Physical Properties

1. Atomic size decreases

As you move across a period, the number of protons in the nucleus increases. This stronger positive charge pulls the electrons closer to the nucleus, making the atom smaller.

• Example: Sodium (Na) has a larger atomic radius than chlorine (Cl)

2. Melting and boiling points generally increase then decrease

• Groups I to IV: Melting and boiling points increase due to stronger metallic or covalent bonding
• Groups V to VIII: Melting and boiling points decrease because elements form simple molecules with weak intermolecular forces

3. Physical states vary

• Most metals (left side) are solids
• Some non-metals (right side) are gases
• Bromine is a liquid

Chemical Properties

1. Electronegativity increases

Electronegativity is the ability of an atom to attract shared electrons in a bond. Moving right, atoms have stronger nuclear attraction, so they attract electrons more strongly.

• Fluorine (F) is the most electronegative element

2. Ionisation energy increases

It becomes harder to remove an electron from left to right because the nucleus has a stronger pull on its electrons.

• Noble gases (Group VIII) have the highest ionisation energies

3. Metallic character decreases

Metals tend to lose electrons, while non-metals tend to gain electrons. As you move across a period, elements become less metallic and more non-metallic.

• Example: Sodium (metal) → Silicon (metalloid) → Chlorine (non-metal)

4. Reactivity changes differently for metals and non-metals

• Metals: Reactivity decreases from left to right (easier to lose electrons at the start)
• Non-metals: Reactivity increases from left to right (easier to gain electrons)

Physical Properties

1. Atomic size increases

Each element down a group has one more electron shell than the element above it. The outermost electrons are farther from the nucleus.

• Example: Lithium (Li) → Sodium (Na) → Potassium (K)

2. Density generally increases

Atomic mass increases more than atomic volume, so elements become denser down a group.

3. Melting points of metals generally decrease

As atoms get larger, the metallic bonding becomes weaker, so less energy is needed to melt the metal.

4. Ionisation energy decreases

The outermost electrons are farther from the nucleus and are shielded by inner electron shells. This makes them easier to remove.

Chemical Properties

1. Electronegativity decreases

With larger atomic size, the outer electrons are farther from the nucleus and less strongly attracted.

• Example: Fluorine (most electronegative) → Iodine (less electronegative)

2. Metallic character increases

Metals become more reactive down a group because they lose electrons more easily.

3. Reactivity trends differ

• Metals: Become more reactive down a group (e.g., sodium is more reactive than lithium)
• Non-metals: Become less reactive down a group (e.g., fluorine is more reactive than iodine)

Question: Explain why sodium (Na) is more reactive than magnesium (Mg) when both are metals.

Answer:

Sodium and magnesium are both in Period 3. Sodium is in Group I and has one valence electron in its outer shell (2.8.1). Magnesium is in Group II and has two valence electrons (2.8.2).

When reacting, metals lose their valence electrons. Sodium needs to lose only one electron to achieve a stable configuration, while magnesium needs to lose two electrons. Therefore, sodium loses its electron more easily than magnesium. This makes sodium more reactive than magnesium.

Additionally, sodium has a larger atomic size and lower ionisation energy than magnesium, so it is easier to remove its outer electron. This explains why sodium reacts more vigorously with water and oxygen than magnesium.

In Tanzania, understanding these trends helps in selecting materials for construction and industry. For example, aluminium (Group III, Period 3) is widely used for cooking utensils because it has a high melting point and is a good conductor of heat — properties that follow the trends across periods. Similarly, potassium fertilizers (from Group I elements) are used in farming in regions like Arusha and Kilimanjaro to boost crop yields, because potassium becomes more reactive down its group, making it effective in supporting plant growth.