ATOM & MOLECULES CLASS - 9
All
the matter is made up of small indivisible particles called
ATOM.
The
word atom is derived from the Greek word “Atomos” which means
indivisible.
Atom:
It is the smallest
particle of an element which may or may not have independent existence. The atoms
of certain elements such as hydrogen, oxygen, nitrogen, etc .do not have
independent existence whereas atoms of helium, neon, argon, etc. do have
independent existence. Thus we can say that all elements are composed of atoms.
Atoms
Combine together chemically to form larger particles which may exist
independently called MOLECULES
Molecules
further combines together to form Element or Compounds. Elements contain same
kind of atoms whereas compounds contain different kind of atoms.
How
and why elements combine and what happens when they combine. Antoine L.
Lavoisier laid the foundation of chemical sciences by establishing two
important laws of chemical combination.
(a) The Law of conservation
of mass was stated by Antoine L. Lavoisier as” Mass can neither be created nor
destroyed in a chemical reaction”
(b)
The law of constant proportions which is also known as the law
of definite proportions was stated by Proust as “In a chemical substance the
elements are always present in definite proportions by mass”.
According
to Dalton’s atomic theory, all matter, whether an element, a compound or a
mixture is composed of small particles called atoms. The postulates of this
theory may be stated as follows:
(i) All matter is made of very tiny
particles called atoms.
(ii) Atoms are indivisible
particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are
identical in mass and chemical properties.
(iv) Atoms of different elements
have different masses and chemical properties.
(v) Atoms combine in the ratio of
small whole numbers to form compounds.
(vi) The relative number and kinds
of atoms are constant in a given compound
AAtom
is considered to be a divisible particle.
=Atoms
of the same element may not be similar in all respects. eg: Isotopes (17Cl35,17Cl37
)
=Atoms
of different elements may be similar in some respects eg. Isobars (18Ar
40 , 20Ca 40 )
=Atom
is the smallest particle which takes part in chemical reactions.
=The
ratio of atoms in a molecule may be fixed and integral but may not be simple
e.g., C12H22O11 is not a simple ratio
(Sucrose)
=Atoms
of one element can be changed into atoms of other element by transmutation.
=The
mass of an atom can be converted into energy. This is in accordance with
Einstein’s
equation E = mc2
Molecule: A molecule is the smallest or the
simplest structural unit of an element (or) a compound which contains one (or)
more atoms. It retains the characteristics of an element. A molecule can exist
freely.
Atomicity: The number of atoms present in one
molecule of an element is called the atomicity of an element. Depending upon
the number of atoms in one molecule of an element, molecules are classified
into monoatomic, diatomic, triatomic or poly atomic molecules containing one,
two, three, or more than three atoms respectively.
Isotopes ⇒ These are the atoms of same element
with same atomic number (Z) but different mass number (A). Example (17Cl35,17Cl37
)
Isobars ⇒ These are the Atoms of the
different element with same mass number but different atomic number. Example (18Ar40,
20Ca 40 )
Isotones ⇒ These are the atoms of different
elements with same number of neutrons Example : (6C13, 7N14
)
WHAT IS AN ION?
An
ion is a charged particle and can be negatively or positively charged.
A
negatively charged ion is called an ‘anion’ and the positively charged ion, a
‘cation’. For example, sodium chloride (NaCl). Its constituent particles
are positively charged sodium ions (Na+) and negatively charged chloride ions
(Cl–).
Ions
may consist of a single charged atom or a group of atoms that have a net charge
on them.
A
group of atoms carrying a charge is known as a polyatomic ion e.g. Calcium oxide (Ca+2 O-2)
Ionic compound A compound which
consist of ions is called ionic compound like NaCl (Na+1 Cl-1)
While performing a reaction, to know
the number. of atoms (or) molecules involved, the concept of mole was
introduced. The quantity of a substance is expressed in terms of mole.
Definition of mole : Mole is defined as the amount of
substance that contains as many specifi ed elementary particles as the number
of atoms in 12g of carbon-12 isotope.
One mole is also defined as the
amount of substance which contains Avogadro number (6.023 x 1023) of
particles.
Avogadro number: Number of atoms or molecules or ions
present in one mole of a substance is called Avogadro number. Its value is
6.023 x 1023.
Therefore, one mole of any substance
= 6.023 x 1023 particles may be atoms, molecules, ions
For e g. One mole of
oxygen atoms represents 6.023 x 1023 atoms of oxygen and 5 moles of
oxygen atoms contain 5 x 6.023x1023 atoms of oxygen.
To find the number of moles, the
following formulae are useful
(a) Number of moles = Given Mass
/atomic mass = Given Mass /molecular mass
(b) Number of moles = No.of
atom/(6.023 x 1023) = No. of molecules/(6.023 x 1023)
RELATIVE ATOMIC MASS (RAM):
Relative atomic mass of an element
is the ratio of mass of one atom of element to the 1/12th
part of mass of one atom of carbon. Relative atomic mass is a pure ratio and
has no unit. If the atomic mass of an element is expressed in grams, it is
known as gram atomic mass.
e.g., Gram atomic mass of hydrogen = 1g
;
Gram atomic mass of carbon = 12g
Gram atomic mass of nitrogen = 14g
;
Gram atomic mass of oxygen = 16g
Atomic mass is expressed in atomic
mass unit (amu).
One atomic mass unit is defi ned as 1/12th part of the mass of one atom of
carbon.
Chemical Formulae: The chemical formula is a symbolic representation of a
compound of its composition.
Eg - HCl , KCl etc.
Valency : The combining power (or capacity)
of an element is known as its valency. Valency can be used to find out how the
atoms of an element will combine with the atom(s) of another element to
form a chemical compound.
RELATIVE MOLECULAR MASS (RMM) : The relative molecular mass of an
element or a compound is the ratio of mass of one molecule of the element or a
compound to the mass of 1/12 th part of mass of one atom of carbon. Relative
Molecular mass is a pure ratio and has no unit. If the molecular mass of a
given substance is expressed in gram, it is known as gram molecular mass of
that substance.
Molecular mass is the sum of the masses of all the
atoms present in one molecule of the compound or an element.
---By M.K.Bajpai (09839281700)
TEST YOUR SELF
1.
Avogadro’s number represents the number of atoms in
(a) 12g of C12
(b) 320g of sulphur
(c) 32g of
oxygen
(d) 12.7g of iodine
2. The number of moles of
carbon dioxide which contain 8 g of oxygen is
(a) 0.5 mol
(b) 0.20 mol
(c) 0.40 mol
(d) 0.25 mol
(3) The total no of ions
present in 111 g of cacl2 is
(a) One mole
(b) Two mole
(c) Three
mole
(d) Four moles
(4) Which of the following
weighs the most ?
(a) one g-atom of nitrogen
(b) One mole of water
(c) One mole of sodium
(d) One molecule of H2So4
(5) 5.0 litre of 0.4 M
H2SO4 Contains-
(a) 2.0 Mole Of H2So4
(b) 0.4 mole H2So4
(c) 5.0 mole H2So4
(d) 2.0 moles H2O
Ans: (1) a (2) d (3) c (4) c
(5) a
1. Find the ratio by mass
of the combining elements in the compound – C2H5OH.
Ans: C = 2x12=24; H = 6x1=6 ; O=
1x16=16
C : H:O =24:6:16 = 12:3:8
2. Give the formula of the
compound formed by the elements calcium and fluorine.
Ans: Ca+2 F-1
CaF2
3. What is the acid
radical present in sodium peroxide?
Ans: The acid radical present in
sodium peroxide (Na202) is peroxide
radical (02-2)
4. Carbon and silicon have
the same valency. What is the formula of sodium silicate?
Valency of silicon and carbon is
4.
The formula of sodium silicate is
Na+1 SiO3 -2 = Na2 SiO3
5. What is the ratio by
number of atoms in mercurous chloride?
Ans: Formula of mercurous chloride
is HgCI. Ratio of the atoms of Hg and CI in HgCI is 1: 1.
6. Name the element whose
Latin name is Stibium.
Ans: Antimony (Latin:
stibium); Mercury (Latin: hydragyrum); Gold (Latin: aurum); Lead (Latin:
plumbum).
7. What is the valency of
a sulphide ion?
Ans: Valency of sulphide ion
So 4 -2 is —2 eg.H2SO4
8. How many atoms of
oxygen are present in 50g of CaCO3?
Molecular mass of CaCO3
= 40 + 12 + 3 X 16=100g
Atoms of oxygen are present in 100 g of CaCO
Atoms of oxygen are present in 100 g of CaCO
= 3 x 6.022 x 1023 atoms
Atoms of oxygen are present in
50 g of CaCO =
{( 3 x 6.022 x 1023)
/100} x 50 atoms = 9.033 X 1023
9. How many molecules are
present in 1 ml of water?
Molecular mass of H2O
=18gm also, Mass of 1 mole of water= 1gm
18gm of water contain = 6.022 x 1023
molecules
1gm of water contain = (6.022 x 1023
)/18molecules = 3.34 x 1022
10. What is the unit of
measurement of atomic radius?
Ans:
picometers (pm) or Angstroms (Å)
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