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Molecules of Elements and Compounds

  •  How do atoms exist?
    Atoms of elements exist in nature either in uncombined form or in combined form.
    (a) The atoms of some elements called the noble-gas elements (helium, neon, argon, krypton, xenon) are normally found in nature as isolated (single) atoms as they are chemically unreactive.
    (b) The atoms of most elements are chemically very reactive and do not exist in the free state or as single atom.
    Thus, atoms usually exist in two forms –
    (i) molecules
    (ii) ions.

Molecules are small particles that make up all living and non-living things. They are made up of even tinier particles called atoms. Molecules are made from as few as two atoms to hundreds of millions of atoms.
“A molecule is an electrically neutral group of two or more atoms which are chemically bound together by attractive forces. It can also be defined as the smallest particle of a substance (element or a compound) that can exist in a free state and show all the properties of that substance”.

Comparison between atoms and molecules

  1.  Atoms are singular whereas molecules are 2 or more combined atoms.
  2. Atoms are not bonded whereas molecules are atoms that are bonded.
  3. Atom is some thing which can’t be split whereas molecule can be divided into many atoms.
  4. Atom consists of protons, neutrons and electrons whereas molecule is combination of mixture of atoms like H2O, H2, O2, NaCl, etc.

 Molecules can be formed either by the combination of atoms of the ‘same element’ or of ‘different elements’. On the basis of this, molecules can be of two types –
(a) molecules of elements, and (b) molecules of compounds.

(1) Molecules of elements
(i) An element is the simplest form of matter which is made up of one kind of atoms.
(ii) Different elements are made up of different kinds of atoms.
(iii) An element may exist as an individual atom (e.g. argon, helium etc.) or as a molecule made up of the same two or more atoms which are chemically combined (e.g. most of the non-metals).
(iv) The number of atoms present in one molecule of an element is called its atomicity. Elements may be monoatomic, diatomic or polyatomic.

(a) The monoatomic element / molecule contains only one atom and is able to exist nonbonded to other neighboring species.
He, Ne, Ar, Kr, Xe,  – atomicity – 1 (monoatomic)

(b) The diatomic molecule is a molecule that is composed of two atoms and is able to exist nonbonded to other neighboring species.

H2, N2, O2, Cl2 etc. – atomicity – 2 (diatomic)

(c) The polyatomic molecule have more than two atoms in their molecules.
O3– atomicity – 3  ( tri-atomic)
P4 – atomicity – 4 ( tetra-atomic)
S8 – atomicity- 8  ( poly-atomic)

(2) Molecules of compounds
(i) The molecule of a compound is a pure substance consisting of two or more different atoms / elements that can be separated into simpler substances by suitable chemical methods.
(ii) The molecule of a compound has a unique and defined chemical structure; It consists of a fixed ratio (by weight) of atoms that are held together in a defined spatial arrangement by chemical bonds.
(iii) The properties of a molecule of a compound is entirely different from the elements / atom from which it is made.

Example –     
Compound              –     Combining   –        Ratio by     – Ratio by
                                                elements                mass            number
Water                           –    Hydrogen, Oxygen     –    2:16     –      2:1
(H2O)                                                                         or 1:8
Carbon mono-oxide  –    Carbon, Oxygen         –    12:16    –      1:1
(CO)                                                                            or 3:4
Carbon dioxide           –    Carbon, Oxygen        –     12:32   –      1:2
(CO2)                                                                           or 3:8
Ammonia                    –     Nitrogen, Hydrogen  –     14:3    –      1:3
Hydrogen chloride     –    Hydrogen, Chlorine  –    1:35.5  –      1:1

  • Compounds that are composed of molecules are called molecular compounds and contain more than one type of atom. Example -water (H2O), carbon mono-oxide (CO), carbon dioxide (CO2), ammonia (NH3), hydrogen chloride (HCl) etc.

 Chemical formula

It is a symbolic representation of a molecule’s composition. It indicates the actual numbers and types of atoms in a molecule. A chemical formula is also called molecular formula. The symbols in the molecular formula tell us about the elements and the subscript tells us how many atoms of that element are present in one molecule.
For example,
a molecule of water (H2O), consists of two hydrogen atoms and one oxygen atom. It is therefore represented by the chemical formula H2O.  Lack of a subscript on the O indicates one atom of O per water molecule.
Another compound composed of these same elements (in different relative proportions) is hydrogen peroxide, H2O2. It consists of two hydrogen atoms and two oxygen atoms. The properties of these two compounds are very different.

Molecular Mass
The molecular mass of a substance is the sum of the atomic masses of all the atoms present in one molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).

(a) The relative molecular mass of water (H2O) can be calculated as –
We know, atomic mass of hydrogen = 1 u
atomic mass of oxygen = 16 u

So, the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is
= atomic mass of two hydrogen atoms + atomic mass of one oxygen atom
=  2  x 1  +  1 x 16   =   18 u


(b) The molecular mass of HNO3
=  1 x the atomic mass of H + 1 x the atomic mass of N + 3 x the atomic mass of O
=  1 x 1 + 1 x 14 + 3 x 16
=  1  +  14  +  48
=  63 u

(c)  The molecular mass of H2    =  2 x the atomic mass of H
=  2 x 1  = 2 u

(d) The molecular mass of Cl2  =  2 x the atomic mass of Cl
=  2 x 35.5  = 71 u




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