1-5 Bonding In Compounds (part 2)

V. Bonding in compounds

Part 2

Page 1

Topic

V.

Bonding in compounds

Part 2

Reference Reading

Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 87–92

Objectives

1.5.1.2 – 1.5.1.3 – describe the formation of covalent bonds in molecules as sharing of electrons in the outermost shell e.g. H2, O2, N2, CH4, NH3, H2O and HCl – deduce the formulae of simple covalent molecules and draw electronic diagrams

Notes

Introduction to chemical bonding Why do particles tend to stick together in nature ? The formation of bond will make the particles relatively more stable. Except noble gas, all isolated atoms are unstable in nature, they tend to stick together with other atoms and become more stable. And through the formation of these attractions, the atom will attain a completely filled outermost shell. The strong attractions among the particles are called chemical bond. There are 3 kinds of chemical bond. 1. Covalent bond – electrostatic attractions formed by sharing of outermost shell electrons. 2. Ionic bond – electrostatic attractions among oppositely charged ions. 3. Metallic bond – electrostatic attractions among mobile electrons and positively charged ions. B. Covalent bonding 1.

Existence of molecule

Covalent bond is formed because isolated atoms are not stable. In covalent bond, the outermost shell electrons (valence electrons) are shared between atoms. The atoms are held together because of the attraction among the nucleus– shared electron–nucleus.

electrostatic electrostatic attraction attraction

positive nucleus

negative bonding electrons

positive nucleus

Covalent bond is electrostatic in nature (attraction between positive and negative static charges).

In nature, two unstable chlorine atoms joins together by a covalent bond to form a stable chlorine molecule. Chlorine molecule is known as a kind of diatomic molecule because it consists of 2 atoms.

V. Bonding in compounds 2.

Part 2

Page 2

Electron sharing

Covalent bond only forms between non-metal atoms. Non-metal atom can achieve a completely filled outermost shell by acquiring electrons. Sharing electron among two non-metal atoms is one way to do this. Covalent bond – a chemical bond formed by sharing of outermost shell electron. (co- means together, jointly; -valent means outermost shell.)

Isolated chlorine atom has only 7 outermost electrons but in a chlorine molecule, each chlorine atom is surrounded by 8 outermost electrons (electronic configuration of an noble gas). The two shared electrons are known as a bond pair. They form a single covalent bond. The electron pairs which are not involved in the bonding are called lone pairs. N.B.

In an electron diagram, electron may be represented by a cross or a dot. Therefore, electron diagram is also called cross & dot diagram. However, electrons are indistinguishable in nature, using different symbols for the same kind of electron may be misleading. It may be preferable to use only 1 single symbol to represent all the electrons.

It can be observed that sharing of each electron will save the use of one electron. Therefore, there is a direct relationship between the no. of electron to be shared and the no. of outermost shell electron present. No. of electron to be shared = 8 - no. of outermost shell electron e.g. Cl has electronic configuration 2, 8, 7 No. of electron to be shared = 8 - group no. = 8 - 7 = 1 No. of electron to be shared by group VII atom e.g. F, Cl, Br, I No. of electron to be shared by group VI atom e.g. O, S No. of electron to be shared by group V atom e.g. N, P No. of electron to be shared by group IV atom e.g. C, Si

=8-7=1 =8-6=2 =8-5=3 =8-4=4

Metal atoms do not form covalent bond, they form ionic bond by losing their outermost shell electron instead.

V. Bonding in compounds 3.

Part 2

Page 3

Predication of the formulae of simple covalent molecules

Covalent bond can be visualized as holding the hands with each other. Each electron to be shared may be considered as a hand. In a stable molecule, all hands have to be held.

= =

No. of electrons to be shared 8 - no. of outermost electrons No. of hands

N.B.

Unlike other atoms, hydrogen atom has only 1 outermost shell electron and will use only this to form covalent bond.

This explain why hydrogen molecule must be H2 and water molecule must be H2O.

4.

Electronic diagram of covalent compound Electron diagram

hydrogen molecule

Molecular formula H2

Cl2

Structural formula

H H

Cl Cl

chlorine molecule HCl

H Cl

hydrogen chloride molecule

O2

O O

oxygen molecule

N2 nitrogen molecule

H2O

water molecule

N N

H

O

H

V. Bonding in compounds

Part 2

Page 4

Electron diagram

Molecular formula

Structural formula

H CH4

H C H H

methane molecule

Cl CCl4

Cl

C Cl Cl

tetrachloromethane molecule

NH3

H N H H

ammonia molecule

Molecular formula – a chemical shorthand showing the actual no. of atoms in a molecule Structural formula - a chemical shorthand showing the actual connection of atoms in a molecule. Depending on the no. of atoms in a molecule, molecular substance can be classified into simple molecular structure (< 100 atoms) or macromolecule (> 1000 atoms).

Glossary

covalent bond bond pair lone pair triple covalent bond cross and dot diagram structural formula methane ammonia

single covalent bond double covalent bond electron diagram molecular formula simple molecular structure macromolecule

V. Bonding in compounds

Past Paper Questions

Part 2

Page 5

92 I 3 b ii 93 I 4 a iii

92 I 3 b ii 3b Neon, a monatomic gas, occurs naturally as a mixture of three isotopes. The relative abundance of these isotopes is tabulated below: 20 21 22 Isotope Ne Ne Ne 10

ii

C

10

10

Abundance (%) 90.52 0.31 9.17 Explain why neon gas is monatomic. 1 A neon atom has a completed outermost shell / eight electrons (octet structure) in its outermost shell, therefore, it is stable. 1 mark [Do NOT accept stable structure.] Candidates were expected to point out that a neon atom owed its stability to the octet structure in its outermost shell. However, most of them only gave a partial answer.

93 I 4 a iii 4a The following table gives some information about W, X, Y and Z which represent particles of some elements. These particles are either atoms or ions. Mass number Atomic number No. of proton No. of electron No. of neutron W 12 12 12 X 12 10 12 Y 35 17 17 Z 17 17 20 iii Molecules of Y and of Z are both diatomic. (1) Draw the electronic structure of a molecule of Y, showing electrons in the outermost shells only. (2) Do molecules of Y and of Z have the same chemical properties ? Explain your answer. (1)

1 mark (2) Yes, because Y and Z have the same electron structure / are the same element / have the same atomic number / are isotopes. 1 mark (Do not accept Y and Z have same no. of e- in outermost shell)

2

V. Bonding in compounds Part 2 94 3 A 3 Which of the following molecules has the greatest number of lone pairs of electrons ? A. fluorine B. hydrogen C. nitrogen D. oxygen 96 44 C

44 Which of the following elements can react together to form a covalent compound ? (1) argon (2) nitrogen (3) oxygen (4) calcium A. (1) and (2) only B. (1) and (4) only C. (2) and (3) only D. (3) and (4) only

98 1 D

1

An element X exists as molecules. X has an atomic number of 7 and a molecule of X has a formula X2. Which of the following can represent the electronic structure of X2 ?

Page 6