HS Chemistry - Periodic Table Trends

Group 17 Elements' Properties

Overview of The Page

This page will cover:

Common properties among Group 17 elements.

The elements in Group $column$ 17 of the Periodic Table $aka the Fluorine group, or the **halogens**$ exhibit specific properties and trends due to their similar electron configuration.

Atomic Radius & Ionic Radius

Each element further down a group in the Periodic Table has an additional electron shell. For example, the element in Period (row) 3 of the Periodic Table has 3 electron shells, while the element in Period 5 has 5 electron shells. Since they have more electron shells, their atomic and ionic radii will be larger. Thus, halogens have larger atomic and ionic radii the further down the group they are.

Electrical Conductivity

Group 17 elements are all non-metals. Therefore, they are all incapable of conducting electricity, unless they are in a liquid or dissolved ionic compound.

Melting Points

Generally, non-metal atoms with a larger atomic radius have stronger intermolecular bonds, as there is more space for intermolecular bonds to form, and therefore more intermolecular bonds form between molecules. To overcome all these intermolecular bonds, more energy is needed, raising the melting and boiling points. This causes non-metals with a larger atomic radius to have higher melting and boiling points than non-metals with a smaller atomic radius. Following this trend, halogens further down the Group have higher melting and boiling points than those higher up in the Group.

In particular:

Fluorine and Chlorine are gases at room temperature and pressure $RTP$
- Fluorine is a yellow-green gas.
- Chlorine is also a yellow-green gas, albeit a darker one.
Bromine is a liquid at RTP
- Bromine is a brownish-red liquid.
Iodine and Astatine are solids at RTP
- Iodine is a purple solid. When heated, it becomes a purple gas.
- Astatine is a dark-colored solid.

In general, halogens further down the group have a darker color than those higher up in the group.

1^st Ionization Energy

The 1^st ionization energy of an atom is the amount of energy required to remove the first valence electron from the atom $i.e. the amount of energy required to turn neutral atom X into ion X^\+^$ . As the atomic radius and number of protons in the nucleus is different for each atom, and the first ionization energy depends on these two things, the first ionization energy is also different for each atom.

In general, as the atomic radius increases, the first ionization energy decreases, as it becomes easier to remove that first electron from the atom's valence shell. Thus, the first ionization energy decreases for Group 17 elements as you go down the group.

Electronegativity

The electronegativity of an atom is how tightly it can hold onto its atoms. This means that a larger first ionization energy corresponds to a larger electronegativity. Thus, the electronegativity for Group 17 elements also decreases as you go down the group.

Reactivity

The reactivity of an element is its ability and tendency to react. Non-metals typically try to complete their valence shell when reacting, as they have more than 4 valence electrons. Therefore, non-metals that have a higher electronegativity are more reactive, as non-metals with a higher electronegativity can more easily pull electrons to complete their valence shell. Thus, the reactivity of halogens decreases down the Group.

Group 17 Ionic Compounds

As you go further down the group, each Group 17 element has one more electron shell, and its atomic radius is therefore larger than the previous halogen's atomic radius. This causes the bond lengths between the metal and non-metal atoms in ionic compounds to be larger, which makes the bonds weaker, as the atoms are farther apart. Therefore, compounds with Group 17 elements further down the Group have weaker intermolecular bonds $and therefore lower melting and boiling points$ than ionic compounds that contain Group 17 elements higher up in the Group.

This is the reason why Hydrogen Iodide $HI$ will easily decompose when subjected to heat, but Hydrogen Chloride $HCl$ and Hydrogen Fluoride $HF$ won't decompose in temperatures under 1500°C.

Reactions with Sulfuric Acid

When ionic compounds that contain Chloride, Bromide or Iodide ions react with $concentrated$ sulfuric acid $H~2~SO~4~$ , they form poisonous gases. These experiments must therefore be undertaken with great care in a fume cupboard. The color of the gases is different depending on which halogen is used - therefore, to determine which halogen is present in an ionic compound, we can observe their reaction with concentrated sulfuric acid.

If an ionic compound with Chloride ions is used:
- A white gas will be produced $HCl$ and the new ionic compound $a type of bisulfate$ will be formed. HCl gas is poisonous.
- If, for example, the ionic compound is LiCl (lithium chloride), the reaction equation would be:
  - LiCl $s$ + H₂SO₄ → LiHSO₄ $s$ + HCl $g$
If an ionic compound with Bromide ions is used:
- Reactions with sulfuric acids and bromide compounds are more complicated, and their exact equations will not be covered here. The products produced in this reaction are:
  - Red-brown Bromine gas $Br~2~$ . The temperature of the substance is above room temperature $20°C$ , as Br₂ is a liquid at 20°C.
  - A type of bisulfate, in the solid state
  - Water $H~2~O$ , in the liquid state
  - Sulfur Dioxide $SO~2~$ , in the gas state

If an ionic compound with Iodide ions is used:
- Reactions with sulfuric acids and iodide compounds are more complicated, with multiple steps, and their exact equations will not be covered here. The products produced in this reaction are:
  - Dark purple Iodine $I~2~$ gas. This is sometimes so dark that it appears black, but in reality it is dark purple. It's temperature is above room temperature $20°C$ , as I₂ is a solid at 20°C.
  - A type of bisulfate, in the solid state
  - Water $H~2~O$ , in the liquid state
  - Sulfur $S$ , present as a yellow solid.
  - Hydrogen Sulfide $H~2~S$ , in the gas state.
  - Sulfur Dioxide $SO~2~$ , in the gas state