Atoms and Molecules
Laws of Chemical Combination
There are two main laws of Chemical Combination as established by Lavoisier and
Joseph L. Proust.
(a)Law of Conservation of Mass
(b) Law of constant or definite proportion
Law of Conservation of Mass
It states that mass cannot be created or destroyed in chemical reaction. So mass of
reactants is mass of the Product.
A + B -> C +D
Mass of Reactants=Mass of (A+B)
Mass of Products = Mass of (C+D)
Mass of Reactants = Mass of Products
Law of constant or definite proportion
It states that elements combine in their definite proportion of mass to give
compounds. or In a chemical substance the elements are always present in definite
proportions by mass
Example
Compound CO2CO2 can be obtained using various ways
C+O2−>CO2C+O2−>CO2
CaCO3−>CaO+CO2CaCO3−>CaO+CO2
The ratio of Carbon and Oxygen is always same ie. 12:32
Similarly For Water H2OH2O, the ratio of the mass of hydrogen to the mass of
oxygen is always 1:8, whatever the source of water
Dalton's atomic theory
There was explanation for the above theories.
British chemist John Dalton proposed the atomic theory which provided the
explanation for the above laws
All matter is made of very tiny particles called atoms.
Atoms are indivisible particles
Atoms can neither be created nor be destroyed
Atoms of same elements are similar.
Atoms of different elements are different.
Atoms combine in the ratio of small whole numbers to form compounds.
The relative number and kinds of atoms are constant in a given compound.
What is an Atom
Matter is made up of Atoms
Atom are the smallest particle of elements
Atoms are very small, they are smaller than anything that we can imagine or
compare with
The size of the atoms is measured by the Atomic Radius .Atomic radius is
measured in nanometres ( 1 nm=10−9 m1 nm=10−9 m)
Atomic Radius of the Hydrogen is 10−10 m10−10 m. And it is the smallest of
all
Symbols of Atoms or Elements
Dalton proposed the below symbols for the atoms
Berzilius suggested that the symbols of elements be made from one or two
letters of the name of the element.
Now-a-days, IUPAC (International Union of Pure and Applied Chemistry)
approves names of elements
Now generally symbols are the first one or two letters of the element's name
in English. The first letter of a symbol is always written as a capital letter
(upper-case) and the second letter as a small letter (lower-case).Symbols of
some elements are formed from the first letter of the name and a letter,
appearing later in the name
Example
Hydrogen - H
cobalt - Co
Chlorine - Cl
There are few elements where the symbols were taken from names of
element in Latin ,German & Greek
Example
Fe from its Latin name ferrum
Atomic Mass
Each element had a characteristic atomic mass
The mass of one atom is called as atomic mass
We define One atomic mass unit is a mass unit equal to exactly one-twelfth
(1/12th) the mass of one atom of carbon-12
relative atomic masses of all elements have been found with respect to an
atom of carbon-12.
So, Atomic mass of atom is measured in amu. amu is written as 'u' � unified
mass as per latest IUPAC recommendations
What is Molecule
A molecule is a group of two or more atoms chemically bonded together. The
atoms with in the molecules are held of force of attraction.
It is smallest particle of an element or a compound that is capable of an free
state and that has all the properties of that substance.
Molecules of Elements
-Generally atoms of most of the elements exists as molecules Like Oxygen exists
as O2O2, Hydrogen exists as H2H2. The number of atoms constituting a molecule
is known as its atomicity
Molecules of Compounds
- The molecule of compounds contains two or more different atoms chemical bonded
together.
Example
HClHCl
H20H20
Ions
Compounds composed of metals and non-metals contain charged species
The charged species are known as ions.
An ion is a charged particle and can be negatively or positively charged.
Anions are negatively charged ion while cations are the positively charged ion
Anions are formed by gain of electrons while cations are formed by loss of
electrons
example Compound Sodium chloride NaCl consists of Positively
charged Na+Na+ and negatively charged ion Cl−Cl−
An Ion can have multiple atoms which has net charge on it. These are called
polyatomic ions
Valency
The combining power (or capacity) of an element is known as its valency. For ions,
the charge indicates the valency of the ions
Chemical Formulae
The chemical formula of a compound is a symbolic representation of its composition.
The chemical formula can be written based on these information
(a) Symbols of the elements involved
(b) The valency of the elements or ions and this must be balanced in formula
(c) when a compound consists of a metal and a non-metal, the name or symbol of
the metal is written first
(d) in compounds formed with polyatomic ions, the ion is enclosed in a bracket
before writing the number to indicate the ratio. In case the number of polyatomic ion
is one, the bracket is not required.
Molecular Mass
The molecular mass of a substance is the sum of the atomic masses of all the atoms
in a molecule of the substance.
This is also expressed in terms of u
FORMULA UNIT MASS
formula unit mass is used for those substances whose constituent particles
are ions
it is sum of the atomic masses of all atoms in a formula unit of a compound
example
Mole Concept
o Wilhelm Ostwald Introduce the word "mole" in 1896 .It is
derived from Latin word moles meaning a 'heap' or 'pile'
o 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
o The number of particles (atoms, molecules or ions) present in
1 mole of any substance is fixed, with a value
of 6.022×10236.022×1023
o The above number is called Avogadro constant.
o The mass of 1 mole of a substance is equal to its relative
atomic or molecular mass in grams. This is called Molar
Mass .This is also called gram atomic mass
o So we just need to replace u in atomic mass or Molecular with
gm to get the Molar mass of the substance
5. Complete the following crossword puzzle (Fig. 3.1) by using the name of the
chemical elements. Use the data given in Table 3.2.
Across Down
The element used by Rutherford during his α– A white lustrous metal used for making
scattering experiment ornaments and which tends to get tarnished
black in the presence of moist air
An element which forms rust on exposure to moist air Both brass and bronze are alloys of the
element
A very reactive non–metal stored under water The metal which exists in the liquid state at
room temperature
Zinc metal when treated with dilute hydrochloric acid An element with symbol Pb
produces a gas of this element which when tested with
burning splinter produces a pop sound.
