Characteristics of metals

chemistry

2. Learning outcomes

Students will be able to

1. describe the general characteristics
of alkali and alkaline earth metals.

2. explain the general trends of s-
block elements.

3. know the relative reactivity of s-
block elements.

3. Introduction: .

The metals of group IA and IIA are
called s-block elements, due to the fact
that the valence electrons occupy the s-
orbitals. The elements of IA group except
hydrogen are called "Alkali Metals", while

those of IIA group are called "Alkaline
Earth Metals".

is an arabic word, which
means "The Ashes". This term was used
by Alchemists because they had found
that the plant ashes contain compounds

4. of sodium and potassium.

IIA group elements are called alkaline
earth metals, due to the fact that they
produce bases in water and are widely
distributed in earth's crust.

5. General Characteristics:

1. Alkali metals are soft and silvery white
in colour. The hardness of metals
decreases down the group so that
cesium is liquid at room temperature.
Alkaline earth metals are also white in

colour and are relatively harder than
alkali metals.

6. 2. Melting points of alkali metals are low,
the highest being that of lithium(186°C).
Melting points of alkaline earths are
much higher than those of alkali metals.
Beryllium melts at 1285°C and
Magnesium at 650°C.

. S-block metals are very reactive, highly
electropositive and strongly metallic.
This is due to their low ionization
energies. Hence, they only form positive
ions and their compounds are ionic.

7. 4. Because of their high reactivity, these
metals are never found free in nature.
They quickly tarnish in air due to
oxidation by atmospheric oxygen.
Beryllium, however does not tarnish as
quickly.

. Both alkali metals and alkaline earth
metals react with water to give
hydrogen. But the reaction with alkali
metals is very vigorous.

8. Sodium element is placed Sodium reacts with water
in water and begins to melt

Hydrogen produced self- .. and burns with a bright
ignites flame

9. 6. They also react with halogens quite
explosively.

7. Their oxides are strongly basic
although the oxides of alkaline earths
are less basic than those of alkali
metals due to their lower solubility in

water.
8. They impart characteristic colour to
the flame.

Magenta - Bright Cane
carmine yellow fae.
flame flame originates
originates originates oa

from from calcium
lithium sodium

10. General trends of s-block elements:
These are the elements in which the last
electron enters the s-orbital. These s-
electrons are involved in the bonding of
these elements. Some of the group trends
of these elements are discussed below:

1. Atomic and ionic Radii

Atomic and ionic radii increase down
the group due to the addition of new
shells. Although nuclear charge also
increases down the group, and this
factor should tend to reduce the atomic
and ionic radii, but its effect is

11. practically neutralized by a
corresponding increase in the shielding
effect of inner shells.

. Hardness, Melting points and Boiling
points:
All these properties of s-block elements

decrease down the group. These
properties are determined by the
strength of bonding between atoms in
the metal crystals.

Alkaline earth metals are harder, with
higher melting and boiling points than

12. alkali metals. This general trend is due
to the fact that in the same period,
atoms of IIA group are smaller and also
they conribute two electrons each to the
metallic bond . This accounts for the
greater hardness, melting and boiling

points of IIA group relative to IA group
metals.

. Heats of Hydration:
“Heat of hydration is the amount of heat
evolved when one mole of the ions is
hydrated by excess of water".

13. Heats of hydration decrease with the
increase in the ionic sizes of s-block
elements.

Li* ion of group IA and Be** ion of group
IIA have smallest ionic sizes in their
respective groups. Hence, their heats of

hydration are also the highest among
the members of their own families.

. lonization Energy:
In s-block elements ionization energies
decreases down the group due to
increase in atomic size. lonization
energies of alkali metals are the lowest

14. in their own periods. Lower members of
this have extremely low ionization
energies. For example, cesium emits
electrons by mere exposure to light
energy.

Because of low ionization energy, metals
of group IA and IIA are strong reducing
agents. They can easily lose electrons to
other atoms or ions, thus causing their
reduction.

15. Relative reactivity of s-block elements:
Elements of this block contains one or
two s-electrons in their outer most shells.
The electronic configuration of their
valency shells are:

Alkali Metals :ns'e.g. for Na: 1s?2s? 2p° 351
Alkaline earths:ns? e.g. for Mg: 1s? 2s? 2p* 3s?

As atomic sizes of alkaline earths are
smaller than alkali metals, therefore the
ionization energies of alkali metals are
relatively lower to the alkali earth metals.

16. These factors favour the alkali metals to
show greater reactivity than alkaline earth
metals. Following reactions illustrate the
reactivity of s-block elements:
1. Reaction with water:

Alkali metals react more vigorously

with water than the alkaline earth
metals. Thus, sodium reacts violently
with cold water while magnesium
reacts only with steam on heating.
2Na+2H20 > 2NaOH + H;
Mg + 2H20 > Mg(OH) + H2

17. 2. Action of ai
Alkali metals are rapidly attacked by
the atmosphere. Carbon dioxide and
moisture present in air convert them to
their oxides, hydroxides & carbonates.
Alkaline earths, on the other hand, are
only tarnished in air forming a protective

coating of the oxide.

Lithium pellets covered in white lithium hydroxide (left)
and ingots with a thin layer of black oxide tarnish (right)

18. 3. Combustion in air:
Alkaline earths burn in oxygen forming
normal oxides. Alkali metals, except Li,
form higher oxides.
2MgO +0, > 2MgO
2Na + 0 > Na202

K +02 > KO,
4. Reducing nature:
Alkali metals are relatively stronger
reducing agents than the alkaline
earths. Thus, sodium can reduce MgO
to metallic magnesium.
MgO +2Na > Mg + Naz0

20. MCQ

1. IA group elements are called Alkali
metals, because their hydroxides

A. increase the pH of water.

B. decrease the pH of water.

C. do not affect the pH of water.
D. are insoluble in water.

21. 2. Which of the following elements
is more reactive?

A. Lithium

B. Beryllium
C. Magnesium
D. Potassium

22. 3. Which of the following elements is
expected to be stronger reducing

agent?

A. Lithium
B. Beryllium
C. Calcium
D. Cesium