Atoms and Molecules

DESIGNED

By

Edustudy point
Laws of chemical combination: There are two laws of chemical combination.
1) Laws of conservation of mass: The law states that mass can neither be created nor
destroyed in a chemical reaction.
i.e. Total masses of reactants is equal to the sum of masses of products and the masses
of unreacted reactants.

2) Laws of constant proportion: The law states that in a chemical substances the
elements are always present in a definite proportion by mass. It is also termed as law of
definite proportion.

For instance, the ratio of hydrogen and oxygen in water obtained from any source is 1:8
i.e. 1g of hydrogen and 8g of oxygen together make up 9g of water.

Atoms: According to John Dalton- all elements, compounds or mixtures are composed of
some small particles which cannot be further divided into smaller particles. They are known
as atoms.

• They are very small and their radius can be measured in nanometers. i.e 1m = 109 nm.

➢ Dalton’s atomic theory: The postulates of this theory may be stated as follows:
• Matters are composed of very tiny particles called atoms.

• These particles are too small to see and cannot be created or destroyed in a chemical
reaction.

• Atoms of a specified element are identical in mass and chemical properties.

• Atoms of different elements have different masses and chemical properties.

• 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.

➢ Naming atoms: There are 118 elements available in periodic table.

• Symbols of elements are either the first letter or first two letters of the name of
the elements.

• First letter of the symbols is always capital. For instance, H for hydrogen, Al not AL
for aluminum, Co for cobalt.
 • Some of the names involves the first letter of the element name and the letter that
appears later in it. For instance, Cl for chlorine, Zn for zinc etc.

• Some of the elements have their symbols derived from their names in latin or greek.
For instance, Na (natrium) for sodium, Fe (Ferrous) for iron, K (kalium) for potassium.

➢ Atomic mass: Atomic mass refers to the mass of an atom.

• It depicts how many times an atom of an element is heavier than one-twelth (1/12th)
the mass of one atom of carbon-12 of mass of one carbon atom.

• It is measured in unit called amu (atomic mass unit).

Mathematically, mass of one atom of an element = atomic mass X (1/12th) of the mass of

one atom of carbon.

atomic mass = mass of one atom of an element / (1/12th) of the mass of one atom of

carbon.

Easy way to calculate atomic mass of oxygen atom.

The Proton number Z = 8

The Neutron number N = 8
The atomic mass of the oxygen atom is given by 15.9994 amu i.e. nearly 16.

Molecules: when two or more atoms chemically bonded together is called molecule.
Ex- water (H2O), Ammonia (NH3)
➢ Atomicity: The number of atoms present in a single molecule is termed as its atomicity.
It can be classified into different types.

1) Monoatomic: The molecules of many elements are composed of only a single atom of
that element. They are termed as monoatomic.
For example, Ar (Argon), He(Helium). So their atomicity is one.

2) Diatomic: The molecules of many elements are composed of two atoms of that
element known as diatomic. So its atomicity is 2.
For example, oxygen can never exist independently and exist as O2 molecule.

3) Triatomic: The molecules of many elements are composed of three atoms of that
element. They are termed as triatomic.
For example, O3, CO2. So their atomicity is 3.

4) Tetra atomic: The molecules of many elements are composed of four atoms of that
element. They are termed as tetra atomic.
For example, P4, SO3. So their atomicity is 4.

➢ Molecules of compounds: Atoms of different elements associate in definite

proportions to form molecules of different compounds. For Example,

• In sodium chloride NaCl is 1:1.

• In water (H2O) ratio of hydrogen and oxygen is 1:8.

➢ Ions: Metals and non-metals leads to the formation of Compounds that possess charged
species. These charged species are called ions.

• These charged species can be either positively charged called cation or negatively
charged called anion. For instance, In sodium chloride (NaCl), Na exist as cation
Na+ whereas Cl exist as anion Cl-.
➢ Valency: It is the ability of an atom to gain or lose electron in order to achieve the
noble gas configuration.

• The every element tries attain stability by acquiring noble gas configuration for which
it tries to either gain electron or donate electron.

For Example sodium has 1 electron in its outermost shell and hence valency of sodium is 1.

• Atomic number of sodium is 11. So the

electronic configuration stands out to be
2,8,1 i.e. there is one electron in the
outermost shell.
In order to gain inert gas configuration it is
better for sodium to lose one electron and
achieve the nearest noble gas configuration
of neon with atomic number 10.

• On the other hand atomic number of

chlorine is 17. So electronic configuration
stands out to be 2,8,7.
In order to achieve noble gas configuration
to become stable it requires one electron
then it will acquire the configuration of neon
(noble gas). Therefore valency of chlorine is
1.

Writing chemical formula:

• RULE I: Cross multiply the valencies of the elements to

form the formula of the respective compound.

• RULE II: If a compound consists of both metallic as

well non-metallic elements then name or symbol of the
metal is considered first.
• RULE III: In case compounds are formed from polyatomic ions then the ion is enclosed in
a bracket before writing the number to indicate the ratio. But in case the number of
polyatomic ion is one then there is no need for bracket.

Molecular mass: Molecular mass of a substance is defined as the sum of the atomic
masses of all the atoms present in a molecule of a substance. Therefore, the relative mass
of a molecule is expressed as atomic mass units (amu).

Let us calculate the molecular mass of H2O. we will find it out by following process.

• We know the atomic mass of hydrogen is 1u and that of oxygen is 16u & H2O contains
two hydrogen atoms and single oxygen atom-

2 X 1 + 1 X 16 = 18u

Formula unit mass: Formula unit mass of a substance is the sum of the atomic masses of
all the atoms present in the formula of a respective compound.

Let us calculate the formula unit mass of NaCl. we will find it out by following process

• We know atomic mass of Na is 23 and that of Cl is 35.5, So Formula unit mass of NaCl
is 1 X 23 + 1 X 35.5 = 58.5 u

KHATAM