atoms combining pages from shadwick skills 3 chemistry

8/7/2019 Atoms Combining Pages From Shadwick Skills 3 Chemistry

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UNIT 17

ATOMS

AND IONS

COMBINING

OUTCOMES

• increased understanding of atomic structure

• increased understanding of atoms and ions

• increased understanding of how chemical bonds form• improved skills in using the periodic table of elements

• improved skills in interpreting electron shell diagrams

• improved skills in applying knowledge to new situations

ACTIVITY.

Read the information, then answer each of the questions in your book. You may need to look

back at the periodic table in Unit 15 for information to help you answer some of these questions.

HOW COMPOUNDS FORM

Ionic compounds form when ions join together. Ions are formed when atoms lose or gain

electrons. Metals form positive ions by losing electrqns (usually to nonmetal atoms) and

nonmetals form negative ions by gaining electrons (from metal atoms). Figure 17.1 shows

you how ions form.

The compound forms when the oppositely charged ions attract each other an d join

together by an electrostatic ionic bond. We often say that ionic compounds form by

electron transfer.

Not all elements combine with each other by transferring electrons. Ionic bond

-formation only works between metals an d nonmetals. When two nonmetals combine, the

process is different.

m



UNIT 17 • ATOMS AND IONS COMBINING m

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Lithium atom (Li)

3 protons

4 neutrons

3 electrons

Net charge zero

Fluorine atom (F)

9 protons

10 neutrons

9 electrons

Net charge zero

lases 1 electron

Lithium ion (Li+)

3 protons

4 neutrons

2 electrons

Net charge + 1

gains 1 electron

Fluorine ion (F-)

9 protons

10 neutrons

10 electrons

Net charge -1

FIGURE 17.1 How ions form

Two nonmetals combine by sharing electrons. The chemical bond which is formed is

called a molecular (or covalent) bond and a molecular (or covalent) compound

results.

To see how molecular bonds form we again

need to look at the electron arrangements in the

atomswhich

arebonding.

Forexample, let 's lookat the molecule formed when two chlorine atoms

combine with each other. Figure 17.2 shows the

structure of a chlorine atom.

FIGURE 17.2

The chlorine atom has

7 electrons in its outer

orbit. To be chemicallystable it needs 8, that is,

it needs one more



m SCIENCE SKILLS IN PERSPECTIVE 3

When two chlorine atoms meet, they each share one of their electrons with the other

chlorine atom. In this way, each atom then has eight electrons in the outer orbit, and a

stable molecule containing two chlorine atoms (a diatomic molecule) is formed (see

Figure 17.3). The shared electrons are called a bonding pair and this is what forms th e

molecular bond and holds th e atoms together. This is why chlorine exists as Cl2

molecules

rather than CI atoms.

Shared electron

FIGURE 17.3 A molecular bond is formed when atoms share a pair of electrons. Both atoms

get eight electrons in their outer orbit

If we wanted to show the bond in the chemical formula, we would represent it as

shown in Figure 17.4.

CI -C lo rC I2

FIGURE 17.4 The small line between the two CI symbols represents the single molecularbond which is formed when the two atoms each share an electron

QUESTIONS

a With the use of examples, briefly explain the difference between an atom

and an ion.

b How does an ion differ from a radica l? Give examples to demonstrate your

explanation.

c A radical is often referred to as a polyatomic ion. Explain, using examples,

how this name makes sense.

2 a How many electrons does a lithium atom have?

b Draw an electron shell diagram to represent a lithium atom.

c How many electrons does a lithium ioIl have?

d Draw an electron shell diagram of a lithium ion.

e How does a lithium atom become a lithium ion?

How many protons are there in a lithium atom nucleus?

9 How many protons are there in a lithium ion nucleus?

h What charge does an electron have?

What charge does a proton have?



UNIT 17 • ATOMS AND IONS COMBINING m

I What charge does a lithium sodium ion have?

k Explain why a lithium ion has this charge in terms of your answers to the

other parts of this question.

3 a How many electrons does a fluorine atom have?

b Draw an electron shell diagram to represent a fluorine atom.

c How many electrons does a fluoride ion have?

d Draw an electron shell diagram to represent a fluoride ion.

e How does a fluorine atom become a fluoride ion?

f How many protons are there in a fluorine atom nucleus?

9 How many protons are there in a fluoride ion nucleus?

h What charge does an electron have?

What charge does a proton have?

I What charge does a fluoride ion have?

k Explain why a fluoride ion has this charge in terms of your answers to the

other parts of this question.

4 a Imagine a lithium atom reacting with a fluorine atom. Where does the

electron each fluorine atom gains to form it s ion come from?

b In what ratio do lithium and fluorine combine to form lithium fluoride?

c What determines this ratio?

d Write the chemical formula for lithium fluoride.

5 a Compare th e charge on a calcium ion with that on a lithium ion.

b Can you explain the difference?

c Compare the charge on an oxide ion with that on a fluoride ion.

d Can you explain this difference?

6 a What difference is there between the arrangement of the electrons in the ion

of an element and it s atom?

b Use this information to invent a general rule to say what happens when a

metal atom forms an ion.

c Use the information to invent a general rule to say what happens when anonmetal atom forms an ion.

7 a Aluminium has an atomic number of 13. Draw a diagram of an aluminium

atom and an aluminium ion similar to the diagrams of the other atoms and

ions above.

b Use your diagrams, and your general rule above (question 6c), to explain the

charge on the aluminium ion.

c Sulfur has an atomic number of 16. Use this information, and your general

rule, to draw an electron diagram of a sulfur atom and a sulfur ion.

d Use this information to work out what charge the sulfur ion has.

e Use this information to write a formula for aluminium sulfide and to explain

the ratio in which the elements combine.

Repeat (a) to (e) above for the elements magnesium and bromine.



SCIENCE SKILLS IN PERSPECTIVE 3

8 a Hydrogen gas exists as diatomic molecular molecules. Draw diagrams to

show how th e molecular bond forms between the hydrogen atoms.

b Explain what happens when this bond forms.

e How would this bond be represented in a chemical formula?

9When hydrogen and sulfur

combine to formhydrogen

sulfide, twohydrogen

atoms combine with one sulfur atom. Each hydrogen atom shares it s electron

with the sulfur atom and the sulfur atom shares a different electron with each

hydrogen atom.

a Draw a diagram to show how the two molecular bonds are formed in hydrogen

sulfide.

b How would you represent this molecule in a diagram?

lOa The bonds in the carbon dioxide molecule are also molecular. Knowing that

the formula is CO2, draw a diagram to show the structure of a carbon dioxide

molecule.

b Explain how th e bonds form.

e Draw a diagram to show how you would represent the bonds in a chemical

formula.

d What is the ratio in which carbon and oxygen combine to form carbon dioxide?

e What determines this ratio?

11 The chemical formula for the chemical phosphine is PH3

. I t is a molecular compound.

a Draw a diagram to show the bonding in this compound.

b What is th e ratio in which th e phosphorus and hydrogen combine in this

compound?

e What determines this ratio?

12 The compound formed when carbon an d chlorine combine is molecular. Draw

diagrams to show it s structure and to work out it s chemical formula. Explain

th e ratio in which the atoms combine.

13 a What is the difference between an ionic compound and a molecular compound?

b How does an ionic compound form?

e How is a molecular compound formed?

d What is an ionic bond?

e How does an ionic bond form?

f What is a molecular bond?

9 How is a molecular bond formed?



UNIT 18

COMBINING

POWERS OF

ELEMENTS

OUTCOMES

• increased understanding of atomic structure

• increased understanding of how elements combine

• increased understanding of how chemical bonds form

• improved skills in using the periodic table of elements

• improved skills in applying knowledge to new situations

• improved skills in writing chemical equations

ACTIVITY.

Read the information, then answer the questions.

"-

• When elements combine with each other, the ratios in which they combine are

determined by the number of electrons they use in forming chemical bonds.

• I t doesn't matter whether the compounds are molecular or ionic, the same idea

holds for both.

• To work ou t the combining powers or valencies of elements, we need to know the

arrangement of electrons in their shells around the atom.

• The position of an element in th e periodic table allows us to work out it s electron

arrangement.

m



m SCIENCE SKILLS IN PERSPECTIVE 3

For example:

Group 1 elements (Li, N a, K, Rb, Cs) all have one electron in their outer orbit. They all

transfer one electron to make an ionic bond with other elements. They have a combining

power or valency of + 1. We call the valency + 1 because the ion formed when the metal

transfers it s electron is a + 1 ion.

Group 7 elements, the halogens (F, CI, Br, I, At), all have seven electrons in their outer

orbit. They all need to receive one electron to make an ionic bond, or share one electron

to make a molecular bond. They have a combining power or valency of -1 .

We can write ionic equations to show how ions are formed. For example:

Na -+ Na+ + 1e-

CI + 1e- -+ CI-

Group 8 elements, the inert gases (He, Ne, Ar, Kr, Xe), all have eight electrons in their

outer orbit. That is why they are chemically inert or chemically inactive. They do not

transfer or share electrons at all. They have a combining power or valency ofO.

Elements can actually have two valencies, but they usually only use one of them when

they form chemical compounds. Hydrogen, for example, can have a valency of + 1 (as in

HCI - hydrochloric acid), or -1 (as in NaH - sodium hydride). Bromine could have a

valency of - l o r +7, but it is rarely seen as a +7 ion.

Many metals do have more than one valency which often they do exhibit . For exam

ple, copper can form +1 or +2 ions. Iron can form +2 or +3 ions. We show th e valency the

element is using with roman numerals like this: Cu(!), Cu(II), Fe(l!) , Fe(II!). The valen

cies of elements in groups 1 to 8 of the periodic table are shown in Figure 18.1.

D ;---

.- - N C") C") N .- -

+ + + + I I I 0~ ~ ~ ~ u u u u u u u uc c c c c c c cQ) Q) Q) Q)

TRANSITION ELEMENTS ~ ~ I I (more than 1 electron orbit not full) I I I I I I

.-- N Most common valency + 2 C") L.l) -0

'"co

0... 0... 0... 0... 0... 0... 0... 0...

::::> ::::> ::::> ::::> ::::> ::::> ::::> ::::>

0 0 r-- 0 0 0 0 0 0~ ~ ~ ~ ~ ~ l ') l ' ) L l' ) l ') l ') l ') l ') l ')

I

L Lanthanum series of metals - Valency + 2

A Actinium series of metals - Valency + 2

FIGURE 18. 1 Diagram of the periodic table showing the valency of elements in groups 1 to 8

QUESTIONS

a Which elements have a valency of +2?

b Which elements have a valency of -2?

c What valency do the group 5 elements have? (Careful!)



UNIT 18 • COMBINING POWERS OF ELEMENTS .

2 Write down the names of at least 25 elements you know. Use the periodic table

in Unit 15 to find the group in the periodic table for each and Figure 18.1 to

work out the valency for each. Put al l this information in a table.

3 Write ionic equations to show how each of the following form ions:

a A, valency -3

b B, valency +2

c C, valency-l

d D, belonging to group 2

e E, belonging to group 7

F, which forms FC2

with C above

9 G, which forms B2G with B above

h H, which forms H 3 ~ with B above

4 Use the periodic table to identify elements which would behave like those inquestion 3.

5 Use diagrams and your own words to explain how sulfur could have valencies of

+6 or -2. Write equations to show the formation of the two ions.

6 Bromine could have a valency of -1 or +7. Explain how this could happen. Write

equations to show the ions which would be formed in each case.

7 What are the two possible valencies for nitrogen? Explain, using appropriate

equations, how you work them out.

8 Using at least two examples from the periodic table for each, write chemical

formulae for the compounds which would be formed if:

a a group 1 element combined with a group 6 element

b a group 2 element combined with a group 5 element

c a group 1 element combined with a group 7 element

d a group 2 element combined with a group 7 element

e a group 3 element combined with a group 4 element

f a group 3 element combined with a group 5 element

atoms combining pages from shadwick skills 3 chemistry

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