Groups I, II and VII elements

Learning Objectives

• Describe metallic bonding • Describe and explain trends in atomic radii, melting (and boiling) points and density for Groups 1, 2 and 7 and Period 3.

Where are the alkali metals?

The elements in group 1, on the left of the periodic table, are called the

alkali metals

.

lithium Li sodium potassium rubidium caesium francium Na K Rb Cs Fr

These metals are all very reactive and are rarely found in nature in their elemental form.

Why are they called the ‘alkali metals’?

The alkali metals are so reactive that, as elements, they have to be stored in oil. This stops them reacting with oxygen in the air.

Alkali metals are soft enough to be cut with a knife, and the most common alkali metals, lithium, sodium and potassium, all float on water.

The elements in group 1 also react with water and form alkaline compounds. This is why they are called

alkali metals

.

What are the properties of the alkali metals?

The characteristic properties of the alkali metals are:  They are

soft

and can be cut by a knife. Softness increases going down the group.

 They have a

low density

.

Lithium, sodium and potassium float on water.

 They have

low melting and boiling points

.

These properties mean that the alkali metals are different to typical metals. However, alkali metals do also share some properties with typical metals:  They are

good conductors

of heat and electricity.

 They are

shiny

. This is only seen when alkali metals are freshly cut.

What are the trends in density?

What is the trend in density?

The alkali metals generally become

more dense

going down the group, but the trend is not perfect because potassium is less dense than sodium.

Element lithium sodium potassium rubidium caesium Density (g/dm 0.53

0.97

0.86

1.53

1.87

3 )

Water has a density of 1 g/dm 3 . Which elements in group 1 will float on water?

What are the trends in melting point?

What are the trends in boiling point?

What is the trend in melting and boiling points?

The melting points and boiling points of alkali metals

decrease

going down the group.

Element Melting point ( °C) Boiling point ( °C) lithium sodium potassium rubidium caesium 181 98 64 39 28 1342 883 760 686 671

The melting and boiling points decrease going down group 1 because the atoms get larger. Melting points are lower than for typical, transition, metals, because alkali metals only have 1 electron in their outer shell. Not much heat energy is needed for this electron to be lost.

Electron structure All alkali metals have 1 electron in their outer shell. This means that:

lithium 2,1

 They can easily obtain a full outer shell by losing 1 electron.

sodium 2,8,1

 They all lose their outer shell electron in reactions to form

positive ions

with a +1 charge.

potassium 2,8,8,1

 They have similar physical and chemical properties.

Explaining the trend in atomic radius The atomic radius of the elements

increases

down group 2 from beryllium to barium.

Element Atomic radius (nm)

The number of protons increases down the group; however, so does the number of shielding electrons. Effective nuclear charge therefore remains approximately constant.

beryllium magnesium calcium strontium barium 0.112

0.145

0.194

0.219

0.253

The increase in radius is due to

higher

principle energy levels being filled, whose orbitals are located

further

from the nucleus.

Electron structure and reactivity The reactivity of alkali metals increases down the group.

What is the reason for this?

Li Na K Rb Cs

 The size of the element’s atoms, and the number of full electron shells, increases down the group.

 This means that, down the group, the electron in the outer shell gets further away from the nucleus and is shielded by more electron shells.

 The further away an electron is from the positive attraction of the nucleus, the easier it can be lost in reactions.

 This means that reactivity increases with the size of the atom.

Group 7 – the halogens

The elements in group 7 of the periodic table, on the right, are called the

halogens

.

F fluorine Cl chlorine Br I bromine iodine At astatine

Why are they called the ‘halogens’?

Halogens are very reactive non metals.

They are all toxic or harmful because they are so reactive. Before antiseptics, iodine was used to clean wounds as it is harmful to all things, including bacteria.

They are also never found free in nature because of their reactivity – they are found as compounds with metals.

These halogen-metal compounds are salts, which give halogens their name – ‘halo-gen’ means ‘salt-former’.

What are the general properties of the halogens?

All the halogens are:  non-metals and so do not conduct electricity  brittle and crumbly when solid  poisonous and smelly.

They become darker in colour down the group: is pale yellow is yellow-green is red-brown is grey

Halogens – what do they look like?

Chlorine Bromine Iodine

What is the physical state of the halogens?

The melting and boiling points of the halogens increase down the group, as the molecules become bigger.

Halogen Relative size Melting point ( °C)

-220

Boiling point ( °C)

-118

State gas

-101 -34

gas

-7 59

liquid

114 184

solid

What is the state of each halogen at room temperature?

Melting and boiling points of halogens

Halogen vapours

Bromine and iodine are not gaseous, but have low boiling points. This means that they produce vapour at relatively low temperature. They are

volatile

.

Bromine produces some red-brown vapour, seen here above the liquid bromine in the jar.

When iodine is heated gently, it changes directly from a solid to a gas without first becoming a liquid.

This is called

sublimation

.

What is the electron structure of the halogens?

All halogens have

seven electrons

in their outer shell. This means that:

fluorine 2,7

 They can easily obtain a full outer shell by gaining one electron.

 They all gain an electron in reactions to form

negative ions

with a -1 charge.

chlorine 2,8,7 bromine 2,8,8,7

 They have similar chemical properties.

How does electron structure affect reactivity?

The reactivity of alkali metals

decreases

going down the group. What is the reason for this?

 The atoms of each element get larger going down the group.

F

 This means that the outer shell gets further away from the nucleus and is shielded by more electron shells.

Cl

 The further the outer shell is from the positive attraction of the nucleus, the harder it is to attract another electron to complete the outer shell.

Br

 This is why the reactivity of the halogens decreases going down group 7.

How do halogen molecules exist?

All halogen atoms require one more electron to obtain a full outer shell and become stable.

Each atom can achieve this by sharing one electron with another atom to form a single

covalent bond

.

F + F



F F

This means that all halogens exist as

diatomic

molecules:

F 2

,

Cl 2

,

Br 2

and

I 2

.

Reactions of the Group I and Group VII elements

How do the alkali metals react with oxygen?

All alkali metals react with oxygen in the air to form

metal oxides

. This produces a layer of dull oxide on the surface of the metal, called

tarnish

.

The speed with which alkali metals react with oxygen in the air increases going down the group:  lithium – tarnishes slowly   sodium – tarnishes quickly potassium – tarnishes very quickly.

Why are alkali metals stored in

oil

?

The oil prevents them from reacting with oxygen and tarnishing.

What is the equation for the reaction with oxygen?

The reaction between an alkali metal and oxygen is an example of an

oxidation

reaction:

alkali metal + oxygen



alkali metal oxide

The word and chemical equations for the reaction between lithium and oxygen are:

lithium + 4Li

(s)

+ oxygen



O 2

(g) 

lithium oxide 2Li 2 O

(s) What are the word and chemical equations for the reaction that causes sodium to tarnish?

sodium + 4Na

(s)

+ oxygen



O 2

(g) 

sodium oxide 2Na 2 O

(s)

What does the reaction with water produce?

All the alkali metals react vigorously with water. The reaction with water becomes more vigorous as you go down the group.

It is an

exothermic

reaction as it releases a lot of heat.

The reaction produces a gas that ignites a lighted splint with a squeaky pop. What is this gas?

When green universal indicator is added to the reaction mixture, it turns purple. What does this tell you about the products of this reaction?

What is the equation for the reaction with water?

This reaction creates

alkaline hydroxide

ions. This is why the group 1 elements are called the alkali metals.

The general equation for the reaction between an alkali metal reacting with water is:

alkali metal + water



alkali metal + hydrogen hydroxide 2M(s) + 2H 2 O(l)



2MOH (aq) + H 2 (g)

Lithium with water

How does lithium react with water?

Lithium is the least reactive of the alkali metals. When added to water, it fizzes and moves around slowly across the surface of the water.

What is the equation for this reaction?

lithium + water



2Li

(s)

+ 2H 2 O

(l) 

lithium hydroxide + 2LiOH

(aq)

+ hydrogen H 2

(g)

Sodium with water

How does sodium react with water?

When added to water, sodium fizzes more than lithium, and moves quickly across the surface of the water. The sodium melts as it reacts, and it becomes spherical and shiny, like a ball bearing.

The hydrogen sometimes catches fire because of the heat from the reaction.

What is the equation for this reaction?

sodium + water



2Na

(s)

+ 2H 2 O

(l) 

sodium hydroxide + 2NaOH

(aq)

+ hydrogen H 2

(g)

Potassium with water

How does potassium react with water?

When added to water, the potassium moves across the surface of the water very quickly.

The reaction produces so much heat that the hydrogen given off catches alight. What colour would the flame be?

Like sodium, it melts with the heat of the reaction.

What is the equation for this reaction?

potassium 2K

(s)

+ water



+ 2H 2 O

(l) 

potassium + hydroxide 2KOH

(aq)

+ hydrogen H 2

(g)

How do alkali metals react with chlorine?

Alkali metals burst into flames when heated and added to chlorine. They form

metal chlorides

.

alkali metal + chlorine



alkali metal chloride

The word and chemical equations for the reaction between lithium and chlorine are:

lithium + chlorine



lithium chloride 2Li

(s)

+ Cl 2

(g) 

2LiCl

(s) What are the word and chemical equations for the reaction that produces sodium chloride?

sodium + chlorine



sodium chloride 2Na

(s)

+ Cl 2

(g) 

2NaCl

(s)