1. Give one word/words for the statements given below :
(a) Arrangement of electrons around the nucleus of an atom. [1]
(b) A compound formed by the actual exchange of electrons from
the valence shell of a metal to the valence shell of a non-metal. [1]
(c) An electrostatic bond between a metallic and a non-metallic ion.
(d) A bond formed between two non-metallic elements by mutual
sharing of electrons. [1]
(e) A compound in which shared pair of electrons are unequally
distributed between the reacting atoms. [1]
(f) A compound in which shared pair of electrons are equally
distributed between the reacting atoms. [1]
(g) The process due to which an atom or an ion loses an
electron/electrons. [1]
(h) The substance which loses an electron or electrons. [1]
(i) Weak electrostatic forces between the molecules of polar
covalent compounds. [1]
(j) A chemical reaction in which gain and loss of electrons takes
place simultaneously. [1]
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Ans. (a) Electronic configuration (b) Ionic compound
(c) Ionic bond (d) Covalent bond
(e) Polar covalent compound
(f) Non-polar covalent compound
(g) Oxidation (h) Reducing agent
(i) Intermolecular forces (j) Redox reaction
2. Choose the correct word/words from the brackets to complete the
sentences given below.
(a) In the formation of sodium chloride, the sodium atom loses an
electron from its valence shell and hence is __________ .
(oxidised/reduced). [1]
(b) In the formation of magnesium oxide, the magnesium atom
loses two electrons from its valence shell and hence is a
_________. (oxidising agent/reducing agent). [1]
(c) In the formation of calcium oxide, the oxygen atom gains two
electrons in its valence shell and hence is ______________ .
(oxidised/reduced) [1]
(d) In the formation of sodium sulphide, the sulphur atom gains two
electrons in its valence shell and hence is a ___________ .
(oxidising agent/reducing agent) [1]
(e) Higher difference in the value of ______________ between the
reacting atoms, leads to the formation of ionic bond.
(electronegativity/electopositivity)
(f) When the participating atoms in a chemical reaction are two
__________ a covalent bond is formed. (metals/non-metals) [1]
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(g) The ______________ compounds in fused state or aqueous
solution are good conductors of electricity. (ionic/covalent) [1]
(h) If participating atoms in a chemical reaction are a metal and a
non-metal, the compound so formed has _________boiling
point. (higher/lower) [1]
(i) When an atom or an ion loses an electron, it is said to be
_________ . (oxidised/reduced). [1]
(j) A covalent compound in which a shared pair of electrons are
________ distributed between the atoms is called polar covalent
compound. (equally/unequally) [1]
Ans. (a) oxidised (b) reducing agent
(c) reduced (d) oxidising agent
(e) electronegativity (f) non-metals
(g) ionic (h) higher
(i) oxidised (j) unequally
3. Draw the geometric representation of the following
(i) Period 2, group IV A element.
(ii) Period 2, group V A element.
(iii)Period 2, group VI A element.
(iv)Period 3, group II A element.
(v) Period 3, group VII A element.
(vi)Period 4, group I A element.
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Ans. (i) Period 2, group IVA element is
carbon. Its atomic number is 6.
It has 2 electron in K-shell and
its L-shell has 4 electrons.
(ii) Period 2, group VA element is
nitrogen. Its atomic number is 7.
It has 2 electrons in K-shell and
5 electrons in L-shell.
(iii) Period 2 group VIA element is
oxygen. Its atomic number is 8.
It has 2 electrons in K-shell and
6 electrons in L-shell.
(iv) Period 3, group IIA element is
magnesium. Its atomic number
is 12. It has 2 electrons in K-shell,
8 electrons in L-shell and
2 electrons in M-shell.
(v) Period 3, group VIIA element is
chlorine. Its atomic number is 17.
It has 2 electrons in K-shell,
8 electrons in L-shell and
7 electrons in M-shell.
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(vi) Period 4, group IA element is
potassium. Its atomic number is 19.
It has 2 electrons in K-shell,
8 electrons in L-shell, 8 electrons
in M-shell and 1 electron in N-shell.
4. State why are noble gases unreactive while atoms of elements
other than noble gas are chemically reactive. [3]
Ans. It has been established that if any element has two electrons in all,
and these electrons are in the valance shell (duplet structure) or eight
electrons in its valance shell, then the element is in the minimum
state of energy. Such an element cannot donate/accept/share
electrons with other elements and hence does not form a chemical
bond.
As noble gases have duplet or octet structure, therefore, they are
chemically inactive. However, all other elements do not have
structure like that of noble gases. They can donate/accept/share
electrons from their valance shell and hence are chemically active.
5. What is the charge on a chloride ion? How does this charge come
about? [2]
Ans. (i) The chloride ion has unit negative charge.
(ii)When the chlorine atom (2, 8, 7) accepts one electron so as to
have a stable argon like structure, there is one electron in excess,
as compared to the number of protons in the nucleus. This in
turn makes chloride ion negatively charged.
6. Sodium oxide [Na 2O] contains ionic bonding. Write down the
formulae of ions in sodium oxide. What changes in electron
arrangement occur when these ions are formed from sodium and
oxygen atoms? What type of force hold these ions together? [4]
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Ans. The sodium oxide contains the following ions :
(1) Two sodium ions, each having a unit positive change, i.e., 2Na + .
(2) One oxide ion, having 2-units negative charge, i.e., O2–
. The
sodium atom has electronic configuration (2, 8, 1). It loses one
electron from its valence shell to have a stable electronic
configuration (2, 8) like that of the neon gas.
Na – e – –→ Na+
(2, 8, 1) (2, 8) [Neon-like configuration]
Sodium atom Sodium ion
The oxygen atom has electronic configuration (2, 6). It accepts
two electrons in its valence shell to have a stable electronic
configuration (2, 8) like that of the neon gas.
O + 2e– –→ O2–
(2, 6) (2, 8) [Neon-like configuration]
Oxygen atom Oxide ion
The electrostatic forces between the sodium and oxide ions hold
them together.
7. Use the list given below to answer the following questions :
List : diamond, sodium chloride, silicon dioxide, methane.
(a) Which substance in the list is an example of a non-polar
molecule?
(b) Which substance in the list is an example of an element with
giant structure of atoms?
(c) Which substance in the list is an example of giant structure of
ions?
(d) Which substance in the list is an example of a compound with
giant structure of molecules? [4]
Ans. (a) Methane (b) Diamond
(c) Sodium chloride (d) Silicon dioxide
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8. Why are metals good conductor of electricity? [2]
Ans. All metals have one to three electrons in their valence shell, which
are very loosely held by the nucleus. When an electric p.d. is
applied, these almost free electrons start drifting in one particular
direction, thereby making the metals good conductor of electricity.
9. (i) Draw diagrams to show the arrangement of electrons in sodium
23
11 Na⎡ ⎤⎣ ⎦ atom and fluorine atom 19
9 F⎡ ⎤⎣ ⎦ . [1]
(ii) Draw a diagram to show valence electrons in a fluorine molecule
[F2 ]. [2]
(iii)What type of bonding is present in the fluorine molecule? [1]
(iv) When sodium and fluorine combine, electron transfer takes
place and ions are formed. What kind of electron transfer takes
place in each atom? [2]
(v) Write down one similarity and one difference between sodium
ion and fluoride ion. [2]
(vi) What is a giant structure? [1]
(vii) Suggest two physical properties of sodium fluoride. [2]
Ans. (i)
(ii)
Fluorine molecule [F2 ]
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(iii) Covalent bonding is present in the fluorine molecule.
(iv) Sodium atom loses an electron to form sodium ion. Fluorine
atom gains an electron to form fluoride ion.
(v) Similarity : Both have electronic configuration of (2, 8), i.e.,
like neon gas.
Difference : Both have different number of protons and
neutrons.
(vi) When large number of particles are joined together to form a
single network, it is called giant structure. In sodium fluoride
it is a giant structure of Na+ and F– ions.
(vii) Sodium fluoride is likely to have high melting point. It is
likely to conduct electricity in aqueous solution or fused state.
10. What is meant by the term “electrovalency”? State why sodium (at.
no. 11] has electrovalency +1 and chlorine (at. no. 17) has
electrovalency of –1. [4]
Ans. Electrovalency : The number of electrons donated (lost) or
accepted (gained) by an atom of an element, from its valence shell,
such that the ion formed (residual particle) has an electronic
configuration of nearest noble gas is called electrovalency of the
element.
If the electrons are donated, then the electrovalency is said to be
positive. If the electrons are accepted, then the electrovalency is
said to be negative.
Sodium atom [at. no.—11] has 11 protons in its nucleus and its
electronic configuration is (2, 8, 1). The sodium atom donates one
electron from its valence shell to have the electronic configuration
of nearest noble gas neon. However, in doing so, it has 11 protons
in its nucleus and 10 electrons. Thus, the residual atom (sodium
ion) has a unit positive charge or has electrovalency + 1.
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Chlorine atom (at. no. 17) has 17 protons in its nucleus and its
electronic configuration is [2, 8, 7]. The chlorine atom accepts one
electron in its valence shell to have electronic configuration of
nearest noble gas argon. However, in doing so, it has 17 protons in
its nucleus and 18 electrons. Thus, the residual atom (chloride ion)
has a unit negative charge or has electrovalency –1.
11. State three differences between an atom X and its ion X 1+ [4]
Ans. 1. In atom X, the number of protons in its nucleus are equal to
number of electrons revolving around its nucleus.
In ion X1+ , the protons in the nucleus are 1 more than the
electrons revolving around its nucleus.
2. In atom X, its valence shell does not have electronic
configuration like noble gases.
In ion X1+ , its valence shell has electronic configuration like
noble gases.
3. Atom X is chemically active and hence can enter into a
chemical reaction. It can exist independently. Ion X1+ is
chemically inactive and hence cannot enter into a chemical
reaction. It cannot exist independently.
12. (a) Why do elements form ions in certain chemical reactions? [2]
(b) What kind of elements form positively charged ions? Support
your answer by two examples. [2]
(c) What kind of elements form negatively charged ions? Support
your answer by two examples. [2]
Ans. (a) With the exception of noble gases which have eight electrons
in their valence shells (He has 2 electrons) and are in the
minimum state of energy, all other elements have one to seven
electrons in their valence shells. Thus, to attain a state in which
they have minimum energy, they either lose or gain electrons,
so that their valence shell has eight electrons. However, in
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doing so their positive and negative charges get unbalanced
and hence they form charged ions.
(b) Metals form positively charged ions.
Na – e – –→ Na 1+
Mg – 2e– –→ Mg2+
(c) Non-metals form negatively charged ions.
Cl + e– –→ Cl1–
S + 2e– –→ S2–
13. Define the terms :
(a) Electrovalent or ionic bond.
(b) Molecular or covalent compound.
(c) Coordinate covalent bond.
Ans. (a) Electrovalent bond :The electrostatic bond formed between
two different atoms (usually a metal or a non-metal), such that
metallic atom donates all its electrons from the valence shell
and the non-metallic atom/atoms accept these donated
electrons, with the formation of cations and anions which
mutually bind each other is called electrovalent bond or ionic
bond.
(b) Molecular bond or Covalent bond : A chemical bond formed
between two non-metallic elements by sharing electron
pair/pairs from the valence shells of reacting atoms, such that
both the atoms acquire the electronic configuration of nearest
noble gas is called molecular bond or covalent bond.
(c) Coordinate covalent bond : The bond formed between an ion
and an atom of polar covalent compound, having a lone
pair/pairs of electrons, such that the ion accepts the lone pair is
called coordinate covalent bond.