Chemical Families - Chemistry Form 2
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.
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.
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.
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.
Sodium is used in the extraction of titanium metal.
Sodium chloride is used as food additive and food preservative.
ALKALINE EARTH METALS
Alkaline earth metals are members of group II in the periodic table. They have two electrons in their outermost energy level.
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.
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.
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 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.
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.
The electron arrangement of halogens is as follows.
Show the above illustration
The above illustration shows that halogens have seven electrons in their outermost energy level.
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.
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