Topic: Families of Elements
Group 1: Alkali Metals
The members of this family include: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr). They are soft metallic solids, with metal properties such as:
- One valence electron
- Shiny and lustrous
- High thermal and electrical conductivities
- Low densities, increasing with atomic mass
- Relatively low melting points, decreasing with atomic mass
- Hydrogen is not considered an alkali metal because the gas does not exhibit the typical properties of the group. Where alkali metals lose their valence electron, hydrogen normally shares its valence electron. The fact that hydrogen can actually gain an electron when forming hydrides further illustrates that it is not truly a member of the alkali metal family.
- Ionize to lose their electron, so the ion has a +1 charge. They are electropositive.
Na ——-> Na+ + e–
- Vigorous exothermic reaction with water to produce hydrogen gas and an alkalis, so they are known as alkali
2K(s) + 2H2O(l) ———-> 2KOH(aq) + H2(g)
- The oxides of alkali metals also dissolve in water to form very strong alkalis.
Na2O(s) + H2O(l) ———–> 2NaOH(aq)
- On heating, their trioxonitrate (V) salts decompose with difficulty to give oxygen and dioxonitrate (III) salts.
2KNO3(s) ————> 2KNO2(s) + O2(g)
Group 2: The Alkaline Earth Metals
The members of the alkaline earth metals include: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra).
- The alkaline earth metals have 2 valence electrons.
- They have characteristic metal properties but are harder, more dense, and melt at higher temperatures than the alkali metals.
- Shiny, lustrous, oxidize easily
- High thermal and electrical conductivities
- Ionize to lose their valence electrons, so the ion has a +2 charge
Ca ———-> Ca2+ + 2e-
- Their reaction with water increases down the group.
- Beryllium does not react with water, even when heated red-hot. Magnesium does not react with liquid water but does react with steam. Calcium and the elements below it react with water at room temperature, although more slowly than the alkali metal adjacent to them on the periodic table.
Ca(s) + 2H2O(l) ————-> Ca(OH)2(aq) + H2(g)
Mg(s) + H2O(g) ————–> MgO(s) +H2(g)
- Magnesium and calcium are essential for living organisms. 99% of the calcium in the human body is found in the skeletal system.
- Their trioxonitrate (V) salts decompose readily on heating to give the oxides, oxygen and the brown gas, nitrogen (IV) oxide.
2Ca(NO3)2(s) ———-> 2CaO(s) + O2(g) + 4NO2(g)
- Their trioxocarbonates (IV) are insoluble in water. They decompose on heating to give the oxides and carbon (IV) oxide.
MgCO3(s) ————> MgO(s) + CO2(g)
Group 3: Boron Family
The boron family is not as well-known as some of the other element families.
- Members of this group have 3 valence electrons. Being a potential donor of three electrons. It is reducing in nature and form electrovalent compounds.
Al ———> Al3+ + 3e–
- Aluminum, the most familiar and useful member, is found in nature as bauxite – Al2O3.
- Aluminium reacts with steam above 600oC to liberate hydrogen
- Aluminium trioxonitrate (V) decomposes to give the oxide, oxygen and nitrogen (IV) oxide gas.
4Al(NO3)3(s) ———–> 2Al2O3(s) + 3O2(g) 12NO2(g)
Group 4: Carbon Family
The carbon group is made up of elements called tetrels, which refers to their ability to carry a charge of 4. Members of this group have 4 valence electrons which include carbon, silicon, germanium, tin and lead. The best known member of this group is carbon, which commonly forms 4 bonds. The compound of Group 4 elements exhibit two oxidation states, +2 and +4.
Carbon is unique among the elements because of its ability to combine with itself and other elements to form an almost limitless number of compounds. The hydrides of carbon are very numerous and are known as hydrocarbons. Examples are methane, CH4, ethane, C2H4, benzene, C6H6 etc.
Carbon is found in nature in the form of coal and diamonds. The difference between them is their crystalline structure, which gives them very different physical properties. Diamond is the hardest of all known substances.
Group 5: The Nitrogen Group (Pnictogens)
Nitrogen and phosphorus belong to this group. They are non-metals and show two common valencies of 3 and 5. The best known member of this group is nitrogen.
Nitrogen makes up about 80% of the air. At normal temperatures and atmospheric pressures, molecular nitrogen is almost inert. They form several oxides such as:
- Nitrogen (III) oxide, N2O3,
- Phosphorus (III) oxide, P4O6,
- Nitrogen (V) oxide, N2O5
All these oxides are acidic and will combine with water to form acids. Nitrogen and phosphorus also form similar hydrides, NH3 and PH3.
Group 6: Oxygen Family or Chalcogens
There is a change from non-metallic to metallic character as we move down this group. Oxygen, sulphur, and selenium are typical nonmetals. Tellurium has some metallic properties and is classified as a metalloid. Polonium, which is radioactive and quite rare, is a metal. They have 6 valence electrons and best known member of this group is oxygen.
Both oxygen and sulphur combine directly with hydrogen to yield water and hydrogen suphide respectively.
2H(g) + O2(g) ————-> 2H2O(l)
H2(g) + S(s) —————> H2S(g)
Group 7: The halogen family
The halogen family is a group of reactive non-metals. The elements included are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The halogen means salt former.
Their melting and boiling points increase with increasing atomic number. Fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid.
Halogen shows great similarity in their properties because each of their atoms has the same number of valence electrons, i.e. seven. Some of these similarities are as follow:
- They are all non-metals.
- They exist as diatomic molecules.
- They are coloured.
- They ionized to form univalent negative ions which react with dissolve readily In water to form acids.
- Halogens are highly active with high electron affinities. In fact, the letter X is sometimes used in chemical equations to indicate any one of the halogen elements. Fluorine gas is so reactive that it is difficult and dangerous to use, requiring specialized lab equipment. Unlike fluorine, chlorine reacts slowly with water to form relatively stable aqueous solutions. Chlorine is often added to drinking water and swimming pools, where the HOCl(aq) that is produced serves as a disinfectant. Halides are all very soluble in water and dissolve to form the hydrohalic acids.
Group 8 or 0: Noble Gases or Inert Gases
The noble gases are a family of nonreactive non-metals. Stable electron octet makes these elements unreactive under ordinary circumstances. The list includes helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). They have 8 valence electrons and typically exist as monoatomic gases, although these elements do (rarely) form compounds.
The transition metals are placed in the centre of the periodic table, between groups 2 and 3. They are generally hard and dense, and less reactive than the alkali metals. Iron, copper, silver and gold are important transition metals. The lanthanides and actinide which are in period 6 and 7 are known as the inner transition elements.
The transition metals have the following properties in common:
- They form coloured compounds
- They are good conductors of heat and electricity
- They can be hammered or bent into shape easily
- They are less reactive than alkali metals such as sodium
- They have high melting points – but mercury is a liquid at room temperature
- They are usually hard and tough
- They have high densities
- These large atoms exhibit a range of oxidation states.