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Chemical Families

CHEMICAL FAMILIES (PATTERNS IN PROPERTIES)

 

Group I elements are called alkali metals. This is because they have only one electron in their outermost energy level. However, they differ in the number of energy levels. Lithium has two energy levels, sodium has three while potassium has four energy levels. This is because of their increasing atomic numbers.

Objectives

By the end of this lesson, you should be able to

:

i) Identify alkali and alkaline earth metals and write their electron arrangement.
ii) Explain the trends in properties of alkali and alkaline earth metals.
iii) State the uses of alkali and alkaline earth metals.

The atomic radius of an element is the distance between the centre of the nucleus of its atom and the outermost energy level.The trend in atomic radius of Lithium, Sodium and Potassium can be illustrated as follows.The atomic radii of group 1 elements increases down the group due to increase in the number of energy levels.

CHARACTERISTICS OF GROUP 1 ELEMENTS

The atomic radii of alkali metals increase down the group. This is due to increase in the number of energy levels.

ANIMATION

Shows the structure of lithium, sodium and potassium atoms containing only energy levels.use arrows to indicate atomic radius in each case i.e.

 

The ionization energy is the minimum amount of energy needed to remove the outermost electron from an atom in gaseous state.

The ionization energy of group 1 elements decreases down the group.

The table below illustrates the trend in melting and boiling points of group 1 elements.

Melting and boiling points of alkali metals decrease down the group. This is because as you move down the group, the atomic radius increases leading to decrease in strength of the metallic bonds.

When sodium reacts with water, hydrogen gas is evolved and sodium hydroxide solution formed.Click to play the video to observe the reaction between Sodium and water.

The video clip shows the reaction between potassium and water.Potassium reacts more vigorously with water than sodium.Click to play the video to observe the reaction between Potassium and water.

This experiments shows that reactivity of alkali metals increases down the group. This is because alkali metals react by losing electron and the ease with which the electrons are lost increases down the group.

Na(s) + H2O(l) w NaOH(aq) + H2(g)

 

K(s) + H2O(l) w KOH(aq) + H2(g)

Alkali metals react readily with chlorine to form metal chlorides.

Click to play the video to observe the reaction between Lithium and Chlorine gas.

Alkali metals react readily with chlorine forming white solids of their chloride. Lithium forms a white solid of lithium chloride while sodium forms a white solid of sodium chloride.The following equations show the products formed when alkali metals react with Chlorine gas.

Alkali metals form ions with one positive charge in their compounds. They combine with hydroxides, oxides and chlorides ion to form their respective components.The table below shows the different compounds formed by alkali metals.

 

Molten sodium is used as a coolant in nuclear reactants.

Nuclear reactant

Sodium vapour is used in lamps for street lighting. Click to play the video to observe lighting within a street.

Lithium Chloride and lithium fluoride are used in the manufacture of glass.They determine the melting point of glass and reduce the chances of glass cracking at high temperatures.

Glass apparatus

Sodium is used in the extraction of titanium metal.

Titatium metal

Sodium chloride is used as food additive and food preservative.

Food additive

ALKALINE EARTH METALS

Alkaline earth metals are members of group II in the periodic table. They have two electrons in their outermost energy level.

QUIZ

Atomic radius is the distance between the nucleus of the atom and the outermost energy level.The atomic radii of alkali earth metals increases down the group.The more the energy levels in an atom, the higher the atomic radius. The atomic radius of alkaline earth metals increases down the group, with increase in atomic numbers.

CONCLUSION

 

First ionization energy is the energy needed to remove an electron from an atom in gaseous state. The second ionization energy is the energy required to remove an electron from a gaseous ion with a single positive charge.Below is a summary of the first and the second ionization energies of alkaline metals. The first and second ionization energies of alkaline earth metals decrease down the group.

Below is a summary of the first and the second ionization energies of alkaline metals. The first and second ionization energies of alkaline earth metals decrease down the group.

CONCLUSION

 

The first and second ionization energies of alkaline earth metals decrease down the group

ACTIVITY

 

Alkaline earth metals have relatively high melting and boilng points than corresponding alkali metals. Melting and boiling points of alkaline earth metals generally decrease down the group with increase in atomic number.

Alkaline earth metals atoms lose two electrons during chemical combination to form a positive ion.The ease of forming ions increase down the group and so does chemical reactivity.Alkaline earth metals tarnish slowly on exposure to air due to formation of oxide coatings. Click to play the video to observe.

Magnesium burns in air with a brilliant flame forming a white solid of magnesium oxide.The solution formed when magnesium oxide is dissolved in water turns red litmus blue but does not change blue litmus, magnesium hydroxide is therefore alkaline.Click the video below to observe this reaction.

Calcium burns in air in a brick _red flame forming calcium oxide

Video

A video clip

Shows burning of calcium in deflagrating spoon to form brick red flame and cooling to form a white solid.

Calcium + air                         calcium oxide

2ca (s)+ 02(g)                       2cao(s)

The oxide of calcium is slightly soluble in water

Animation

Video clip

Calcium oxide prepared in the clip before is dissolved in water and both red and blue litmus paper are dipped in the resulting solution

Calcium oxide is slightly soluble in water to form a solution of calcium hydroxide. The solution turns red litmus paper blue while the blue litmus paper remains blue.

Calcium oxide + water                             calcium hydroxide solution

2cao(s) + H2o(l)                          Ca(OH)2(aq)

Calcium burns in air with a brick red flame.Calcium oxide prepared is dissolved in water and both red and blue litmus paper are dipped in the resulting solution
Calcium oxide is slightly soluble in water to form a solution of calcium hydroxide. The solution turns red litmus paper blue while the blue litmus paper remains blue.

Magnesium reacts slowly with cold water. The illustration below shows Magnesium ribbon in water. Roll the mouse over each test tube to view the explanation.

Calcium reacts vigorously with water. Click to play the video to observe how Calcium reacts with water and how Hydrogen gas produced is tested.

As we move down the group the reactivity of group 2 elements increases.This is due to the ease with which electrons are lost.

Magnesium reacts rapidly with steam. Magnesium reacts with steam to produce a white solid and hydrogen gas.

Burning Magnesium ribbon reacts with chlorine gas forming a white solid of Magnesium chloride.

Magnesium and calcium react vigorously with dilute acids.The following illustration shows the reaction of calcium in an acid.

All alkaline earth metals have a valency of two and this compounds show similarities in their chemical formulae. Click on the blank spaces in the table provided to confirm the correct formulae for the various compounds.

The following are some of the uses of alkaline earth metals. Magnesium is used in flashlight photography and in flares and fireworks.The illustration below shows fireworks.

Magnesium Hydroxide is used as an anti-acid.

Calcium compounds ( Calcium Oxide, limestone and gypsum are used in the manufacture of cement.

HALOGENS

By the end of this lesson you should be able to:

i) State and explain trends in physical and chemical properties of halogens.

ii) Explain the unreactive nature of noble gases and state their uses.

ELECTRON ARRANGEMENT.

The electron arrangement of halogens is as follows.

ELEMENT

SYMBOL

ELECTRON ARRANGEMENT

FLOURINE

CHLORINE

F

CI

2.7

2.8.7

 

Show the above illustration

The above illustration shows that halogens have seven electrons in their outermost energy level.

 

ACTIVITY

 

 

The atomic radii of halogens increases down the group.

Atomic radii of halogens increase down the group as the atomic number increases.

Halogens form ions by gaining one electron in their outermost energy level.

The animation below shows the comparison between atomic radii and ionic radii of halogens.

Note that the ion formed is larger than its atom.

When an atom gains an electron, the overall attraction of the electrons by the protons is reduced because of the increased repulsive effect between electrons. In addition the ion has more electrons than protons. Therefore each electron experiences a smaller pull towards the nucleus.Atomic radius of halogens is smaller than ionic radius.

Halogens exist as diatomic molecules .Chlorine molecules, Bromine molecules and Iodine molecules.The forces of attraction between individual molecules are very weak leading to low melting and boiling points. The table below shows the melting points and boiling points of some halogens.

 

The melting and boiling points of halogens increases down the group. This is because the force of attraction between individual molecules increases with size of the atom. Therefore fluorine and chlorine are gaseous; Bromine is a liquid while iodine is a solid at room temperature. The ability to gain an electron decreases as the size of the atom increase. This is because when the size of the atom is small the nucleus is less shielded and can easily attract an electron. When the atomic radius is big the nucleus is more shielded hence the ability to gain an electron is less.

The melting and boiling points of halogens increases down the group. This is because the force of attraction between individual molecules increases with size of the atom. Therefore fluorine and chlorine are gaseous; Bromine is a liquid while iodine is a solid at room temperature.
The ability to gain an electron decreases as the size of the atom increase. This is because when the size of the atom is small the nucleus is less shielded and can easily attract an electron. When the atomic radius is big the nucleus is more shielded hence the ability to gain an electron is less.

Halogens are non conductors of both heat and electricity. Note that the bulb does not light.

ACTIVITY

 

 

Chlorine reacts with water as shown in the following videos.Click to play the video to observe the reaction of Chlorine gas and water.

 

By the end of this lesson you should be able to

State and explain trends in physical and chemical properties of halogens Explain the unreactive nature of noble gases and state their uses.

ACTIVITY 

HALOGENS

LESSON OBJECTIVES:

By the end of this lesson you should be able to:

  1. Identify halogens and noble gases in the periodic table.
  2. State and explain trends in properties of halogens and noble gases.
  3. State the uses of halogens and noble gases.

 

The procedure below is used to test the reaction between bromine and water. Add 2cm3 of bromine liquid to about 250cm3 of water in a large beaker. Place red and blue litmus papers in the resulting solution and note the colour changes.

Bromine liquid in a flask

Conlusion:

When halogens react with water they form acidic solutions that have bleaching effects. The blue litmus paper change to red to show the acidic nature of the solution. Both papers finally turn white because they are bleached.

The equations are:

Animation:

Show the equation below

CL2(g) + H2O(l)                            HCL(aq) + HCLO(aq)

And                                               chloric(I)acid

Br2(l) + H2O(l)                                 HBr(aq) + HBrO(aq)

                                                      Bromic (I) acid

Bromic (I) acid (HBrO aq) and Chloric (II) acid (HClO aq) are responsible for the bleaching action of the solution. Iodine is sparingly soluble in water.

Fluorine reacts most vigorously with water.

 

ACTIVITY

 

 

Halogens react with water to form acidic solutions with bleaching properties. Both red and blue litmus papers finally turn white showing that they are bleached .The bleaching action is due to formation of chloric (I) acid and bromic (I) acids which are bleaching agents.Bromic (I) acid and Chloric (II) acid are responsible for the bleaching action of the solution. Iodine is sparingly soluble in water.Fluorine reacts most vigorously with water.

 

Sodium is heated until it starts to burn. The sodium metal is then lowered into a gas jar containing Chlorine gas. Zinc and Iron are heated until they are redhot and lowered into separate gas jars containing Chlorine gas.

Chlorine gas reacts with heated sodium, zinc and iron to form the corresponding metal chloride. The equations for the reactions are:

The set-up below may be used to investigate the reaction between metals and bromine .

Heated Iron glows when bromine vapour is passed over it. Brown Iron (III) bromide solid is formed. Iodine vapour reacts with heated iron to form Iron II iodide.Heated Zinc reacts with Bromine and Iodine vapour to form white Zinc Bromide and Zinc Iodide solid respectively.

ACTIVITY

 

.

Compounds of halogens have similar formulae since all halogens have a valency of one.

Chlorine is used in water treatment, to chlorinate drinking water, swimming pools and sewage in order to kill micro-organisms.Click to play the video to observe how Chlorine is used to treat water in a swimming pool.

Chlorine is used for making bleaches

Jik bleach

Chlorine is used to manufacture hydrochloric acid

Chlorine is used in manufacturing plastic known as P.V.C that is used in manufacture of pipes.

Hydrogen fluoride is used to engrave words or pictures on glass items.

Some fluorides are used in the manufacture of tooth paste.

Noble gases are members of group (VIII) of the periodic table.

Atomic radius of noble gases increases down the group.

The word noble gases is used to refer to elements of group VIII or Group  (0) of the periodic table.

The atoms of the noble gases have their outermost energy levels completely filled with electrons. Helium has a stable duplet while Neon and Argon have stable octets.Therefore, noble gases are generally non-reactive.

The following table shows some members of this group.

Noble gases have various uses.

 

 

 

 


 

 



 Chemical Families 

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