chapter4

Chapter 4 structure of molecules

ADNAN SOHAIL M. Phil chemistry

1. Why does sodium form a chemical bond with chlorine?

Ans. An ionic bond is formed between sodium and chlorine. Sodium being metal loses one electron. Chlorine being non metal accepts that electron. In this way, both atoms attain noble gas electronic configuration and become stable.

2. Why does sodium lose an electron and attains +1 charge? Sodium has only one electron in its outer most shell. It loses that electron to attain noble gas electronic configuration.

3. How do atoms follow octet rule? Ans. An atom can accommodate 8 electrons in its valence shell in three ways: i. By giving valence shell electrons (if they are less than three) to other atoms. ii. By gaining electrons from other atoms (if the valence shell has five or more electrons in it). iii. By sharing valence electrons with other atoms.

4. Which electrons are involved in chemical bonding? Ans. Valence electrons are involved in chemical bonding.

5. Why does group 1 elements prefer to combine with group 17 elements? Ans. Group 1 elements are metals. They have only one electron in their outermost shells. They form positively charged ions by losing one electron. Group-17 elements are non metals. They are electronegative elements with high electron affinities. They have the tendency to gain electrons. If atoms belonging to these two groups are allowed to react, they form a chemical bond.

6. Why chlorine can accept only 1 electron? Ans. Chlorine has seven electrons in its valence shell. It needs only one electron to complete its octet. So, it accepts only one electron and become stable.

1. Give the electronic configuration of carbon atom? Ans. The electronic configuration of carbon atom is 1s2, 2s2, 2p2

2. What type of elements have tendency of sharing of electrons? Ans. When non metals allowed react with each other. They form a chemical bond by sharing of electrons.

3. If repulsive forces dominate to attractive forces will a covalent bond form? Ans. If repulsive forces dominate to attractive forces no bond will be bond formed.

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4. Considering the electronic configuration of nitrogen atom, how many electrons are

involved in bond formation and what type of covalent bond is formed. Ans. In N2 molecule three electrons are involved in bonding. There is triple covalent bond between two nitrogen atoms.

5. Point out the type of covalent bonds in the following molecules? CH4 , C2 H4 , H2 , N2 , and O2

Ans. Single covalent bond CH4, H2 Double covalent bond C2 H4 , O2

Triple covalent bond N2

6. What is a lone pair? How many lone pairs of electrons are present on nitrogen in ammonia? Ans. The non-bonded electron pair available on an atom is called a lone pair. There is one lone pair of electrons present on nitrogen in ammonia

7. Why is the BF3 electron deficient? Ans. Boron atom has three electrons in its outer most shell. It forms three single covalent bonds with fluorine and attains six electrons. Boron needs still two more electrons to complete its octet. It accepts a lone pair through coordinate covalent bond from another molecule to complete its octet.

8. What types of electron pairs make a molecule good donor? Ans. Lone pair of electrons available on an atom in a molecule make it a good donor.

9. What is difference between bonded and lone pair of electron and how many bonded pair of electrons are present in NH3 molecule? Ans. The valence electrons, which are involved in chemical bonding, are termed as bonding electrons. The non-bonded electron pair available on an atom is called a lone pair.

10. What do you mean by delta sign and why it develops? Ans. The delta ( ) sign indicates partial positive or partial negative charge that is developed due to unequal sharing of shared pair or bonded pair of electrons.

11. Why does oxygen molecule not form a polar covalent bond? Ans. Both atoms in oxygen molecule (O2) attract share pair of electron equally. That’s why, covalent bond in oxygen molecule is a not a polar.

12. Why has water polar covalent bonds? Ans. Oxygen atom in water molecule being electronegative atom attracts shared pair of electrons towards itself. Due to this, covalent bond between hydrogen and oxygen becomes polar.

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1. What type of elements form metallic bonds? Ans. Metallic bond exists in atoms of metallic elements.

2. Why is the hold of nucleus over the outermost electrons in metals weak? Ans. The hold of nucleus over the outermost electrons is weak because of large sized atoms and greater number of shells in between nucleus and valence electrons. Furthermore, because of low ionization potentials, metals have the tendency to lose their outermost electrons easily.

3. Why the electrons move freely in metals? Ans. Because of low ionization potentials, metals have the tendency to lose their outermost electrons easily. These loose or free electrons of all metal atoms move freely in the spaces between atoms of a metal.

4. Which types of electrons are responsible for holdings the atoms together in metals? Ans. Free electrons are responsible for holding the atoms together in metals.

5. Why a dipole develops in a molecule? Ans. A dipole is developed in a molecule when shared pair of electrons are attracted unequally by the atoms forming covalent bond.

6. What do you mean by induced dipole? Ans. when non polar molecules come close to each other their electronic clouds repel each other. In this way, temporary dipoles are created in the molecules. These are called induced dipole.

7. Why are dipole forces of attraction not found in halogen molecules? Ans. Halogens form homoatomic molecules. The bonding electrons in homoatomic molecules are equally attracted by both atoms. This is the reason that dipole forces of attraction are not found in halogen molecules.

8. What types of attractive forces exist between HCl molecules? Ans. Dipole-dipole forces exist in between HCl molecules.

9. Define intermolecular forces; show these forces among HCl molecule? Ans. The relatively weak forces which exist in between the molecules are called intermolecular forces. The bonding and intermolecular forces of hydrochloric acid are shown below:

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1. Why the ionic compounds have high melting and boiling points? Ans. Ionic compounds are made up of positively and negatively charged ions. There exist strong electrostatic forces of attraction between oppositely charged ions. So, a great amount of energy is required to break these forces.

2. What do you mean by malleability? Ans. Malleability is the property by virtue of which a metal can be rolled into sheets.

3. Why are ionic compounds easily soluble in water? Ans. Water is a polar compound. When ionic compounds are placed in water, its molecules separate their positive and negative ions and surrounds them. In this way they become soluble.

4. What type of bond exists in sodium chloride ? Ans. An ionic bond exists in sodium chloride.

5. Why the covalent compounds of bigger size molecules have high melting points? Ans. Large molecules with three dimensional bonding form covalent crystals which are very stable and hard. They have very high melting and boiling points. 6. (a): What is the electronegativity difference between the following pair of elements (atoms). Predict the nature of the bond between them? (a) H and CI (b)H and Na (c) Na and I (d) K and CI (b): Comparing the electronegativity differences, arrange these compounds in increasing ionic strength.

Ans. (i) H and CI

E.N of H = 2.2, and E.N of Cl = 3.2

Electronegativity difference = 3.2 – 2.2 = 1.0 < 1.7

The bond between H and Cl will be polar covalent

(ii) H and Na

E.N of H = 2.2 and E.N of Na = 0.9

Electronegativity difference = 2.2 – 0.9 = 1.3 < 1.7

The bond between H and Na will be ionic bond.

(iii) Na and I

E.N of Na = 0.9 and E.N of I = 2.5

Electronegativity difference = 2.5 – 0.9 = 1.6 = < 1.7

The bond between Na and I will be covalent bond.

(iv) K and CI

E.N of K = 0.8, and E.N of Cl = 3.2

Electronegativity difference = 3.2 – 0.8 = 2.4 > 1.7

The bond between K and Cl will be ionic bond.

(b) Arrangement of these compounds in increasing ionic strength.

KCl > NaI > NaH > HCl

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SHORT ANSWER QUESTIONS

1. Why do atoms react?

Ans. It is a universal rule that everything in this world tends to become more stable. Atoms react to achieve stability by attaining electronic configuration of noble gases.

2. Why is the bond between an electropositive and an electronegative atom ionic in nature? Ans. There is complete transfer of an electron when an electropositive and electronegative atom is allowed to react. The bond formed in this way is ionic in nature.

3. Ionic compounds are solids. Justify. Ans. There are strong electrostatic forces of attraction between positive and negative charges of an ionic compound. That’s why they are solid at room temperature.

4. More electronegative elements can form bonds between themselves. Justify? Ans. More electronegative elements attain stability and form non polar covalent bonds between themselves by sharing of electrons.

5. Metals are good conductor of electricity. Why? Ans. Metals are good conductor of heat and electricity due to the presence of free mobile electrons in them.

6. Ionic compounds conduct electricity in solution or molten form. Why? Ans. Because in solid state ions of ionic compound are not free to move. While in molten or solution form their positive and negatives ions can move freely to conduct electricity.

7. What type of covalent bond is formed in nitrogen molecule? Ans. There is triple covalent bond in nitrogen molecule.

8. Differentiate between lone pair and bond pair of electrons? Ans. The non-bonded electron pair available on an atom is called a lone pair. The valence electrons, which are involved in chemical bonding, are termed as bonding electrons or bond pair.

9. Describe at least two necessary conditions for the formation of a covalent bond? Ans.

Electronegativity of elements must be high.

The ionization energy of elements must be high

10. Why HCl has dipole-dipole forces of attraction? Ans. HCl is a polar molecule due to unequal sharing of bonding electrons by two boded atoms. In HCl molecule partial positive and partial negative charges exist. The adjacent molecules will arrange themselves in such a way that negative end of one molecule comes near to positive end of other molecule. It results in net forces of attraction between oppositely charged ends of two adjacent HCl molecules. These attractive forces are called dipole – dipole interactions.

11. What is a triple covalent bond, explain with an example? Ans. When each bonded atom contributes three electrons, three bond pairs are involved in bond formation. This type is called triple covalent bond. Three small lines are used to indicate these three pairs of electrons between those atoms in the molecules of such compounds. The examples of molecules having triple covalent bonds are nitrogen (N2 ) and ethyne (C2 H 2).

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12. What is difference between polar and non-polar covalent bonds, explain with one

example of each? Ans. If a covalent bond is formed between two similar atoms (homo-atoms), the shared pair of electrons is attracted by both the atoms equally. Such type of bond is called non-polar covalent bond. For example, bond formation in H2 and CI2. If the covalent bond is formed between two different types of atoms (hetro atoms) then the bond pair of electrons will not be attracted equally by the bonded atoms. Such type of bond is called polar covalent bond. For example: water, hydrogen fluoride and hydrogen chloride.

13. Why a covalent bond becomes polar? Ans. When there is difference of electronegativity between two covalently bonded atoms, there will be unequal attraction for the bond pair of electrons between such atoms. It will result in the formation of polar covalent bond.

14. What is relationship between electronegativity and polarity? Ans. The ability of an atom to attract shared pair of electrons toward itself is called electronegativity. Polarity is partial positive or partial negative charge that is developed on covalently bonded atoms due to unequal sharing of bonded pair of electrons.

15. Why does ice float on water? Ans. The density of ice at 0 °C (0.917 gcm-3) is less than that of liquid water at 0°C (1.00 gcm-3). In the liquid state water molecules move randomly. However, when water freezes, the molecules arrange themselves in an ordered form, that gives them open structure. This process expands the molecules, that results in ice being less dense as compared to water. Due to this reason ice floats on water.

16. Give the characteristic properties of ionic compounds? Ans.

Ionic compounds are mostly crystalline solids.

Ionic compounds in solid state have negligible electrical conductance but they are good conductors in solution and in the molten form. It is due to presence of free ions in them.

Ionic compounds have high melting and boiling points.

They dissolve easily in polar solvents like water.

17. What characteristic properties does the covalent compound have? Ans.

They have usually low melting and boiling points.

They are usually bad conductors of electricity.

They are usually insoluble in water but are soluble in non-aqueous solvents like benzene, ether, alcohol and acetone.

Large molecules with three dimensional bonding form covalent crystals which are very stable and hard. They have very high melting and boiling points.

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