero as negative charge of its electrons is neutralized by the positive charge of its protons. The electric charge is measured in unit Coulombs.

One Coulomb  charge = 6*10^18 electrons

 Atomic number

The number of protons or electrons persent in an electrically neutral atom is called atomic number.
Atomic number = number of electrons = number of protons (for neutral atoms)

For example; an atom of oxygen has 8 protons and 8 electrons, so it's atomic number is also 8.

Atomic mass or Atomic Weight

The sum of the number of protons and neutrons in the nucleus of an atom is called atomic weight. (Electrons hasve negligible mass so, it is neglected.) 

Atomic mass = Number of protons + Number of neutrons
Atomic mass = Atomic number + Number of neutrons

For example; an atom of oxygen has 8 proton and 8 neutron, so it's atomic mass is 16 (8+8).


1 Hydrogen(H) 1
2 Helium(He) 4
3 Lithium(Li) 7
4 Beryllium(Be) 8
5 Boron(B) 11
6 Carbon(C) 12
7 Nitrogen(N) 14
8 Oxygen(O) 16
9 Fluorine(Fl) 19
10 Neon(Ne) 20
11 Sodium(Na) 23
12 Magnesium(Mg) 24
13 Aluminium(Al) 27
14 Silicon(Si) 28
15 Phosphorus(P) 31
16 Sulphur(S) 32
17 Chlorine(Cl) 35
18 Argon(Ar) 40
19 Potassium(K) 39
20 Calcium(Ca) 40

Electronic Configuration

Electronic Configuration is the systematic distribution of electrons in different shells around the nucleus of an atom.

The different shells around the nucleus have different electron holding capacity. The maximum number of electrons that can be kept in a shell is determined by 2n² rule, where 'n' is the number of shell.

For first shell (K shell) n=1, its is the first shell from nucleus so, maximum number of electrons in it are 2*1²= 2

For second shell(L shell), n=2, so maximum number of electrons in it are 2*2²=8.

For third shell(M shell), n=3, so maximum number of electrons in it are 2*3²=18.

For fourth shell(N shell), n=4, so maximum number of electrons in it are 2*4²=32.


The smallest partcile of matter(element or compound) which can exist freely and still possesses the properties of that kind is called molecule. Molecules of a matter are similar but molecules of different matter are different.

Molecules of a compound Molecules of an element
It is made up of two or more different types of atoms.  It is made up of similar types of atoms.
Example; H2O. Example; Br₂, O₂, I₂, F₂ N₂, H₂, Cl₂ (BROFINHCL) .


Atom Molecule
The term atom is used only for the smallest particle of an element. The term molecule is used for the small part of element or compund.
An atom may or may not exist freely.   A molecule can exist freely.
It is made up of electron, proton, and neutron. It is made up of similar or dissimilar atoms.
Atoms can combine to give rise to a molecule. A molecule may be decomposed into atoms.
Example; H is an atom of Hydrogen. Example; H2O is a molecule of hydrogen.


The short and abbrebiated name from the full name of an element is called symbol. 

1. Symbol of some elements are derived from the first letter of their names. For example; H(Hydrogen), N(Nitrogen), S(Sulphur), B(Boron), P(Phosphorous), etc.

2. Name of some elements begin with same letter, in such condition next significant letter from their name is taken with first letter to avoid confusion. For example; Cr(Chromium), Cl(Chlorine), Cobalt(Co), Ca(Calcium). First letter is written in capital and second letter is written in small.

3. Symbols of some elements are made from their names


Element Latin name Symbol
Sodium Natrium Na
Pottasium Kalium K
Iron Ferrum Fe
Copper Cuprum Cu
Silver Argentum Ag
Mercury Hydragium Hg
Lead Plumbum Pb
Tin Stannium Sn
Gold Aurum Au


A radical is an atom or a group of atoms of different elements having positive or negative change which behaves as a single unit through out the reaction. Radical acts as an atom and has its own symbol and valency. Radicals are of two types. 

a. Electropositive radical or Basic radicals

They are those radicals which carry npositivative charge. example; Mg⁺⁺, Ca⁺⁺, Al⁺⁺, NH4⁺.

b. Electronegative radical or Acidic radicals

They are those radicals which carry positive charge. example; Sulphate(SO4), Sulphite(SO3), Carbonate(CO3), Bicarbonate(HCO3), Bisulphate(HSO4), Nitrate(No3), Nitrite(NO2), Hydroxide(OH), Chlorate(ClO3), Phosphate(PO4).

Sulphate, Sulphite, and Carbonate have 2 valency, each valency of phosphate is 3 other compound radicals given above have valency each.


We know, that the number of positively charged proton and the number of negatively charged electrons are equal in an atom. Due to this, atom gives electron from other atom it becomes electrically charged atom.

When an atom ives electron present in it to other atom, it becomes positively charged, whereas when an atom takes electron form other atom, it becomes negatively charged. The positively charged ion is called cation and the negativelyc harged ion is called anion.

Duplet and Oclet Rule

The principle of attaining 8 electrons in the outermost orbit of an atom to have stable electronic configuration is called oclet. For example; Ne: 2, 8 and Ar: 2, 8, 8.

In case of element having only one orbit (K orbit), two elements in that gives stable elctronic confiuration is called duplet. For example; He: 2.

Those elements which have complete outermost shell either by oclet or duplet do not take part in chemical reaction, so they are called inactive elements or noble gases.

Valency electrons and Valency

The outermost shell of an atom is called valence shell. The electrons are called valence electrons because they determine the valency of an atom. Only valency electrons of an atom take part in chemical reaction

The combining capactity of an atom is called valency. The valency of an element is determined by the number of its valence electrons. If an atom contains 1, 2, 3 or 4 electrons in its outermost shell, then valency is equal to the valence electrons. When the number of electrons in the outermost shell of an atom is 5, 6 or 7, then the valency is equal to 8 minus valence elctrons. All the inert gases have 0 valency because their outermost shells are completely filled and they have no combining capacity.

Symbol K L M N Valency
H 1       1
He 2       0
Li 2 1     1
Be 2 2     0
B 2 3     3
C 2 4     4
N 2 5     3
O 2 6     2
F 2 7     1
Ne 2 8     0
Na 2 8 1   1
Mg 2 8 2   2
Al 2 8 3   3
Si 2 8 4   4
P 2 8 5   3
S 2 8 6   2
Ar 2 8 8   0
Cl 2 8 7   1
K 2 8 8 1 1
Ca 2 8 8 2 2

Other elements:

Fe: 2,3
Cu: 1, 2
Hg: 1, 2
Fb: 2
Ag: 1
Br: 1
I: 1

Valency of some radicals:

SO4: 2
SO3: 2
CO3: 2
HSO4: 1
HCO3: 1
ClO3: 1
OH: 1
NO3: 1
NO2: 1
NH4: 1
PO4: 3

Molecular formula:

Molecular formula of a substance is the symbolic representation of its molecule or symbolic group of atoms whcih represents a molecule of a substance.

Information from molecular formula:

  1. It gives name of a substance. For example; H2O means water.
  2. It shows names of constituent elements present in a compound. Example; H2O shows the water is made up of hydrogen and oxygen.
  3. It shows number of atoms of each elements present in a molecule of a susbstance. Example; H2O shows that a molecule of a water consists of 2 atoms of hydrogen and one atom of oxygen.
  4. It helps to calculate molecular weight of a molecule. Molecular weight is the sum of atomic weight of all the atoms present in  a molecule. Molecular weight of H2O is 2+16= 18.

Methods of writing molecular formula:

  • Write the symbols of elements or radicals present in a molecule side by side. The electropositive radical is written first and the electromagnetic radical is written to the right of it. Example; calcium oxide: Ca O
  • Write the valency of each element on the top of the symbol. Ca² O²
  • Remove the common factor of valencies if there is. In above case, common factor of valencies is 2. So, we get Ca O.
  • Exchange  or Criss cross the valencies and write them at the right bottom of the symbol or radical.  So, we get Ca O.
  • If the radical is multi atomic, the radical is enclosed in brackets, the numnber is placed outside the brackets to its lower right side. Example; Magnesium Hydrixide: Mg² OH, after criss coss, we get Mg(OH)²
  • Some elements have more than one valency. this is called variate valency. Example; Iron have 2, 3, Copper 1,2, Mercury 1, 2. In case of variate valency, and if the name of compound is ending with -ic then use bigger valency, otherwise use smaller valency. Example: Ferrous Oxide or Iron Oxide: Fe² O², after criss cross and removing common factors we get FeO. For Ferric Oxide or Iron Oxide: Fe³ O², we get Fe²O³.

Molecular formula of elements 

Molecules of most of the elements are made up of only one atom, such molecules are called monoatomic molecules. In such case atom and molecule appear similar, Example; Na, Mg, Cu, Al, Ag, Ni, Au, etc. Molecules of some elements are made up of  two similar atoms, they are called diatomic molecules. In sucha case atom and molecule appear differnet like Br2, O2, F2.

H and H2: H means one atom of hydrogen, it cannot exist freely but H2 means one molecule of hydrogen, it can exist freely in the nature.

Chemical reaction and chemical equation:

The process by which a chemical change is brought about is called chemical reaction, Chemical equation is the representation of a chemical reaction with the help of symbols and formula of the substances involved in it. The substances which take part in a chemical reaction are called reactants and the substances obtained from a chemical reaction are called products.. In a chemical equation, the reactants are written on the left hand side of the arrow head and the product are written on the right hand side. representation of a chemical reaction by writting the full names of reactants and prooducts is called word equation.

When a chemical reaction is represented in terms of symbols and molecular formula of reactants and products, it is called chemical equation.

Essentials of chemical equation:

  • It should show actual chemical change.
  • It should be balanced.
  • It should be molecular.