O Levels - Periodic Table Guide

There are three groups of elements that we are particularly interested in.

• Alkali metals (Group 1)
• Halogens (Group 17) 
• Noble gases (Group 18)
  

Alkali Metals (Group 1)

Group 1 elements, also known as alkali metals, are characterised by having only 1 valence electron in the valence shell. This singular electron defines many of their physical and chemical properties. 

Physical Properties of Alkali Metals 

• Shiny and slivery 
• Soft and easily cut 
• Low density (Li, Na, K float on water)
• Low melting and boiling point 
• Good conductors of electricity and heat

Chemical Properties of Alkali Metals 

• Tend to lose their valence electron to form positive ions with +1 charge (e.g., Li+) 
• Highly reactive and have to be stored under oil or in vacuum to prevent them from reacting with moisture or air

Types of reactions: 

Trends of Alkali Metals 

Halogens (Group 17)

Halogens, found in Group 17 of the periodic table, are a fascinating group of non-metals known for their high reactivity and distinctive properties. With 7 valence electrons in the valence shell, halogens are just one electron short of achieving a stable noble gas configuration. As such, they exists as diatomic molecule (e.g., F2) to achieve electronic stability. 

Physical Properties of Halogens 

• Brittle in solid state
• Coloured molecules (darker down the group)
• Low melting and boiling points
• Poor conductor of heat and electricity

Chemical Properties of Halogens  

• Tend to gain electron to fill valence shell and form negative ions with -1 charge (e.g., F–, Cl–).
• Very reactive 

Types of reactions:

Trends of Halogens

Halogen Displacement 

Halogen displacement reactions are a key concept in understanding the reactivity trends within Group 17 elements. These reaction occur when a more reactive halogen can displace the less reactive halide ion from its compound or solution.

Reactivity Order: 

The reactivity of halogens decreases down Group 17, with fluorine being the most reactive, followed by chlorine, bromine, and iodine (F > Cl > Br > I). For example, when chlorine gas (Cl2) is added to a solution containing bromide ions (Br–), the chlorine will displace the bromine, forming chloride ions (Cl–) and free bromine (Br2):

Cl2​ + 2Br− → 2Cl− + Br2​

Observation of Displacement: 

The identity of halogens during halogen displacement can be easily inferred from its colour and physical state as shown below. 

Noble Gases (Group 18)

Noble gases, found in Group 18 of a periodic table, are unique due to their fully filled valence electron shells. This configuration allows them to exist as monatomic atoms (e.g., Ar, Ne) and makes them extremely stable and largely unreactive. Here’s a closer look at their properties and uses:

Physical Properties of Noble Gases

• Colourless gases at r.t.p
• Low melting and boiling points 

Chemistry Properties of Noble Gases

• Inert and extremely unreactive (due to their electronic stability)

Uses of Noble Gases 
Noble gases are widely used due to their lack of reactivity. For example, 

• Helium is used to fill balloons and airships, as it is non-flammable, low density, and safe to use. 
• Neon is used in advertising signs and fluorescent lights, as it glows when electrically excited. 
• Argon makes up about 1% of Earth’s atmosphere and is used in bulbs to prevent filament from oxidising and burning out quickly.