Families of Elements – SS2 Chemistry Lesson Note (First Term)

Elements in the same group form a family because their atoms have the same number of valence electrons, so they show similar properties. Within a group, some properties change gradually as atomic number increases. This gradual change is called a group trend. This note describes the properties of Groups I to VII and Group 0.

Behavioural objectives

By the end of the lesson, students should be able to:

• explain why elements in the same group are called a family;
• define group trend;
• name the elements in Groups I to VII and Group 0;
• state the main properties of each group;
• write equations for some reactions of the group elements and their oxides.

What are the Group I elements?

The Group I elements include Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs) and Francium (Fr). They are univalent elements. The properties are as follows:

1. They are good reducing agents since they can readily donate one electron to form a cation.
2. They are metals, so they are good conductors of electricity and heat.
3. They react vigorously with cold water to liberate hydrogen gas and form an alkali, hence they are known as alkali metals. Example:
   2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
4. Their oxides dissolve in water to give very strong alkalis. Example:
   K₂O(s) + H₂O(l) → 2KOH(aq)

What are the Group II elements?

Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra) belong to Group II. They are divalent elements and are also known as alkaline earth metals. Their properties include:

1. They ionise by donating their two valence electrons, so they are good reducing agents.
2. They are hard, ductile and malleable metals, and they conduct both electricity and heat.
3. Beryllium does not react with cold water or steam, magnesium reacts with steam only, while calcium reacts slowly with cold water to liberate hydrogen gas.
4. Their oxides are insoluble in water, except calcium oxide, which dissolves in water to form an alkali. Example:
   CaO(s) + H₂O(l) → Ca(OH)₂(aq)

What are the Group III elements?

The Group III elements are Boron (B), Aluminium (Al), Gallium (Ga), Indium (In) and Thallium (Tl). They are trivalent elements. Their properties are:

1. They are reducing in nature since they can donate their three valence electrons to form electrovalent compounds.
2. Only aluminium reacts with steam, at about 750 °C, to liberate hydrogen gas.
3. The oxide and hydroxide of aluminium are amphoteric, that is, they have both acidic and basic properties. Examples:
   a) Al₂O₃(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂O(l)
   b) Al(OH)₃(s) + NaOH(aq) → NaAl(OH)₄(aq)

What are the Group IV elements?

Group IV elements include Carbon (C), Silicon (Si), Germanium (Ge), Tin (Sn) and Lead (Pb). Their properties are:

1. They form covalent compounds.
2. They exhibit two oxidation states: +2 and +4. Because of the inert pair effect of the electrons in the s-orbital of the valence shell, the +2 oxidation state becomes more prominent down the group.
3. Electropositivity increases down the group. Carbon is a non-metal, silicon and germanium are metalloids, while tin and lead are metals.
4. Carbon does not react with water in any form, but silicon and tin react with steam at red heat to form +4 state oxides and hydrogen. Example:
   Si(s) + 2H₂O(l) → SiO₂(s) + 2H₂(g)

What are the Group V elements?

Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi) belong to Group V. They have the following properties:

1. They exhibit oxidation states of -3 and +5.
2. They show group trends. Nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids, while bismuth is a metal.
3. They are electron acceptors, hence they are oxidising in nature.
4. They form oxides that dissolve in water to form acids, except nitrogen(I) oxide.

What are the Group VI elements?

Elements in Group VI include Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te) and Polonium (Po). Their properties are as follows:

1. They are non-metals and exist as solids at room temperature, except oxygen.
2. They are electron acceptors and oxidising in nature.
3. They do not react with water in any form. However, oxygen and sulphur combine directly with hydrogen to yield water and hydrogen sulphide respectively.

What are the Group VII elements?

Elements in this group include Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). They are known as halogens (salt-makers). Their properties include:

1. They ionise to form univalent anions.
2. They exist as diatomic molecules.
3. As electron acceptors, all halogens are good oxidising agents.
4. They exhibit group trends. Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids at room temperature.

What are the Group VIII (0) elements?

The elements in Group 0 are known as rare or noble gases because they are non-reactive and exist freely as monatomic molecules in the atmosphere. The elements that belong to this group are Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn).

Evaluation

1. Why are elements in the same group said to belong to a family?
2. What is a group trend?
3. Why are the Group I elements called alkali metals?
4. State the oxidation states shown by the Group IV elements and say which becomes more prominent down the group.
5. Describe the physical state of the halogens at room temperature.

Assignment

1. Write the equation for the reaction of sodium with cold water.
2. Explain why the oxide and hydroxide of aluminium are described as amphoteric.
3. List the elements in Group 0 and give two reasons why they are called noble gases.