1972-11-29 — Page 23

REFEREDUCE

29 NOV 1972

CITY

HA

KIU YAT PÔ

報日僑華

三期星

育教僑頁三第張六第 日四廿月十年子壬歷

* 1973英文中學會考試題預習專欄

化學科(六)

Chemistry (6),

Solution for last week

Part

AMARA

6. S

R

Anod

atomic No.15

atomic No.17.

atomic No.18

atomic No.19.

atomic No.20

Ker = os electron

The bonding of atoms

In fact that the "inert gases" absolutely show no chemical activ-

ities and have no tendency to enter into chemical combination with any element. Each of these gases has eight electrons in outermost shell

(octet) or two elett

as in the case of

therefore taken that ai

group of

two or eight electrons constilutes. an electronic arrangment of extren -ce- or exceptional stability. Atome may aequire the stable electronic configuration of an inert gas in two ways:

1 by the loss or gain of electrons

to form ions, this type is called electro-valency

by the sharing of electrons this is called covalency.

Tonic Bonds

A bond which is formed by transfer of one or more electrons from one atom to another is called an electrovalent, ionic, or hetero -polar bond. As an illustration of this type, the sodium and chlorine pan be used for example,

When sodium atom Loss 1 electron, it form a sodium ion

N (2,8,1)

Na 12,8)

The chlorine atom, acquire, electron to complete its octect. In this process, the chlorine atom becomes a negatively charged ion.

01

(2,8,7) ____, (2,8,8,)

The sodium atom olss one electron to form a stable positive ton, Na, and the chlorine atom takes on this electron to form stable negative ion, 01. This electromic interaction produces, the ionic compound

Na C1

• The attraction between these oppositely changed ions the ionic bond that holds the resulting com pound, sodium chloride, together in the Crystal lattice.

Na CL

Na + 01 (2,8,1) (2,8,7) · (2,8) (2,8,8) Similary,

Ca

(2,8,8,2) (2,8,6) (2,8,8) (2,8,8

Notice in the above examples, involving Na, Ca, C1 and S, there- fore electrovalency operates be- tween a metallic element and a non

metallic element. Atoms of the metal

1 each lose a number of elect rons equal to the valency of the metal, atoms of non-metal each accept a number of electrons equa. to the valency of the non-metal. Periodic Table and the Tonie Banɑ.

An atom of an element in group I or II readily loses 1 or 2 electrons respectively to form a positive ion. The atom of an ele- ment in group VII and VI, gain 1 pr 2 electrons to form a negative. Lon. This means, for example, that the atoms of the elements in group I and these in group II can combin With the atoms of the elements in group VII and those in group VI to from ionic compounds.

7.3

Covalent Bondi

A covalent Bond is created between two atoms by sharing of two electrons, one, electron being contribited by each of the atoms concemed.

Covalent compounds are mole- cular and contain no ions. When

atoms of the same element are co- valently bonded, molecules of the element are formed. When atoms of different elements are covalently bonded, molecules of compounds ar fomed

Covalent bonas between atoms or the same element.

The hydrogen molecule,ng

The shared electron pair co.. stitutes the covalent bond between the two atom.

convalent bonds between atoms of different elements

The covalent bond many also link together atoms of different elements to form a molecule of a compound. Following are examples 1. The water molecule HO

2. The ammonia molecule, NH2

H• + H + •N• + H HON

The symbol to represent the electron of the elements,

In molecules of some compounds, two atoms share more than one pair of electrons. Two electron pairs share between two atoms constitutes a double (covalent) bond; three electron pairs so shared constitute a triple bond Examples of each are given in the follows:

The carbon dioxide molecule

(Double bond

10 C 10.

The hydrogen cyanide molecule (Triple bond)

H CN

The differences between the pro- perties of electrovalent and cova- Lent compounds:

Electrovalent Compound

For ion- Crystalline solids Insoluble in organic solvents. High melting point and boiling.

point (ie, non-volatile): Electrolytes. Conduct electr

when melted or dissolved.

Covalant compounds Consists of molecules & folus

ions.

Liquids and gases

Soluble in organic solvents. Low melting point & boiling point

(i.e. Volatile) Non-electrolytes. Do not conduct

electricity.

Co-ordinate covalent Bond

We know that a covalent bond consists of a pair of electrons shared by two atoms. One of the electrons in the pair was supplied by one atom, and the other electron was supplied by the other atom. In some covalent bonds, however, one of the two atoms supplies both electrons. A covalent bond formed in this way la called a co-ordinate covalent bond or dative covalency.

H

A pair of electrons of this sort is often referred to as a lone pair or free pair. Exercise

Part I

1. In which of the following com→ pounds are the atoms joined gether by electrovalent bond

1. Sodium chloride

I calium chloride III. hydrogen chloride.

IV. carbon tetrachloride. A. TV

B. I and II 7 C. II and III D. III and IV

I, II and ITL

2. 10, is formed

A. Sharing of 1 pair of electrons between the carbon and oxygen atom. B. sharing of 2 pair of electrons between the carbon and oxygen atom. Ctransferring of electron rom the carbon atom to each of the oxygen atom.

D: transferring of 2 electrons from the carbon atom to each of

1978

日九廿月一十年二七九一层公年一十六國民華中

中文中學會考試題預習專欄 英文科(六),徐有降。

| English (6)

It is now time to show how a candidate can make use of the knowledge he has gathered from this column so far, to write an essay for the purpose of the Exam- ination under consideration.

Suppose the essay is on "Revenge". He should now consider what is the most suitable theme, with the

of the five differ- ent ways on in this column. In this case, a combination of the first and fifth methods should suffice.

(Refer to column printed on 1st Nov. 1972

popgested to him earlier

Candidates have been told that they can assemble relevant facts or opinion for the essay either by, way of the "Association of Ideas" method or by the use of "Self-questioning technique" Here the latter is used to illustrate. how it is done.

Revenge

What is revenger. It is a per son doing to another person some- thing, which the other person has done into him. To give an extreme example: John murdered Peter's father. Peter murdered John's father at a later stage to get even. This leads to another quest- ion? Should this sort of thing, revenge, be allowed and encouraged or stopped and deprecated?

revenge in mild forms, is indicat- ed and should be allowed, but cer tainly no violence. He does not think that revenge in any shape of form is to be deplored altogether. He believes that ever it is a just

it must be in a -ified revenge, moderated form. What is your opinion?

The following is another illustration to show candidates how to decide on a theme and then write an essay on the topic.

Suppose the topic is "Jure Delinquency in Hong Kong." The central idea or theme in this cu position should be very clear. Candidates should describe some of the crimes committed by the juven- iles or young people of Hong Kong - "hese days, in the order of their relative importance, starting with the major ones for Juvenile Delinquency in Hong Kong.

What is Juvenile Delinquency: In simple language, it means young people committing crimes. The young people hare in Hong Kong used to be very docile and law changed in recent years from good abiding, but not go now. They have,

to bad, and bad to worse, and are still going down. Serious crimes committed by young people are pre- valent. News of these crimes are · printed in the newspapers everyday. These crimes include, murder, arsor rape, robbery, tarfficking danger- ous drugs, smoking dangerous drues and petty thefts.

It is not the purpose of this essay to describe the seriousness of these crimes in detail. To do so, will take volumes. However,. people must made to realize that the present situation is serious, that these crimes committed by th

consroots of Hong Kong. What then is

young people are affecting the

to be done?

To answer the question is te draw a conclusion, which should not be done until the pros and of two schools of thought regard- ing this matter are carefully examined. On the one hand, thero are people who think that revenge is to be advocated. They believe in "An eye for an eye and a tooth for a tooth." They believe that revenge is part of social justice, that it helps to stop people from foing wrong. On the other hand, there are people, who are of the opinion that human beings shou be tolerant, that forgiveness is livine. Some of these people ever Jelieve that if a person's cheek has been spat on, he should not Wipe it dry, but rather allow it to dry through the natural process of evaporation, so as not to be”.

provoking.

A fair-minded person will pro -bably think that both schools of thought are bias, that the answer should come from somewhere between the two, that in certain instances,

the oxygen atom.

E. electrostatic attraction be- tween

vetoms

3. Which of the following compounds is electrovalent?

A. calcium oxide

B. carbon tetrachloride

G. Ammonia gas

D. Ethyl alcohol

E. Sulphur dioxide.

4.

covalent compound

1. is a crystalline solau

2. consists of ions

3. conducts electricity when

melted or dissuved

4. is usually volatile

5. is soluble in inorganic and

organic solvents

Which of these combination is correct?

ALL

2,3 only

1,2,3 and 5 only.

pura water has low electrical nductivity because it

4. is a universel solvent i B. contains few hydrogen ions. C. contains many hydrogen iuns Dnes a maximum density at 4° B. contains many hydroxyl ions

5. Which of the following specie: has the same electronic configure. etion Hs Argon?

P. Naon atom

2. Magnesium ion R. calcium ston. 5. chloride ion. 1. Fluorive atom

7. Single covalent bonda exist between

hydrogen and oxygen in hydrox Tors

First of all, the generation gap must be bridged, as there is a generation gap here. When this is done, the parents will be abla to influence, and indeed, control the children, as was the case many years ago. Secondly, the Government should introduce stringent laws to punish the young criminals, so as

to deter others from committing similar offences. Thirdly, more free recreational facilities shoula be provided, so that the young people do not have to roam the streets, keeping bad company during their leisure time,,

____ There are, of course, other remedies, but the three suggestions should find expressions first. Then and not before then, will the tide of Juvenile Delinquency in Hong Kong be stamped,

0. Nitrogen and hydrogen in

annonia

Fotassium and chlorine in essium chloride..

carbon atoms in parafiang,

wa and oxygen in sodium acetate

8. Carbon compounds are mostly covalent. This is because P. The atomic number of

is six

Q. Carion, exhibius alotrody R. Carbon is a non-metal S. Organic compounds contain

carton

Carbon is a reducing agent,

If a compound A B is formed transferring 2 electrons from atom A to atom a, which of the following is/are true?

1. Bis oxidised

II. B is divalent

IT

is more electropositiva than A

IV. A is reduced

V. A Bis e covalent combound

P. II only

2. II and III only

R. III and V only

3. I, II and III only

I, III and IV only

10. Potessium reacts with chlorine

by

each petassium atom

sharing

one electron with o hlorine aten

each potassium atom "sharing". two electron with a chlorine atomi:

R each chlorine atom passing one

lectron to a potassium atom. each potassium atom passing one electron to a chlorine atom. each potassium atom passing two electrons to a chlorine atom.