1991-10-18 — Page 32

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1992 中學會考預習專欄

三明德出版社:

MILL & DALE PRESS

Chemistry(2)

H. LAM

3.16When the electron transfer in (ionic bond formation) is incomplete or the sharing of bond pairs between two atoms (In coyalent bond formation) is unequal, a bond which is partly tonic and partly covalent is resulted. Such a bond will have properties intermediate between the pure ionic bond and pure covalent bond.

e-g. HCl consists of polar covalent bonds due to the unequal sharing of the bond pairs.

H

dipolé

C1.

or H

electron clouds

Question 1.3

.Chemical formula

3.17A polar molecule is one containing a net dipole.. It has a much greater attractive force among its molecules,. It is affected by an electrostatic field.

3.18structures of the substances are closely related to the types of banding while the properties of the substances are closely related to their structures.

Tonic Compounds

(a) Composing particies.

((b)Nature of.

Tons

[Simple

Covalent. Compounds

molecules

{Giant.

Covalent Structure

atoms

| Metals

strong-

weak

force

particies

(c)Melting

-point/Boiling

point.

very high.

low

strong

atoms

Fusually

Unit 1 continue

-3.13The 3-dimensional shape of simple molecules can be predicted using the "electrán pairs repulsion theory".

(a) The electron

pairs

around the central atom in a molecule arrange themselves in such a way so as to make the repulsion among them a minimum.

(b)The order of repulsion is:

LP-LP>'LP-BP>BP-BP,where

LP is Tone pair and BP is the band pair. N.B. A double bond or, a triple bond is counted as one bond pair.

3,14The final 3-D shape of a molecule is determined by the relative positions: of the bond pairs. The angle between two covalent bonds is called the bond angle.

Examples:

¡Electron paira

J-D shape

force between electrostatic intermolecular covalent bond strong the

attraction

(d)Physical state at room

中學程 親㩗履歷 圍24號 翟小姐

(a) co, 2 double bonds

(16. 28P).

(D) CHA

4 single bonds {4BP)

sear O-C-0 tetrahedral

109.5*

|NH, 13 single bonds

(38P, 1 LP)

pyramidal

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H20-12 single bonds

(2BP, 2LP)

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V-shaped (or angular).

104.5"

The bond angle decreases from CH4 to NH2 to H2O is due to the increased repulsion of the bond pairs by the Increasing number of one pairs.

3.15The metallic " bonding regards

metal as consisting of "the positively-

charged metal jons embedded closely sea of electrons". The

together in electrons

凸

.come from the outermost

shells and are free to move within the.

e.g. Na matal vi

metal crystal lattice.

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Imetallic

bonding

very high

|high.

Phard.solid

| mostly gases/ hard solid.

liquids

sometimes soft

solids

poor

A few

poor. (except. graphite)

temperature

(e)Electrical

poor: when

conductivity solid, good

when molten or conducts in

Question 1.1

in aqueous solution (with}

Isolution)

| chemical (changes)

Six elements lettered A, B, C, D, E and F, together with their respective atomic numbers, were shown in the table below.

usually hard solid

good

(without any

| chemical

| changes)

below with some of the elements being denoted by letters which are NOT the chemical symbols of the elements.

Vil

Element

B

C ▸

E F

rlods,

Atomic number

70 12 15

A D

B

C

X

Y

(a)Give the letter.

element "consisting of manatomic molecules. (b)Give the letter of the element which could be a strong oxidizing agent. (c)Give the letters of TWO elements which are chemically similar. Explaie briefly your choice. (d)Elements @ and E are both solids at room température and one atmospheric pressure. However, D has a much higher melting point than E. Explain briefly..

(e)F can react separately with D and E

X

(a)Give the family names of the elements in groups 1 and VIT respectively.

(b)Give the letter of ̇the element

"having monatomic molecules. (c)Which of the elements shown has the

strongest reducing power?

(d)Why do the elements A, B and C show

: similar chemical properties?

sto-sforms the compounds K and K (e)arrange the elements W. sammiuyin

respectively.

(1)Write down the chemical formulae

of X and Y respectively.

(2)Using the outermost shells only,

draw diagrams ®

to show the electronic structures of X and Y respectively.

(3)Which oneory and Y. would have a higher melting point? Explain your answer.

(4)Which one of X and Y, in liquid state. would be able to conduct electricity.

Question 1.2

Part of the periodic Table is Showg

薄利多銷

the order of increasing reactivity. (F)Which TWO of the elements shown would combine most readily: to form an

Give ionic compound?

-your

reasons. (g)Suggest the chemical formulae. for. the chlorides of E and F respectively, and draw 3-dimensional diagram to show the shapes' of these. compounds..

(h)Would the oxide of F. give an acidic, alkaline Gr neutral solution on dissolving in water? Give 匪 reason for your answer.

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No. of electrons in the

Lizo

Beo 8,0, CO2

N2O Oz F20

2

3

4

5

outermost shell of the element

6 7.

Bolling potnt (°C)

1700 2500

450 -57.

鸿

2300 3900 1500 178 -21

-219 -224

-183 -146

poor

Melting point ("C)

Electrical conductivity of Biquid oxide.

Information about the oxides of the elements in the 2nd period of the Periodic Table is summarized in the table above.

(a)Choose from the above table an oxide which BEST fits the descriptions. below. Explain briefly you choice in.. each case.

(1)It is a gas at room temperature

and one atmospheric pressure. (2)It is most likely to be a liquid

at room temperature and atmospheric pressure.

Solution to Q.1.1.

(a)Element C. It has

outermost shell..

one

complete

(b)Element F. It only has to gain one more electron to attain the stable octet. (c)Elements

B and E.t 15 because they have the same number of -electrons in their outermost shel}. (d)D has a higher melting point. Disa.

̇metal and its atoms.afe. held together by: strong metallic bonding while E is a molecular solid, with its molecules being held together by the weak intermolecular forces only. (e)(1)The formula of x is DF, and that

of Y is EF (2)Compaund X:

O

Compound

點

[O

(3)x would have a higher melting point. X is an ionic compounds. and its fons are held strongly together by the electrostatic force. Y is a covalent compound and its molecules. are held together -by... the weak. intermolecular forces only.

conduct electricity. Liquid x consists of mobife ions which are. responsible for conducting electricity through Tiquids. Liquid Y consists of -molecules only, so, does not

conduct electricity

Solution to Q, 1.2

(a)Group I is the alkali metals. Group VLTks the halogens.

(b)Element R

(cfflement c. It loses electrons most

readily.

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(3)It will sublime when heated. (4)It is likely composed of ions. (5)It has a giant covalent lattice.

(b)using the outermost shells only, draw diagrams to show the electronic structure af (1) F0 (2)L120 respectively.

(c)Arrange F20, CO2, Li20 and N20, in

the order of increasing acidity.

{d}lt is because they have the same:

number

electrons in their outermost shell.

of

(e) The reactivity order is: Y CX CH. (f)Elements C and H. It is because Cis the most reactive metal and Wis the ̇most reactive non-metal

(g)The formulae are ECT and FC13

ECIA

C1

C1

Tetrahedral

FC),

Pyramidal (h)The oxide of F would be acidic

because it is an oxide. of a non- metal.

Solution to Q.1.3

(a)(1)C0,/F20; because their boiling

points: are far below temperture (about 25°C).

the room

(2)N.0, because its melting point the room

very close to 'temperature.

(3)C0, because its melting point is

below its boiling point. (4)Li,0; because it can conduct electricity when in molten state, indicating the presence of mobile ions.

(5)8e0/8,0,; because they have high boiling points and melting points but do not conduct electricity: when in liquid state..

(b)(1)F0 - covalent compound

(2)Lię0 - ionic compound

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(c)Acidity: Li20 € CO2 < N22 < F20

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