# 9th Class Science Atoms and Molecules Atomic Mass

Atomic Mass

Category : 9th Class

### Atomic Mass

It is the mass of an atom of an element or it is defined as the quantity of mass equal to the one twelfth of the mass of an atom of carbon 12.

Relative Atomic Mass

It is defined as the number of times an element of an atom is heavier than one twelfth of an atom of carbon 12.

Relative atomic mass $=\frac{\text{Mass of one atom of the element}}{\frac{1}{12}\times \text{Mass of one atom of C}-\text{12}}$

Gram Atomic Mass

The atomic mass of an element expressed in gram is called the gram atomic mass of that element.

Molecules

Molecules are the smallest particle of the substance, element or compound which can exist in Free State under the normal condition of temperature and pressure. The total number of atoms, of all types, present in one molecule of a substance is called its atomicity.

For example:

Atomicity of ${{O}_{2}}$ is 2 and that of $C{{O}_{2}}$ is 3. There are different types of molecules. The molecules having only one kind of atom is called monoatomic molecules, such as $He,\,Ne,\,\,Ar$ etc.

The molecules containing two atoms are called diatomic molecules, such as ${{O}_{2}},\,\,{{H}_{2}},\,\,C{{l}_{2}}$ etc.

The molecules containing three atoms are called triatomic molecules, such as $C{{O}_{2}},\,\,{{O}_{3}},\,\,{{H}_{2}}O,\,\,N{{O}_{2}}$ etc.

The molecules containing four atoms are called tetratomic molecules, such as ${{H}_{2}}{{O}_{2}},\,\,N{{H}_{3}},\,{{P}_{4}}$ etc.

The molecules containing five atoms are called pentatomic molecules, such as $NH{{O}_{3}},\,C{{H}_{4}}$ etc.

The molecules of an atom contains same kind of atoms. For example ${{O}_{2}},\,\,{{H}_{2}},\,C{{l}_{2}}$ are all molecules of the atoms.

The molecules of the compound contains atoms of different elements. For example $C{{O}_{2}},\,\,{{H}_{2}}O,$ etc.

Molecular Mass

The molecular mass of a substance is defined as the sum of atomic mass of all the atoms present in the substance.

For example:

The molecular mass of $C{{O}_{2}}=1\times 12+2\times 16=44\,\,\text{amu}$.

Gram Molecular Mass

The molecular mass of the substance expressed in gram is called the gram molecular mass.

Mole Concept

A mole is a unit for expressing the smallest unit of matter, such as proton, electron, atoms, ions, etc. The number of atoms present in one gram atom of an element is equal to the $6.023\times {{10}^{23}}$. Thus one mole of a substance is equal to $6.023\times {{10}^{23}}$ atoms. This constant is also called the Avogadro's constant or

Avogadro’s number. Hence we can define mole as the amount of substance which contains same number of particles as the number of carbons atoms present in 12g of C-12 isotopes of carbon.

Thus, 1 mole = gram atomic mass of atom $=6.023\times {{10}^{23}}$ atoms of the substance.

The best standard for atomic mass is:

(A) H – 1

(B) 0 – 16

(C) C – 12

(D) S – 32

(E) None of these

The percentage of oxygen in water is:

(A) 88.9%

(B) 11.1%

(C) 91.2%

(D) 8.8%

(E) None of these

One mole of any substance contains how many numbers of atoms?

(A) $6.00\times {{10}^{23}}$

(B) $6.023\times {{10}^{23}}$

(C) $6.023\times {{10}^{22}}$

(D) $6.023\times {{10}^{24}}$

(E) None of these

Modern atomic symbol was proposed by:

(A) Berzelius

(B) Dalton

(C) A. Lavosier

(D) Bhor

(E) None of these

Empirical Formula

The formula, which represents the simplest whole number ratio of atoms of various elements, present in one molecules of the substance.

For example:

Find the emperical formula for the compound which contains 54.35 % of carbon and 36.58% of oxygen and rest as hydrogen.

 Elements Percentage Atomic mass Atomic ratio Simplest ratio Whole No. Ratio C 54.35 12 54.35/12 = 4.53 4.53/2.28 = 1.98 2 O 36.58 16 56.58/16 = 2.28 2.28/2.28 = 1 1 H 9.07 1 9.07/1 = 9. 07 9.07/2.28 = 3.98 4

Thus, the emperical formula of the compound is ${{C}_{2}}{{H}_{4}}O$

Molecular Formula

Molecular formula of a compound represents the actual number of atoms of each element presents in a molecule of given compound.

Molecular formula $=n\times$ Emperical formula

For example:

The emperical formula of glucose is $C{{H}_{2}}O$.

The molecular mass is 180 amu

The emperical formula mass $=1\times 12+2\times 1+1\times 16=30$ amu

The value of $n=\frac{\text{Molecular mass}}{\text{Emperical formula mass}}$

$=\frac{180}{30}=6$

Therefore, $n=6$

Hence, molecular formula

= n x Emperical formula

$=6\times C{{H}_{2}}O={{C}_{6}}{{H}_{12}}{{O}_{6}}$.

Ions and Valency

An ions is formed by gaining or losing of electrons. If an element lose electrons then it forms positive ion called cations and if an elements gains electrons then it forms negative ion called anoins.

Valency is defined as the number of electron loosed or gained to attain the stable electronic configuration of an atom.

Valencies of Some of the Elements and Radicals

 Valency Name of Cations Symbols Name of anions Symbols 1                   2               3 Monovalent Hydrogen Sodium Potassium Silver Cuprous Ammonium       Divalent Cations Calcuim Barium Cupric Cadmium Magnesium Ferrous   Trivalent Cations Aluminium Ferric Chromic ${{H}^{+}}$ $N{{a}^{+}}$ ${{K}^{+}}$ $A{{g}^{+}}$ $C{{u}^{+}}$ $N{{H}_{4}}^{+}$           $C{{a}^{2+}}$ $B{{a}^{2+}}$ $C{{u}^{2+}}$ $C{{d}^{2+}}$ $M{{g}^{2+}}$ $F{{e}^{2+}}$     $A{{l}^{3+}}$ $F{{e}^{3+}}$ $C{{r}^{3+}}$ Monovalent Chloride Bromide Iodide Hydride Fluoride Hydroxide Nitrate Nitrite Acetate Diavelent Anions Oxide Sulphide Carbonate Sulphate Sulphite     Trivalent anions Phosphate B orate Nitride $C{{l}^{-}}$ $B{{r}^{-}}$ ${{I}^{-}}$ ${{H}^{-}}$ ${{F}^{-}}$ $O{{H}^{-}}$ $N{{O}_{3}}^{-}$ $N{{O}_{2}}^{-}$ $C{{H}_{3}}CO{{O}^{-}}$   ${{O}^{2-}}$ ${{S}^{2-}}$ $C{{O}_{3}}^{2-}$ $SO_{4}^{2-}$ $SO_{3}^{2-}$       $P{{O}_{4}}^{3-}$ $B{{O}_{3}}^{3-}$ ${{N}^{3-}}$

The chemical formula for ferric carbonate is:

(a) $FeC{{O}_{3}}$

(b) $F{{e}_{2}}C{{O}_{3}}$

(c) $F{{e}_{2}}\,{{(C{{O}_{3}})}_{2}}$

(d) $F{{e}_{2}}{{(C{{O}_{3}})}_{3}}$

(e) None of these

Write the name of the compound given by the formula ${{(N{{H}_{4}})}_{2}}S{{O}_{4}}$.

(a) Ammonium sulphate

(b) Ammonium sulphite

(c) Ammonium sulphide

(c) Ammonium sulphurous

(e) None of these

The valency of phosphate is:

(a) 1

(b) 2

(c) 3

(d) 4

(e) None of these

Find the number of moles in 128 g of oxygen molecules.

(a) 4

(b) 8

(c) 12

(d) 16

(e) None of these

• The smallest particles of matter is called atom.
• A combination of atoms is called a molecule.
• The sum of proton and neutron present in the nucleus of an atom is called atomic mass of an atom.
• 1 mole of an substance is equal to $\mathbf{6}\mathbf{.023\times 1}{{\mathbf{0}}^{\mathbf{23}}}$ atoms..

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