Chemical Bonding2016

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CHEMICAL BONDING

Ans The smallest particle of an element or compound that is capable of an

independent existence and shows all the properties of the substance is called a molecule.

Q2 What is a chemical bond?

Ans When atoms of elements combine to form molecules a force of

attraction is developed between the combining atoms which holds them together .This force is called chemical bond.

Q3 What are the different types of molecules? Give one example

of each.

Ans Molecules of an element:

Molecules of an element is composed of atoms of the same kind. Eg: A molecule of hydrogen consists of two atoms of hydrogen

Molecules of compound:

Atoms of different elements combine together definite proportions to form molecules of compound.

Eg: hydrogen chloride is a compound whose molecules are made up of one atom of hydrogen and one atom of chlorine.

Q4 Why do atoms form bonds?

Ans Atoms form bonds to become energetically favourable and lead to a

minimum energy .

Q5 What is octet?

Ans Eight electrons in the outermost shell is called octet of an electrons.

Q6 What is duplet?

Ans Two electrons in the outermost shell of helium is called is called duplet.

Q7 State Octet rule.

Ans The principle of attaining eight electrons in the valence shell of atoms

is called octet rule.

Q8 What is electronic theory of valency?

Ans According to Electronic theory of valency, atoms combine to acquire

the nearest noble gas electronic configuration by losing, gaining or sharing electrons .This lead to the formation of chemical bonds between the combining atoms.

Q9 Explain why Argon atom is stable?

Ans The electronic configuration of Argon atom is 2,8,8. Since its

outermost shell has eight electrons, this atom is stable.

Q.10 Why is hydrogen molecule generally diatomic and not monoatomic? 2000

Ans: The atomic no. of hydrogen is 1.Therefore, its electronic configuration is 1. It is required 1 electron more to get stable configuration or duplet. The only means is to combine two hydrogen atoms.Hence, hydrogen is diatomic in nature.

Q11 Why is the element with atomic numbers 10 stable?

Ans The electronic configuration of the element is 2, 8.Thus, this element

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Q 12 Why do elements other than noble gas combine to form

molecules? What type of chemical bonding is expected when atom of hydrogen combines with an atom of chlorine?

Ans Because these elements combine with other atom so as to get stable

configuration of its nearest inert gas.

When an atom of hydrogen combines with an atom of chlorine, the type of chemical bond expected is ‘covalent’ or ‘electrovalent’ bonding

Q13 How do atoms combine together to form molecules?

Ans All atoms other than noble gases have less than eight electrons in

their outermost shells. So, the outermost shells of atoms of these elements do not have stable configurations. Therefore, they combine with each other or with other atoms to acquire stable nearest noble gas electronic configuration

Q14 What are the ways by which an atom acquires the electronic

configuration of the nearest inert elements.

Or How can an atom of an element acquire stable electronic

configuration?

Or write the conditions for the formation chemical bond.

Ans The two ways by which an atom acquires the electronic configurations

of the inert gas are given below:

a) By transference of valence electron or electrons from an atom to another enabling in the formation of electrovalent or ionic bond. E.g: NaCl

b) By sharing of electrons between the bonded atoms or sharing of valence electrons of atom with other atoms resulting in the

formation of covalent bonds. E.g: H2

ELECTROVALENT OR IONIC BOND

Q15 What is electrovalent or ionic bond?

Ans The type of combination between atoms which depend upon the

transfer of electrons and in which oppositely charge ions are held together by electrostatic force of attraction is called electrovalent or ionic bond.

Q16 What is electrovalency of atom?

Ans The number of electrons lost or gained by an atom to attain the stable

electronic configuration is known as the electrovalency of the atom. Q17 What is an electrovalent compound?

Ans A compound which contains electrovalent bond formed by transfer of

one or more electrons is known as an electrovalent compound.

Q18 Give any three characteristics of Ionic or electrovalent

compounds.

Ans The characteristic properties are as follows:

a. Ionic compounds are generally crystalline in nature b. Ionic compounds have high melting and boiling points.

c. They conduct electricity in molten state and in their aqueous solution.

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Q19 How is an electrovalent bond or ionic formed?

Ans An ionic or electrovalent bond is formed by the transfer of one or

more electrons from one atom to another to attain the stable noble gas configuration.

Q20 Describe the formation of NaCl molecule from its elements.

Ans When sodium and chlorine react together, the outer electron of the

sodium atom is transferred to the chlorine atom to produce sodium

ion Na+ _{and chloride ion Cl}-_{. The two ions (Na}+_{, Cl}-_{), being oppositely}

charged, are now held together by the electrostatic forces of attraction. Since both sorts of atoms attain a very stable inert gas configuration, the process is energetically favourable and sodium

chloride, Na+_Cl- _{is formed.}

.Q21 ( 2014) Sodium atom loses an electron to form sodium ion.

Identify the compound form by sodium ion with another ion having the same number of electrons. Predict the solubility of the compound. 2 marks

Ans: Ionic compound (i.e.Sodium chloride). The ionic compounds are

soluble in water and other polar solvents .

Q22 Explain the formation of MgCl from its elements.

Ans The electronic configuration of Mg ( at. No. ) is 2,8,2.It has 2 electrons

in its outermost shell. Thus the Mg atom transfers its two valence electrons to two Cl atoms and changes into a stable magnesium ion

(Mg2+_{), with a stable octet of electrons in its outer shell.The positively}

charged magnesium ion (Mg2+_{) and negatively charged chloride ions}

(Cl-_{) are now held together by the electrostatic force of attraction.}

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Q23 Desribe the formation of CaO from its elements.

Ans The atomic number of Ca and O are 20 and 18 respectively. So, , their

electronic configuration are 2,8,8,2 and 2,6, respectively, so that ,Ca atom has two valence electrons whereas the O atom has 6.When the atoms combine , the two valence electrons of Ca are transferred to the valence shell of oxygen. So, calcium as well as oxygen atom, achieve a stable octet of electrons and thus attain stable noble gas configuration.

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Q24 How is molecule of calcium oxide formed? (2002)

Ans Same as Q17

CHARACTERISTIC S OF ELECTROVALENT COMPOUNDS

Q25 Write three Characteristics of Ionic compound.

Ans the three characteristics of ionic compounds are as follows:

a. They are crystalline in nature.

b. They have high melting and boiling points.

c They conduct electricity in molten state and in their aqueous

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Q26 Ionic compounds have high melting and boiling points. Give

reason why?

Or, why do ionic compounds possess considerably high

melting and boiling points?

Ans This is due to the strong electrostatic force of attraction between the

oppositely charged ions. As a result of this, large amount of energy is needed to break the bond and hence melting and boiling points are high.

Q27 Ionic compounds are good conductors of electricity in fused

state or aqueous solution.Give reason why

Ans Solid ionic compounds do not conduct electricity as the ions are fixed

at particular positions and are not free to move. However, when these are melted or dissolved in water, the ions become fee and, therefore, conduct electricity.

Q28 Why is sodium soluble in water but not in an organic solvent

like benzene?

Ans Since sodium chloride (NaCl) is an ionic compound; hence it is soluble

in polar solvent like water and insoluble in non polar solvent like benzene. That is why NaCl is soluble in water but not soluble in benzene.

Q29 Draw the crystalline structure of NaCl and label the ions.

2014 2 mark

COVALENT COMPOUNDS

Q30 What is the limitation of ionic bond?

Ans The formation of ionic bond cannot explain the chemical combination

between identical atoms or atoms of similar chemical nature to form molecules. Thus it cannot account for the formation of molecules like

H2,O2,Cl2,Co2 and also for the organic molecules like methane etc.

Q31 What is covalent bond?

Ans The bond formed by mutual sharing of electrons between the

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CHEMICAL BONDING

Q32 How is a covalent bond formed ? Why are the covalent compounds non-conducting and low melting /boiling point ? 3 marks (2013)

Ans: Covalent bond is formed by the mutual sharing of electrons between

the combining atoms.

The covalent compounds are non-conducting because they are made up of neutral molecule.

The covalent compounds have low melting and boiling point because the force between the molecules is weak; a comparatively small amount of heat energy is required to overcome these forces.

Q33 What are covalent compounds?

Ans The type of combination between atoms which results from the

mutual sharing of pair of electrons between the combining atoms is called covalent bond and the compounds formed are called covalent compounds.

Q34 What is covalency?

Ans The number of electrons contributed by an atom for sharing in the

formation of a covalent molecule is known as covalency. Q35 What are the different types of covalent bond?

Ans The different types of covalent bond are single bond, double bond and

triple bond.

Q36 Explain the formation of Chlorine molecule (Cl2) from two

atoms of Chlorine.

Ans The electronic configuration of Chlorine atom 3, 8, 7.The two

combining atoms of chlorine gives one of its electrons to be shared between them and constitute a shared pair of electons.The two shared electrons are counted for both the atoms .Thus, each Cl atom

in a chlorine molecule (Cl2) acquires a stable configuration.

Q37 Give the electron dot representation of chlorine molecule.What covalency of chlorine in HCl. (2012)

Ans:

The covalency of chlorine in HCl is one.

Q 38 Explain the formation of hydrogen Chloride molecule .Draw

the electronic structure.

Ans Hydrogen atom has only one electron and chlorine atom has seven

electrons in its valence shell. Hence by mutual sharing of electron pair between them both the atoms acquire nearest noble gas configuration.

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Q39 Explain why Methane ( CH4) Molecule has four pairs of shared

electron?

Ans In a molecule of methane, the carbon atom and each of the four H

atoms acquire the stable configurations of the neon and helium

respectively. In methane, we have four carbon-hydrogen (C-H)

single bonds. Thus, methane has four pairs of shared electrons.

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Q40 Explain how water H2 _{O molecule.}

Ans A molecule of water is made up of one oxygen atom and two

hydrogen atoms. An oxygen atom is two electrons short and hydrogen atoms are one electron short of a noble gas structure. Thus one atom of oxygen shares two of its electrons with two hydrogen atoms thus attaining a stable configuration.

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Q41 Describe the formation of ammonia (NH3) molecule.

A ns A molecule of ammonia is formed by the combination of one atom of nitrogen with three atoms of hydrogen .Therefore five electrons in the valence shell of nitrogen atom. So, it has three electrons short of to complete its octet and to be stable .When it shares three of its electrons with three H atoms, the stable electronic configuration of neon is achieved. The three H atoms acquire the stable configuration of helium

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Q42 How many lone pairs and bonding pairs are represent in NH3

molecule.

Ans One lone pair and three bonding pairs are present in NH3 molecule.

Q43 Covalent compound are generally insoluble in water, but

ammonia is soluble in water, Give reasons.

Ans Ammonia NH3 readily dissolves in water H2O because it reacts with

water to ions.

MULTIPLE COVALENT BONDING

Q44 What is a bonding pair of electrons or bond pair?

Ans The pair of electrons which take part in bonding are called bonding

pair of electrons.

Q45 What are lone pairs?

Ans The pair of electrons which do not take part in bonding are called lone

pairs.

Q46 What is single covalent bond?

Ans If the two atoms share one electrons pair, the bond is known as single

covalent bond and is represented by one dash ( ). Q47 What is double covalent bond?

Ans if the two atoms share two electron pairs ,bond is known as double

covalent bond and is represent by two dashes ( ) . Q48 What is triple bond?

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Ans if the two atoms share three electron pairs ,bond is known as triple

covalent bond and is represent by two dashes ( ) .

Q49 What do you mean by lone pairs of electrons? How many

lone pairs and bonding pairs are present in H2O ,NH3 and CH4 molecules.

Ans Pairs of electrons which do not take part in bonding are called lone

pairs.

In H2O molecule, there are two lone pairs and two bonding pairs .

In NH3 molecules , there are three bonding pairs and one lone pairs.

In CH4 molecules, there are only four bonding pairs and no lone pair.

Q50 Write the configuration of a water molecule and thereafter,

mention the number of lone pairs it possesses.

Ans In the electronic configuration, the central atom is surrounded by four

electron pairs – two of which are bonding pairs and two of which are lone pairs.

The number of lone pairs is two.

Q51 Explain the formation of double bond in Oxygen atom.

Ans Two oxygen atoms combine to form an oxygen molecule (O2) by

sharing two electron pairs. Each oxygen atom has six electrons in the valence shell and requires two electrons to complete the octet. Therefore, in the formation of oxygen molecule, both the atoms contribute two electrons each for sharing resulting in two electrons pairs. Hence there is double bond between the two oxygen atoms. Q52 Using dot(.) and Cross(x) to represent valence electrons

,explain the formation of carbon dioxide ( CO2 ) molecule.

Ans A molecule of carbon dioxide ( CO2 ) is formed when an atom of

carbon combines with two atoms of oxygen. The C atom has dour electrons in its valence shell and thus requires four more electrons to complete its octet while oxygen atom requires two electrons to complete its octet. This is achieved when the carbon atom shares its

four electrons with two oxygen atoms to form a carbon dioxide ( CO2 )

molecule.

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Q53 Explain the formation of Nitrogen molecule (N2).

Ans In the formation of nitrogen molecule, each of the two nitrogen atoms

having five valence electrons provides three electrons pairs for sharing. Thus a triple bond is formed between the two atoms.

Dot and cross: Page 32 COORDINATE OR DATIVE BOND

Q54 What are Coordinate covalent bonds? Why is it called dative

or semi polar bond ?

Ans A coordinate covalent bonds or dative bond is a covalent bond in

which both the electrons of the shared pair came originally from one atom and none from the other.

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Since co-ordinate bond has some polar character ,it is also called as dative or semi polar bond.

Q55 What is a DONOR ?

Ans The atom which contributes the electrons pair is called the DONOR

Q56 What is a ACCEPTOR ?

Ans The atom which only shares the electron pair is known the ACCEPTOR

Q57 Explain the formation of ozone (O3) molecule?

Ans In O2 molecule, each oxygen atom has two pairs of unused valence

electrons. If an atom of oxygen having six valence electrons comes

close to O2 molecule, the new atom may share a lone pair of electrons

of one of the oxygen atoms of O2 molecule. It gives rise to the

formation of a coordinate covalent bond.

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Q58 Describe the formation of hydronium ion (H3O+_{) . Draw the}

electronic structure.

Ans Water , H2O has lone pair of electron s on oxygen atom while

hydrogen ion (H+_{) has empty K- shell thereby can accommodate two}

electrons. Thus, oxygen in H2O can easily provide two electrons while

H+ _{ion can accept these electrons to form a coordinate bond between}

them.

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Q59 Draw the electronic structure of ammonium ion

NH ¿ +¿ ¿ ¿ .

Ans Dot and cross: Page 34

CHARACTERISTICS OF COVALENT BOND

Q60 Why do covalent compounds exist as liquids or gases at

room temperature?

Ans The covalent compounds do not exist as ions but they exist as

molecules. There are weak inter molecular forces between the molecules. Hence they usually exist as liquids or gases at room temperature.

Q61 Give reason why covalent compounds generally have low

melting and boiling points?

Ans Since the forces between the molecules are weak, a comparatively

small amount of heat energy is required to overcome these forces. Hence covalent compounds generally have low melting and boiling points.

Q62 Covalent compound cannot conduct electricity. Give reason,

why?

Ans Because they are made up of neutral molecules, not ions.

Q63 Write three properties of covalent compound.

Ans Three properties of covalent compound are as follows:

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b. They are soluble in polar solvents like water but insoluble in polar solvents like benzene.

c. They do not conduct electricity.

Q64 Why is a hydrogen atom molecule generally diatomic and not

mono-atomic?

Ans Hydrogen atom cannot exist as mono-atomic because H-atom has

one electron in its outermost shell, so it is unstable and possess high energy.

One hydrogen atom can combine with another hydrogen atom by sharing an electron pair and duplet is achieved. Hence, hydrogen molecules are diatomic.

DO COVALENT BONDS SHOWS IONIC CHARACTER ?

Q65 Defined the term the electro negativity of the atom.

Ans The relative tendency of an atom to attract electrons to itself when

combined in a compound is called the electro negativity of the atom. Q66 Give two conditions under which polar covalent bonds formed.

(2000)

Ans: Conditions form polar covalent bonds are as follows:

(i) Atoms should be different

(ii) The bond pairs should be shared unequally to atoms.

Q67 Why do shared pair of electrons in H2 molecule is situated in

between the identical nuclei?

Ans Because the shared pair of electrons is equally attracted by the two

atoms.

Q68 If the atoms forming bonds are different, the electron

attracting tendency of the two atoms may differ. Explain why?

Ans As a result of the difference in electro negativity, the shared electron

pair will be displaced towards the atom with more electron attracting tendency or electro negativity.

Q69 Why does the molecule HCl have a dipolar character? (2000)

Ans Same as Q70

Q70 Why does the molecule HCl have a dipolar character?

Ans Because the shared electron pair will be more attracted towards the

chlorine end of the molecule. So the chlorine end of the molecule will be slightly negative and the hydrogen end slightly positive. Thus, the HCl have a dipolar character.

Q71 What are polar covalent compounds? Give a condition under

which such bonds are formed.

Ans All covalent bonds between unlike atoms are partially polar. The

shared electron pair will be displaced towards the atom with more electron attracting tendency or electro negativity.

Thus, they are called polar covalent compounds.

Condition: A polar covalent bond is formed between different atoms. Q72 What do you mean by partially polar?

Ans Those molecules which develop a partial ionic character even though

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Q73 Explain why HF is considered to be a polar covalent

compounds. (2015) 1 mark

Ans Fluorine (F) has more electro negativity that of Hydrogen (H) .The

shared electron pair would be attracted more towards the Fluorine end of the molecule. So the Chlorine end of the molecule will be slightly negative and the hydrogen end slightly positive, these are

represented as −¿ +¿∧δ¿ δ¿ charges. δ−¿ δ+¿ −H¿ H¿

Q74 Why is water a polar covalent compound?

Ans Water is a polar covalent compound because the shared electron pair

is unequally attracted by hydrogen and oxygen atom i.e. the shared electron pair is more attracted towards the oxygen atom developing

charges

−¿

+¿∧δ¿

δ¿ .

Diagram : page 35

Q75 Why is water molecule V- shaped?

Ans The water(H2O) molecule contains two covalent bonds between the

oxygen and hydrogen atoms. These two bonds are not symmetrically

arranged, and H2O molecule is V-shaped.

Q76 Why is water a polar solvent?

Ans The water (H2O) molecule contains two covalent bonds between the

oxygen and hydrogen atoms. These two bonds are not symmetrically

arranged, and H2O molecule is V-shaped. This result in the polar

character of water .Hence water is a polar solvent.

Q. What is the type of bonding in water? Write down the

electronic structure of it Why is water molecule polar?

Ans: Try yourself

BONDING IN METAL

Q77 What is Metallic bonding?

Ans The simultaneously force of attraction between the positively charged

kernel and the mobile electrons which binds the metal atoms together is called metallic bonding.

Q78 Who proposed the electron sea model?

Ans Lorentz

Q79 Define electron sea model?

Ans According to model, a metal lattice consist of positively charged

kernels arranged in a regular way surrounded by loosely held valence electrons.

Q80 What are kernels? Why is it called electron sea model?

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CHEMICAL BONDING

It is called electron sea model there is a sea of valence electrons in which positively charged kernels are immersed.

Q81 Draw a neat diagram of electron sea model of metallic

bonding and label the kernels and valence electron. 2 marks 2013

.Ans For Diagram see on page 36

TEXTUAL QUESTIONS (page 28)

Ans1 Electronic configuration having eight electrons in their outermost shells (two in case of there is K-shell) favours inert or noble behavior of element.

Ans2 The outermost shells of the atoms of all elements other than noble gases do not have stable configurations. Therefore, they combine with each other or with other atoms to acquire stable nearest noble gas electronic configurations. Thus, Most of the elements form atom.

Ans3 Electropositive elements can form cations.

E.g: (i) Na Na+ _{+ e}

-2,8,1 2,8

(ii) Ca Ca2+_+2e

2,8,8,2 2,8,8

Ans4 Electronegative elements can form anions.

E.g (i) Cl + e- _Cl

-2,8,7 2,8,8

(ii) O+e2- _O

2-2,6 2,8

Ans5 The electronic configuration of Sodium atom is 2,8,1, so the atom can lose one electron from its outermost shell to attain the nearest inert gas configuration. Whereas, the electronic configuration of sodium ion is 2, 8 and get its octet. Thus sodium atom reacts vigorously with water, but sodium ion does not.

Ans6 The type of combination between atoms which depend upon the transfer of electrons and in which oppositely charge ions are held together by electrostatic force of attraction is called electrovalent or ionic bond.

The three conditions for its formation

(i) When electropositive element can lose one or more valence

electron to attain nearest inert gas configuration.

(ii) When electronegative element can accept one or more valence

electrons to attain nearest inert gas configuration.

(iii) When the electrostatic force of attraction develop between the

oppositely charged ion i. e cations and anions.

PAGE 36

Ans 2 Since carbon tetrachloride is a neutral molecules, there is no free electron to move.

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CHEMICAL BONDING

Ans3 A normal covalent bond is formed by mutual sharing of electron pair between the atoms of two similar or dissimilar elements to attain the stable configuration of the nearest inert gas configuration. Whereas, in co-ordinate bond an electron pair bond is formed in which both the electrons of the shared pair came originally from one atom and none from the other.

Ans4 In the formation of hydrogen molecule, H2, the two hydrogen atoms

approach each other, the single electron of both the atoms forma shared pair. This electron pair revolves around the two nuclei of H-atoms and it is contained in the electron shells of both the H-atoms and each hydrogen atom attains the electronic configuration of the

nearest inert gas helium. Thus, H2 exist. Whereas, Helium (He)

has two electrons in the k- shell to get its duplet and become stable.

Thus ,He2 does not exist.

Ans5 The shared electron pair would be attracted more towards the Fluorine end of the molecule. So the Chlorine end of the molecule will be slightly negative and the hydrogen end slightly positive, these are

represented as −¿ +¿∧δ¿ δ¿ charges. δ−¿ δ+¿ −H¿ H¿ Thus, HF is polar.

While H2 and F2 molecules, the shared pair of electrons is

equally attracted by the two atoms. That is why they are not polar.

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Ans1 The outermost shell of an atom of an element is called Valence shell. The number of electrons present in the outermost shell of the atom is called valence electrons.

Ans2 Elements combine chemically because the element other than noble gases have less than eight electron in their outermost shells .So, these element combine with each other or with other atoms to decrease there energy to a minimum level and acquire stable nearest noble gas configuration .

Ans3

Ionic bond Covalent bond

(i) This is formed due to the strong

electrostatic force of

attraction.

(i) This is formed due to the weak intermolecular forces between the molecules.

(ii) These are formed when an

electropositive element

combines with electropositive element.

This is formed by mutual sharing of electrons between the combining atoms.

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Example from text book.

IMPORTANT PREVIOUS YEARS QUESTIONS (5 Marks )

Q 1 Explain how an electrovalent bond is formed. What type of chemical

bond is present in sodium oxide ? Show the type of bond formed by using dot(.) and Cross(X) to represent electrons of different atoms. 3+1+1=5 (1995)

Q2 What is an electrovalent compound Give two properties of an

electrovalent compound.Using ‘ dot (.) ’and ‘cross (x) to represent electronic different atoms, write the formation sodium chloride molecules.

Q3 How is an ionic bond formed Write any three characteristics properties of ionic compound. Using dot(.) and cross(X) to represent electrons of different atoms draw the electronic structure of calcium oxide. 1+3+1=5 1996

Q4 What are the types of bonding in water and calcium oxide Write down the electronic structure of water and calcium oxide. Why is water molecule V shape?

Q5 How is covalent bond formed? Using dot(.) and Cross(x) to represent electrons, describe the formation of the bond that holds two oxygen

atoms together in an oxygen molecule. State two characteristics

properties of covalent compound. 1+2+2=5 .

Q6 An element X(atomic no. 15) combines with another element Y( atomic

no. 1) .What will be the nature of the bond formed? Using dot(.) and

cross(x) to denote valence electrons give the electronic structure of the

compound and suggest any two properties of the compound .1+1+1+2=5 Q7 Two elements A and B have the atomic number 15 and 17 respectively. What type of bonding will be formed when A and B combine and what will be the formula of the compound ? 1+1+3=4