The Halogens Group 17 for the ESAT

The Relative Positions and Configurations of the Halogens

The halogens are found in Group 17 of the Periodic Table. The group includes the elements fluorine ($F$), chlorine ($Cl$), bromine ($Br$), and iodine ($I$). In line with IUPAC conventions, these are vertical columns, as shown in the complete Periodic Table below:

Every halogen atom has seven electrons in its outermost shell. For example, the electron configuration of a fluorine atom (atomic number 9) is 2,7, while a chlorine atom (atomic number 17) is 2,8,7. Because they have similar outer shell arrangements, they share similar chemical properties. They exist naturally as diatomic molecules, such as $F_2$, $Cl_2$, $Br_2$, and $I_2$.

Trends in Physical Properties

As you descend Group 17, there is a clear trend in the physical state and melting/boiling points of the elements at room temperature and pressure (rtp):

1. Fluorine ($F_2$): A pale yellow gas.
2. Chlorine ($Cl_2$): A green-yellow gas.
3. Bromine ($Br_2$): A red-brown liquid.
4. Iodine ($I_2$): A grey-black solid that produce purple vapours when heated.

The melting and boiling points increase down the group. This is because the molecules become larger and have more electrons, leading to stronger intermolecular forces that require more energy to overcome.

Trends in Chemical Reactivity

In contrast to the alkali metals (Group 1), where reactivity increases down the group, the reactivity of the halogens decreases as you go down Group 17.

Halogens react by gaining one electron to form a negative ion with a $1-$ charge (a halide ion), reaching a stable noble gas configuration (e.g., $Cl$ becomes $Cl^-$). The ease with which an atom attracts this electron determines its reactivity. As you move down the group:

• The atomic radius increases (there are more electron shells).
• The outer shell is further from the positive attraction of the nucleus.
• There is increased shielding by inner electron shells.

Consequently, it becomes harder for the nucleus to attract and capture an incoming electron, making the element less reactive. Fluorine is the most reactive halogen, while iodine is significantly less reactive.

Displacement Reactions of Halogens

A displacement reaction occurs when a more reactive element takes the place of a less reactive element in a compound. In Group 17, this is a competition for electrons between a halogen atom and a halide ion.

A more reactive halogen will displace a less reactive halogen from an aqueous solution of its halide ions. For example, if chlorine gas is bubbled through a solution of potassium bromide, the chlorine will displace the bromine because chlorine is higher in the group and more reactive.

Worked Example: Chlorine and Potassium Bromide

• Full Equation: $Cl_2(g) + 2KBr(aq) ightarrow 2KCl(aq) + Br_2(aq)$
• Ionic Equation: $Cl_2(aq) + 2Br^-(aq) ightarrow 2Cl^-(aq) + Br_2(aq)$

In this reaction, the solution changes from colourless to orange-brown as bromine is produced. This is a redox reaction. The chlorine molecules are reduced as they gain electrons to become chloride ions ($Cl_2 + 2e^- ightarrow 2Cl^-$). The bromide ions are oxidised as they lose electrons to become bromine molecules ($2Br^- ightarrow Br_2 + 2e^-$). Therefore, chlorine acts as the oxidising agent.

Predicting Outcomes

You can use the relative positions in the group to predict if a reaction will occur:

• Chlorine + Sodium Iodide: Reaction occurs. Chlorine is above iodine, so it displaces the iodide ions. The solution turns brown ($I_2$ produced).
• Bromine + Sodium Chloride: No reaction occurs. Bromine is below chlorine and is less reactive, so it cannot displace chloride ions.
• Iodine + Sodium Bromide: No reaction occurs. Iodine is the least reactive of the common halogens.