C09 periodicity of elements

Periodicity Of ElementsChapter 9

LEARNING OUTCOMES

Explain the basis of the arrangement of elements in the Periodic Table

Predict the properties of unknown elements based on group trends

Identify trends in Group II Identify trends in Group VII Identify trends in Period 3 using gradation

from metallic to non-metallic properties

The elements are arranged in order of increasing proton (atomic) number.

There are 7 horizontal rows called “Periods”.

Per

iod

1 2 3 4 5 6 7

Chapter 9

Periodicity Of Elements

There are 8 vertical columns called “Groups”.

There is also a block of elements in the middle of the periodic table, called the transition elements.

Transit ion elements

1 2 3 4 5 6 7 0

Chapter 9

Gro

ups


Chapter 9

Group Number and Properties The Group number is related to the number of electrons in the

outermost shell (the valence electrons).

Elements in Group I have 1 outermost electron, elements in Group II have 2 outermost electrons, and so on. The outer (valence) electrons take part in chemical reactions, and are therefore responsible for the chemical properties of the elements.

Li(Group I)

Be(Group 2)

B(Group 3)

C(Group 4)

N(Group 5)


Group Number and Properties Since elements in the same group have the same number of valence

electrons, they exhibit similar chemical properties. For example, elements in Group 0 (noble gases) have a filled outer

shell of 8 electrons, except for helium which has two.

This complete outer shell of electrons makes the atom very stable, and hence Group 0 elements are all chemically unreactive (inert).

Chapter 9

Li(Gp I)

Be(Gp 2)

B(Gp 3)

C(Gp 4)

N(Gp 5)


Group Number and Ionic charge

Group number

I II III IV V VI VII 0

Ionic Charge +1 +2 +3 +4 -3 -2 -1 0

Chapter 9

Elements in the same group have the same number of valence electrons, hence they form ions with the same charge.

The group number is related to the ionic charge in the following manner:


Period number and electron shells The period number is related to the number of electron shells in the atom:

Elements in Period 1 have 1 electron shell, elements in Period 2 have 2 shells, and so on.

Chapter 9


Metals and non-metals

As we go across a period from left to right, the elements change from metalsmetals to metalloidsmetalloids and then to non-metals non-metals.

There is a step-like dividing line in the Periodic Table between metals and non- metals.

Metals are found to the left of this line, while non-metals are found to the right.

Chapter 9

NON-METALSMETALS


Patterns in the Periodic Table

Elements in the same group have the same number of valence electrons. Hence, they have the same ionic charge and therefore have similar chemical properties.

However, as we go down a group, the atoms increase in size and it becomes easier for them to lose their outermost electrons.

This means that the elements further down a group have a greater tendency to form positive ions. They are said to become more metallic in character.

Down a Group

Chapter 9


Patterns in the Periodic Table

Metals, besides having certain physical metallic characteristics, are electropositive. This means that they have a tendency to lose electrons and form positive ions.

Non-metals are elements which have more than 4 electrons in their outermost shell, and have a tendency to gain electrons to form negative ions.

As we go across a period from left to right, the number of valence electrons increases. This means that the elements change from metals to metalloids and then to non-metals.

Across a Period

Chapter 9


Quick check 11. In what order are the elements in the Periodic Table arranged?2. What name is given to a vertical column of elements in the

Periodic Table?3. What name is given to a horizontal row of elements?4. In which Group does the element potassium belong ?5. In which Period does the element arsenic belong ?6. In which group does the element titanium belong ?7. State whether the following are metals or non-metals: caesium,

calcium, boron, silicon, phosphorus, selenium, iodine.8. Explain why potassium, which is in the same group as sodium in

the periodic table, is more reactive than sodium.

Solution

Chapter 9


1. The elements in the Periodic Table are arranged in the order of increasing proton number.

2. Group3. Period4. Group I5. Period 46. Transition elements7. Metals: caesium, calcium

Non-metals: boron, silicon, phosphorus, selenium, iodine8. Potassium is more reactive than sodium because being lower than

sodium in Group I, it is more electropositive and hence it is easier for potassium to lose an electron compared to sodium.

Return

Chapter 9

Solution to Quick Check 1


The elements are shiny and silvery metals which conduct electricity. They are kept under oil to protect them from air and moisture.

The elements are soft and can be cut with a knife.

Lithium, Li

Sodium, Na

Potassium, K

Rubidium, Rb

Caesium, Cs

Francium, Fr

Physical properties

Group I (The Alkali Metals)

Chapter 9


They have low densities e.g. lithium, sodium and potassium float on water.

The density increases down the group.

They have low melting and boiling points.

The melting point decreases down the group.

180

98

64

3928

27Mel

ting

poin

t (o C

)

Li Na K Rb Cs Fr

0.53

0.97

1.53

0.86

Den

sity

(g c

m-3 )

1.87

Li Na K Rb Cs Fr

Group I (The Alkali Metals)Density and melting point

Chapter 9


The alkali metals react vigorously with water to form an alkali and hydrogen.

E.g. 2Na + 2H2O 2NaOH + H2

The metals become more reactive as we go down the group.

Lithium reacts vigorously with water; sodium reacts even more vigorously with water; potassium reacts violently and caesium explodes with water.

Less reactive

More reactive

Group I (The Alkali Metals)Chemical properties

Chapter 9


The alkali metals react with elements in Group VII (halogens) to form salts that are white in colour.

E.g. 2Na + Cl2 2NaCl

The reactivity of the alkali metals increases down the group.

Group I (The Alkali Metals)

Chemical properties

Chapter 9


The elements have low boiling points and melting points, which increase as we go down the group.

Fluorine and chlorine are gases, bromine is a liquid, while iodine and astatine are solids, at room temperature and pressure.

Fluorine, F

Chlorine, Cl

Bromine, Br

Iodine, I

Astatine, At

Gas

Liquid

Solid

Group VII (The Halogens)

Physical properties

Chapter 9


The elements become darker in colour as we go down the group.

Fluorine is an almost colourless gas, chlorine is a greenish-yellowyellow gas, bromine is a reddish liquid, iodine is a purplish-grey solid, while astatine is a dark black solid. Darker Darker

colourcolour

Lighter Lighter colourcolour

Colour of the HalogensGroup VII (The Halogens)

Chapter 9

Fluorine, F

Chlorine, Cl

Bromine, Br

Iodine, I

Astatine, At


The elements are diatomic i.e. each molecule is made up of 2 atoms joined by a single covalent bond. E.g. F2, Cl2, Br2, I2 .

The atoms achieve greater stability by joining up in pairs to share electrons and achieve an octet structure.

fluorine chlorine

bromine iodine

astatine

Chemical properties

Group VII (The Halogens)

Chapter 9


The halogens react with metals in Group I to form white coloured salts. E.g. 2Na + Br2 2NaBr

Unlike Group I, the elements become less reactive as we go down the group.

This is because the positively charged nucleus is farther away from the outer shell and it is therefore harder to attract an extra electron.

Least reactive

Mostreactive

Group VII (The Halogens)Chemical properties

Chapter 9

Fluorine, F

Chlorine, Cl

Bromine, Br

Iodine, I

Astatine, At


Less reactive

Morereactive

2KI(aq) + Cl2(g) 2KCl(aq) + I2(s)

Candisplace

Chapter 9

Group VII (The Halogens)Displacement reactions of halogens

Fluorine, F

Chlorine, Cl

Bromine, Br

Iodine, I

Astatine, At

A more reactive halogen can displace a less reactive halogen from a solution of its salt.

For e.g. when chlorine is passed into an aqueous solution of potassium iodide, iodine is displaced and the mixture turns reddish brown due to the iodine liberated.


All the elements in this group are gases with very low boiling points.

They are monatomic: they made up of single atoms.

Helium has an outer shell filled with 2 electrons, while the rest of the noble gases have a full octet of 8 electrons.

They are chemically very unreactive or inert because of their stable electronic structures.

Helium, He

Neon, Ne

Argon, Ar

Krypton, Kr

Xenon, Xe

Chapter 9

Group 0 (The Noble Gases)Properties


Helium is used to fill airships and balloons. Argon is used for providing an inert atmosphere

e.g. to fill electric light bulbs and in the manufacture of steel.

Neon is used for filling light tubes used in advertising signs.

Xenon is used in making motorcar head lamps and flash lights for photography.

Chapter 9

Group 0 (The Noble Gases)Uses


The atomic radius decreases – nuclear charge increases and the electrons are pulled closer towards the nucleus.

The first ionisation energy increases – electrons are held tightly by the nucleus and become harder to remove.

The electronegativity increases – the elements are more likely to gain electrons than give away electrons; hence, the metallic character decreases.

The melting and boiling points increase and then decrease.

Chapter 9

Period 3Trends in physical properties across the period


The nature of oxides changes from basic (metal oxides) to acidic (non-metal oxides).

The structures of the oxides change from ionic to simple molecular.

The reactivity with water decreases across the period as the metallic character decreases.

Chapter 9

Period 3Trends in chemical properties across the period


1. State 2 physical and 2 chemical properties of the elements in Group I.

2. (a) Why are the metals in Group I called alkali metals?(b) (i) Which is the most reactive metal, (ii) the least reactive metal, in Group I?

3. Write an equation with state symbols for:(a) reaction between potassium and water,(b) reaction between potassium and chlorine.

4. (a) Which is (i) the most reactive, (ii) the least reactive element, in Group VII ?(b) Describe what happens when chlorine is bubbled into an aqueous solution of potassium bromide.

5. What is meant by monatomic ? Why do the noble gases exist as monatomic elements?

Solution

Chapter 9

Quick check 2


1. The metals are soft and have low densities;They react vigorously with water to form an alkali and hydrogen; They react with halogens to form white salts.

2. (a) They react with water to form alkalis.(b) (i) Francium , (ii) Lithium

3. (a) 2K(s) + 2H2O(l) 2KOH(aq) + H2(g)(b) 2K(s) + Cl2(g) 2KCl(s)

4. (a) (i) Fluorine, (ii) Astatine(b) The chlorine displaces bromine from the solution of potassium bromide, and the mixture turns reddish in colour: 2KBr + Cl2 2KCl + Br2

5. Monatomic means that the elements exist as single atoms. The noble gases exist as monatomic elements because they have a full octet structure of electrons, and hence they are stable as single atoms.

Chapter 9

Return

Solution to Quick Check 2


1. http://www.chemicool.com/

2. http://www.wou.edu/las/physci/ch412/perhist.htm

3. http://www.webelements.com/webelements/scholar/

To learn more about the Periodic Table, click on the links below!

Chapter 9


References

Chemistry for CSEC Examinations by Mike Taylor and Tania Chung

Longman Chemistry for CSEC by Jim Clark and Ray Oliver

Science

C09 periodicity of elements