Group VII properties Revision Notes

Halogens are elements found in Group VII (17) of the periodic table and include chlorine (Cl₂), bromine (Br₂), and iodine (I₂). Each halogen atom pairs with another identical atom to form diatomic molecules, represented by the formula X₂, where X is the halogen symbol.

This diatomic nature arises because halogen atoms have seven electrons in their outer shell and require one more electron to achieve a stable octet. By sharing one electron each through a single covalent bond, two halogen atoms form a stable molecule.

For example, chlorine exists as Cl₂ molecules, where two chlorine atoms share a pair of electrons: Cl–Cl. This covalent bond holds the two atoms together, making the molecule diatomic.

• Halogens exist as diatomic molecules (X₂).
• Each molecule contains two atoms covalently bonded.
• Diatomic molecules form due to the need to complete the outer electron shell.

Halogens are non-metals, meaning they have high electronegativity and tend to gain electrons in reactions rather than lose them. They do not conduct electricity in solid or liquid states and have relatively low melting and boiling points compared to metals.

Their non-metallic nature is evident in their physical states at room temperature: chlorine is a pale yellow-green gas, bromine is a red-brown liquid, and iodine is a grey-black solid. These states reflect increasing molecular mass and intermolecular forces down the group.

Non-metallic elements typically form covalent bonds and react with metals to form ionic compounds, such as sodium chloride (NaCl).

• Halogens are non-metals with high electronegativity.
• They form covalent bonds and ionic compounds with metals.
• Physical states vary with molecular mass but all are non-metallic.

Density is defined as mass per unit volume (density = mass/volume). As you move down Group VII from chlorine to iodine, the density of the halogens increases.

This increase occurs because the atomic mass of the halogen atoms increases significantly down the group, while the volume occupied by the molecules does not increase proportionally. For example, chlorine gas has a lower density than bromine liquid, which in turn is less dense than solid iodine.

Approximate densities at room temperature: chlorine gas ~3.2 g/L, bromine liquid ~3.1 g/cm³, iodine solid ~4.9 g/cm³. The large increase in density from chlorine gas to iodine solid reflects the change in physical state and atomic mass.

• Density increases down the group due to increasing atomic mass.
• Physical state changes from gas to liquid to solid down the group.
• Higher molecular mass leads to higher density despite changes in state.

Reactivity refers to how readily an element undergoes chemical reactions. In Group VII, reactivity decreases down the group from chlorine to iodine.

This trend is explained by the atomic structure: as atomic number increases, atoms have more electron shells, increasing the distance between the nucleus and the outer electrons.

The outer electron is less strongly attracted to the nucleus due to increased shielding by inner electrons, making it harder for the atom to gain an electron and react. Therefore, chlorine is more reactive than bromine, which is more reactive than iodine.

• Reactivity decreases down the group.
• Increased atomic size and electron shielding reduce attraction for electrons.
• Halogens become less able to gain electrons down the group.

Consider the reaction of chlorine and iodine with potassium bromide solution. Chlorine can displace bromine from potassium bromide because it is more reactive, but iodine cannot.

Equation for chlorine displacing bromine: Cl₂ (aq) + 2KBr (aq) → 2KCl (aq) + Br₂ (aq)

Observation: The solution changes colour as brown bromine is released, indicating displacement. Iodine does not displace bromine because it is less reactive, so no colour change occurs.

• More reactive halogen displaces less reactive halogen from solution.
• Chlorine displaces bromine; iodine does not.
• Displacement reactions demonstrate reactivity trends.

The halogens are elements found in Group VII (17) of the periodic table and include chlorine, bromine, and iodine among others. At room temperature (approximately 25°C) and atmospheric pressure (1 atm), these halogens exist in different physical states due to variations in their molecular structures and intermolecular forces.

Chlorine exists as a diatomic molecule (Cl₂) and is a gas at room temperature. Bromine (Br₂) is a liquid, while iodine (I₂) is a solid under these conditions. These differences are important for their identification and practical handling.

• Halogens are diatomic molecules: Cl₂, Br₂, I₂.
• Physical state depends on molecular mass and intermolecular forces.
• Room temperature defined as 25°C and pressure as 1 atm.

Chlorine is a greenish-yellow gas at room temperature and pressure. It is denser than air and has a pungent, irritating smell. Chlorine gas is toxic and used in water treatment and disinfectants.

Its pale yellow-green colour is distinctive and can be observed in a well-ventilated laboratory setting using a gas jar or by releasing a small amount from a chlorine source.

The molecular formula is Cl₂, and the gas consists of diatomic molecules held together by covalent bonds.

• Physical state: Gas (Cl₂)
• Colour: Pale yellow-green
• Odour: Pungent and irritating
• Toxicity: Harmful to respiratory system

Bromine is unique among the halogens as it is a liquid at room temperature. It has a deep red-brown colour and emits a strong, choking odour.

Bromine vapour above the liquid is also coloured red-brown. It is volatile and evaporates readily at room temperature, producing visible coloured vapour.

Bromine's molecular formula is Br₂, and it forms a diatomic molecule with covalent bonding. Its liquid state is due to stronger van der Waals forces compared to chlorine, resulting from its larger molecular size.

• Physical state: Liquid (Br₂)
• Colour: Red-brown
• Odour: Strong and choking
• Volatility: Evaporates easily producing coloured vapour

Iodine is a dark grey-black solid at room temperature and pressure. It forms shiny, crystalline crystals that sublimate on heating, turning directly into a violet vapour without melting first.

The solid iodine consists of diatomic molecules (I₂) held together by relatively strong van der Waals forces, making it a solid at room temperature.

Its characteristic colour and sublimation behaviour are useful for identification in the laboratory.

• Physical state: Solid (I₂)
• Colour: Grey-black crystalline
• Sublimation: Solid to violet vapour on heating
• Molecular form: Diatomic I₂ molecules

The following table summarises the physical states and colours of chlorine, bromine, and iodine at room temperature and pressure.

Halogens are elements in Group VII of the periodic table, including fluorine, chlorine, bromine, and iodine. They exist as diatomic molecules (e.g., Cl₂, Br₂) and form halide ions (Cl⁻, Br⁻, I⁻) when they gain an electron.

The reactivity of halogens decreases down the group: fluorine is the most reactive, followed by chlorine, bromine, and iodine. This trend is due to increasing atomic size and shielding, which reduce the attraction of the nucleus for an additional electron.

Halide ions are the negatively charged ions formed when halogens gain electrons. Their reactivity is the opposite of their parent halogens; less reactive halogens form more stable halide ions.

• Halogen reactivity order: F₂ > Cl₂ > Br₂ > I₂.
• Halide ions are formed by halogens gaining one electron.
• Reactivity decreases down Group VII due to atomic structure changes.

A displacement reaction occurs when a more reactive halogen displaces a less reactive halide ion from a compound in solution.

For example, chlorine gas (Cl₂) can displace bromide ions (Br⁻) from potassium bromide (KBr) solution because chlorine is more reactive than bromine.

This reaction involves the transfer of electrons: the halogen molecule is reduced to halide ions, while the halide ions are oxidised to halogen molecules.

• More reactive halogen displaces less reactive halide ion.
• Electron transfer underlies displacement: halogen gains electrons, halide loses electrons.
• Displacement reactions occur in aqueous solutions of halide salts.

The displacement of bromide ions by chlorine can be represented by the balanced equation:

\[ \mathrm{Cl}_2 (g) + 2 \mathrm{KBr} (aq) \rightarrow 2 \mathrm{KCl} (aq) + \mathrm{Br}_2 (aq) \]

In ionic form: \[ \mathrm{Cl}_2 (g) + 2 \mathrm{Br}^- (aq) \rightarrow 2 \mathrm{Cl}^- (aq) + \mathrm{Br}_2 (aq) \]

• Chlorine gas bubbles through potassium bromide solution.
• Solution changes colour from colourless to orange-brown due to bromine formation.
• Similar reactions occur with chlorine displacing iodide ions, and bromine displacing iodide ions.
• No reaction occurs if the halogen is less reactive than the halide ion present.

When bromine water is added to potassium iodide solution, bromine displaces iodide ions forming iodine:

\[ \mathrm{Br}_2 (aq) + 2 \mathrm{KI} (aq) \rightarrow 2 \mathrm{KBr} (aq) + \mathrm{I}_2 (aq) \]

Observations include the solution changing from orange-brown (bromine) to brown (iodine).

• Bromine is more reactive than iodine, so displacement occurs.
• Iodine formed dissolves in solution, causing a colour change.
• This confirms the reactivity order: Cl₂ > Br₂ > I₂.

To predict if a displacement reaction will occur, compare the reactivity of the halogen and the halide ion.

If the halogen is more reactive, displacement happens; if less reactive, no reaction occurs.

Testing involves adding a halogen to a solution containing halide ions and observing colour changes or using starch indicator to detect iodine.

• Use reactivity order to predict displacement.
• Observe colour changes: chlorine (pale green), bromine (orange-brown), iodine (brown).
• Starch indicator turns blue-black in presence of iodine, confirming displacement.

Group VII elements are halogens that exist as diatomic molecules (X₂) under standard conditions. Their physical states vary down the group: fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids at room temperature. This progression from gas to solid is due to increasing molecular mass and stronger van der Waals forces between molecules.

The colours of halogens deepen down the group: fluorine is pale yellow, chlorine is pale yellow-green, bromine is red-brown, iodine is grey-black with a violet vapour, and astatine is predicted to be dark but is rarely observed due to radioactivity.

Melting and boiling points increase down the group, reflecting stronger intermolecular forces as molecular size increases.

• Physical state changes from gas (F₂, Cl₂) to liquid (Br₂) to solid (I₂, At) down the group.
• Colour deepens from pale yellow (F₂) to grey-black (I₂).
• Melting and boiling points increase down the group due to stronger van der Waals forces.

Reactivity of halogens decreases down the group. Fluorine is the most reactive, followed by chlorine, bromine, iodine, and astatine. This trend is due to increasing atomic radius and electron shielding, which reduce the halogen's ability to attract electrons to form negative ions (X⁻).

Electronegativity decreases down the group, making halogens less effective oxidising agents as you move down.

The general reaction of halogens with metals forms ionic halide salts, for example: 2Na + Cl₂ → 2NaCl.

• Reactivity decreases down the group due to increased atomic size and electron shielding.
• Halogens act as oxidising agents, gaining electrons to form halide ions (X⁻).
• Halogens react with metals to form ionic halide salts.

Given the trends observed in chlorine, bromine, and iodine, predictions can be made for other halogens such as fluorine and astatine.

Fluorine, being above chlorine, is expected to be a pale yellow gas with very high reactivity and low melting and boiling points.

Astatine, below iodine, is predicted to be a solid with a dark colour, higher melting and boiling points than iodine, and lower reactivity than iodine due to its larger atomic size and greater electron shielding.

• Fluorine: pale yellow gas, very reactive, low melting/boiling points.
• Astatine: solid, dark colour, higher melting/boiling points than iodine, less reactive.

Halogens can displace less reactive halide ions from solution. For example, chlorine can displace bromide and iodide ions from their salts, but iodine cannot displace chloride ions.

These displacement reactions provide evidence for the reactivity trend: F₂ > Cl₂ > Br₂ > I₂ > At.

Example reaction: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq) (chlorine displaces bromide ions).

• More reactive halogens displace less reactive halide ions from solution.
• Displacement reactions confirm the order of reactivity in Group VII.
• Balanced equation example: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).

Given that iodine is a grey-black solid with melting point 114 °C and boiling point 184 °C, and that melting and boiling points increase down the group, predict the physical state and relative melting point of astatine.

Since astatine is below iodine, it is expected to be a solid at room temperature with melting and boiling points higher than iodine's.

Its reactivity will be lower than iodine's due to increased atomic size and electron shielding, making it a weaker oxidising agent.

• Astatine predicted to be a solid with melting point >114 °C.
• Reactivity decreases down the group; astatine less reactive than iodine.
• Physical properties follow trends of increasing melting/boiling points down the group.