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Class IX, Ncert Cbse Chemistry (Science) Study Materials
ATOMS AND MOLECULES
(All presented in the Question/Answer format for your convenience)
Q.1:
What are the ‘laws of chemical combination’?
Answer: There
are two important laws of chemical combination established by Lavoisier, are as
follows:
(i) Law
of Conservation of Mass
The law of
conservation of mass states that mass can neither be created nor destroyed in a
chemical reaction.
(ii) Law
of Constant Proportions
The
Law of Constant Proportions which is also known as the Law of Definite
Proportions, was established by Proust. According to this law, in a chemical
substance the elements are always present in definite proportion by mass. All
pure samples of a compound contain the same elements combined together in the
same proportion by mass. For example, a sample of water would always contain
hydrogen and oxygen in the ratio of 1:8 by mass irrespective of the source of
water.
Q.2:
Explain the ‘Law of Conservation of Mass’.
Answer: See
answer of Q. 1 above.
Q.3:
What is the Law of Constant Proportion ?
Answer: See
answer of Q. 1 above.
Q.4: What are the postulates of Dalton’s Atomic Theory of matter
?
Answer:
According to Dalton’s atomic theory, all matter, whether an element, a compound
or a mixture is composed of small particles called atoms. The different
postulates of Dalton’s atomic theory are stated as follows:
(1)
All matter is made of very tiny particles called atoms.
(2)
Atoms are indivisible particles, which can not be created or destroyed in a
chemical reaction.
(3)
Atoms of a given element are identical in mass and chemical properties.
(4)
Atoms of different elements have different masses and chemical properties.
(5)
Atoms combine in the ratio of small whole numbers to form compounds.
(6)
The relative number and kinds of atoms are constant in a given compound.
Q.5: What is the full form of IUPAC ? What is the present
accepted norm of IUPAC for naming symbols of elements ?
Answer:
The full name of IUPAC is International Union of Pure and Applied Chemistry.
Many
of the 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 is always a small letter (lower case). For
example, Hydrogen is written as H and Aluminium is written as Al. Some symbols
are formed from the first letter of the name and a letter, appearing later in
the name. For example, Chlorine as Cl and Zinc as Zn. Some other symbols are
taken from the names of elements in Latin, German or Greek. For example, the
symbol of Iron is Fe from Latin name ferrum.
Q.6: Write the names of symbols of five elements where the
symbols are taken from their name in English.
Answer:
The symbols of the following elements have been taken from their name in
English: Calcium (Ca), Oxygen (O), Zinc (Zn), Magnesium (Mg) etc.
Q.7: Write the names of symbols of five elements where the
symbols are taken from their name in a language other than English.
Answer:
The names of symbols of the following elements have been taken from their name
in a language other than English: Silver (Ag), Gold (Au), Lead (Pb), Sodium
(Na), Iron (Fe) etc.
Q.8: Define and explain atomic mass of an element.
Answer:
The atomic mass of an element is the relative mass of its atom as compared with
the mass of a particular atom of Carbon-12 (12C) isotope taken as 12
units. Thus the atomic mass of an element indicates the number of times one
atom of an element is heavier than 1/12 th of a Carbon-12 (12C)
isotope atom. For example, the atomic mass of oxygen is 16 which indicates that
an atom of oxygen is 16 times heavier than 1/12 th of a Carbon-12 (12C)
isotope atom.
Q.9: Distinguish between a ‘molecule’ and a ‘compound’.
Answer:
Molecule - a
molecule is usually formed when at least two atoms of the same or different
kinds combine. If two or three atoms of the same kind of element combine, then
it is referred to as a molecule of an element or simply Molecule. For example,
molecule of oxygen(O2) is formed by the combination of two oxygen
atoms. However, there are exceptions such as argon (Ar), helium (He) which are
made up of only one atom of that element.
Compound - Molecule
formed by union of two or more atoms of different atoms is called a molecule of
compound or simply Compound. For example, a molecule or a compound of carbon
dioxide is formed by the union of one carbon atom and two oxygen atoms.
However, the noble gases consists of
molecules of single atoms only.
Q.10wer: Define: (a) Atomicity (b) Ion (c) Molecular Mass (d)
Relative Formula Mass
Answer:
(a)
Atomicity - Number of atoms in a single molecule of an element is known
as atomicity. This can also be defined as the number of atoms constituting a
molecule.
(b)
Ion - An ion is a charged particle which can be positively or negatively
charged. A negatively charged ion is called an Anion and a positively charged ion is called a Cation. Ions may consist
of a single charged atom or a group of atoms that have a net charge on them
known as Polyatomic Ion.
(c)
Molecular Mass - The molecular mass of a substance is the relative mass
of its molecule expressed in atomic mass unit (u). It is equal to the sum of
the atomic masses of all the atoms present in one molecule of the substance.
For example, one molecule of water (H2O) contains two atoms of
hydrogen and one atom of oxygen.
Molecular
mass of water = 2 x atomic mass of hydrogen + 1 x atomic mass of oxygen
= 2 + 16 =
18 u.
(d)
Relative Formula Mass (or Formula Mass) - The Formula Mass of a
substance is the sum of atomic masses of all atoms in a formula unit of a
compound. Formula Mass is used for substances whose constituent particles are
ions.
Q.11:
What is valency ? Mention use of valency.
Answer: Valency is the combining capacity of an
element.
Valency
can be used to find out how many atoms of an element will combine with the
other element to form a chemical formula. For example, hydrogen has a valency
+1 and chlorine has a valency –1, so one atom of hydrogen combines with one
atom of chlorine to form hydrochloric acid (HCl). Oxygen has a valency –2, so
one atom of oxygen combines with two atoms of hydrogen to form water molecule
(H2O).
Q.12:
What are the rules of writing a chemical formula? or, How is a molecular
formula of a compound written ?
Answer:
In order to write formula of a compound comprised of cations and anions, the
valencies of the ion must be known. Let us consider a compound composed of
cation A with valency x+ and anion B with valency y–. Then
the molecular formula can be written by following the rules given below:
(i)
Write down the symbols of the cation and anion side by side.
A
B
(ii)
Write their valencies at the top corners as
Ax+ By–
(iii)
The valencies or charges on the ions must balance after combining. So,
interchange between the ions their valencies and these are placed on the lower
side of each radical or used as subscripts.
Ay Bx
(iv)
If a radical is multi-atomic, use a small bracket around it before writing the
valency number to indicate the ratio.
(v)
Eliminate the common factor, if any, from the numbers used in subscripts. For
example, calcium phosphate, a compound of calcium ion with valency 2+
and phosphate ion with valency 3– is written as Ca3(PO4)2.
Q.13:
Give the chemical name, chemical formulae for the following compounds having
common names:
Baking soda, Washing soda, Blue
vitriol, Green vitriol, Gypsum, Oil of vitriol or White vitriol, Soda ash,
Marble, Lime water.
Ans:
|
Common
Name
|
Chemical
Name
|
Chemical
Formula
|
|
Baking
Soda
Washing
Soda
Blue
Vitriol
Green
Vitriol
Gypsum
Oil
of Vitriol
Soda
ash
Marble
Lime
water
|
Sodium
bicarbonate
Sodium
carbonate
Copper
sulphate
Ferrous
sulphate
Calcium
sulphate
Sulphuric
acid
Sodium
carbonate
Calcium
carbonate
Calcium
hydroxide
|
NaHCO3
Na2CO3.10H2O
CuSO4.5H2O
FeSO4.7H2O
CaSO4.2
H2O
H2SO4
Na2CO3
CaCO3
Ca(OH)2
|
Q.14: Name the smallest particle that has all the properties of
an element.
Answer:
Atom.
Q.15: How many ions are present in one mole of ions ?
Ans:
6.023 x 1023
Further study on this chapter:
- Class 9, NCERT (CBSE) solutions of Chemistry - Chapter 3, Atoms and Molecules | Answers of NCERT textbook chapter In Text Questions
- Class 9, NCERT (CBSE) Guide | Chapter 3, Atoms and Molecules | Additional Multiple Choice Questions (MCQ)
- CBSE Board Class 9, Atoms and Molecules, Chapter 3, NCERT Solutions of Science Textbook Exercise Questions Solved CBSE Board Class 9, Atoms and Molecules, Chapter 3, NCERT Solutions of Science Textbook Exercise Questions Solved
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