Chem 3.3

Discuss the similarities and differences in the chemical properties of elements in the same group.

Physical properties of alkali metals:

• They are good conductors of electricity
• They have low densities
• They have grey shiny surfaces when freshly cut with a knife

Chemical properties of alkali metals

• They are very reactive metals
• They form ionic compounds with non-metals
• They form single charged positive ions
• They have a low ionization energy
• Reactivity increases down the group as they have lower ionization energies
• Their ability to cunduct electricity is due to the mobility of the outer electron

Physical properties of halogens:

• They are coloured
• They show a gradual change from gasses (F and Cl) to liquid (Br) to solid (I and At)

Chemical properties of halogens

• They are very reactive non-metals
• Reactivity decreases down the group because the nuclei have a high effective charge and exert a strong pull on other atoms, this decreases with the increase in atomic radius
• They form ionic compounds with metals or covalent compounds with other non-metals

Discuss the changes in nature, from ionic to covalent and from basic to acidic of the oxides across period 3.

Bonding of period 3 oxides:

• Ionic compounds are formed between metals and non-metals so the structure of the oxides of elements Na to Al is giant ionic.
• Covalent compounds are formed between non-metals so the oxides of P to Cl are molecular covalent.
• Si is a metalloid and has a giant covalent structure.
• The oxidation number of a period 3 element corresponds to the group number.

Acid base character of the period 3 oxides:

• Na2O and MgO are basic
• Al2O3 and SiO2 are amphoteric
• P4O10/P4O6, SO3/SO2 and Cl2O7/ClO are acidic

Alkali metals react with water to form hydrogen and the metal hydroxide:

• Lithium floats and reacts slowly but keeps its shape.
• Sodium reacts with a vigorous release of hydrogen, the heat released melts the metal that forms a small ball and whizzes around on the water.
• Potassium reacts even more vigorously, it makes enough heat to set fire to the hydrogen making a purple flame and zooms around even faster.

The metals are called alkali metals because the solution formed is alkali due to the presence of hydroxide.

The reaction becomes more vigorous down the group because they form ions more readily.

Halogen reaction with group 1 metals:

• They form ionic halides
• The electrostatic charge between the oppositely charged ions (stable octets of the hlogen and alkali metal) holds the atoms together.
• The outer electron of one atom moves to the other pulling them together.
• The most vigorous reaction occurs between the most reactive halogen and the most reactive metal (Fr and F)

Halogen displacement reactions:

Displacement reactions are a way of determining the more reactive element. Two examples of this are:

• 2KBr + Cl2 -->  2KCl + Br2 when chlorine gas is bubbled through potassium bromide. The colour of the solution changes from colourless to orange due to the production of bromine in the competition for an extra electron.
• 2I- + Br2 --> 2Br- +I2 The reaction causes a colour change from colourless to dark orange due to the formation of iodine. To distinguish between bromine and iodine the solution can be shaken with a hydrocarbon. iodine: violet, bromine: orange

Halides with silver nitrate:

• They form a precipitate useful in identifying the halide because of the colour

Bonding of period 3 oxides:

• The change in the structures of the oxides illustrates the change in bonding along the group.
• The oxides change from giant ionic through to giant covalent through to molecular covalent due to the reduction in difference of electronegativity between the period 3 element and the oxygen molecules
• Conductivity is a way of determining how ionic the oxides are however they only conduct electricity in their liquid state.