1. Periodicity Of Elements
Chapter 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
2. The elements are arranged in order of increasing
proton (atomic) number.
There are 7 horizontal rows called “Periods”.
Period
1
2
3
4
5
6
7
Chapter 9
Periodicity Of Elements
3. 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.
Transition elements
1 2 3 4 5 6 7 0
Chapter 9
Groups
Periodicity Of Elements
4. 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)
Periodicity Of Elements
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)
Periodicity Of Elements
6. 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:
Periodicity Of Elements
7. 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
Periodicity Of Elements
8. Metals and non-metals
As we go across a period from left to right, the elements change from metals to
metalloids and then to 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
Periodicity Of Elements
9. 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
Periodicity Of Elements
10. 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
Periodicity Of Elements
11. Quick check 1
1. 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
Periodicity Of Elements
12. 1. The elements in the Periodic Table are arranged in the order of
increasing proton number.
2. Group
3. Period
4. Group I
5. Period 4
6. Transition elements
7. Metals: caesium, calcium
Non-metals: boron, silicon, phosphorus, selenium, iodine
8. 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
Periodicity Of Elements
13. 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
Periodicity Of Elements
14. 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
39
28
27
Meltingpoint(oC)
Li Na K Rb Cs Fr
0.53
0.97
1.53
0.86
Density(gcm-3)
1.87
Li Na K Rb Cs Fr
Group I (The Alkali Metals)
Density and melting point
Chapter 9
Periodicity Of Elements
15. 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
Periodicity Of Elements
16. 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
Periodicity Of Elements
17. 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
Periodicity Of Elements
18. The elements become darker in
colour as we go down the group.
Fluorine is an almost colourless
gas, chlorine is a greenish-yellow
gas, bromine is a reddish liquid,
iodine is a purplish-grey solid,
while astatine is a dark black
solid.
Darker
colour
Lighter
colour
Colour of the Halogens
Group VII (The Halogens)
Chapter 9
Fluorine, F
Chlorine, Cl
Bromine, Br
Iodine, I
Astatine, At
Periodicity Of Elements
19. 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
Periodicity Of Elements
20. 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
Most
reactive
Group VII (The Halogens)
Chemical properties
Chapter 9
Fluorine, F
Chlorine, Cl
Bromine, Br
Iodine, I
Astatine, At
Periodicity Of Elements
21. Less
reactive
More
reactive
2KI(aq) + Cl2(g) 2KCl(aq) + I2(s)
Can
displace
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.
Periodicity Of Elements
22. 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
Periodicity Of Elements
23. 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
Periodicity Of Elements
24. 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 3
Trends in physical properties across the period
Periodicity Of Elements
25. 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 3
Trends in chemical properties across the period
Periodicity Of Elements
26. 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
Periodicity Of Elements
27. 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
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Solution to Quick Check 2
Periodicity Of Elements
