Halogen Displacement Reactions

• When solid iodine is added to potassium chloride solution, there is no visible change or reaction.

• Iodine is less reactive than chlorine, so it cannot displace chlorine from potassium chloride solution to form potassium iodide and chlorine.

• Chlorine passed through a tube with potassium bromide and potassium iodide results in colored substances at different points:

  • P: Red-brown vapor
  • Q: Violet vapor
  • R: Black solid

• Astatine is at the bottom of Group 17 with bromine above it, bromine is more reactive than astatine.

• Astatine cannot displace bromine from potassium bromide solution.

• Astatine is expected to be a black solid and have no reaction with aqueous potassium bromide.

Displacement Reaction Involving Halogens

• A displacement reaction happens when one element takes the place of another in a compound.
• The more reactive halogen can displace a less reactive halogen from its salt solution.
• Reactivity decreases down the group: fluorine > chlorine > bromine > iodine > astatine

Example 1: Chlorine and Potassium Bromide

• When chlorine water is added to aqueous potassium bromide, potassium chloride and bromine form.
• The solution goes from colorless to reddish-brown when bromine forms.
• Chlorine is more reactive than bromine, so it displaces bromine from potassium bromide to form potassium chloride.
• Chemical equation: Cl2 (aq) + 2 KBr(aq) → 2 KCl(aq) + Br2(aq)
• Ionic equation: Cl2 (aq) + 2 Br-(aq) → 2Cl-(aq) + Br2(aq)

Example 2: Chlorine and Potassium Iodide

• When chlorine water is added to colorless potassium iodide solution, the solution turns brown and forms a black precipitate.
• Chlorine is more reactive than potassium iodide, so it displaces iodine from potassium iodide to form potassium chloride.
• Chemical equation: Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(s)
• Ionic equation: Cl2(aq) + 2 I-(aq) → 2Cl-(aq) + I2(s)

Halogens as Oxidizing Agents

• Halogens are powerful oxidizing agents, meaning they accept electrons.
• Example reaction: 2 Na + Cl2 → 2 NaCl
• Halogen atoms readily gain electrons to form halide ions to achieve a fully-filled valence shell.
• Chlorine acts as an oxidizing agent in the reaction with sodium.

Trends in Group 17 Halogens

• Boiling point increases down Group VII.
• Color darkens down Group 17.
  • Fluorine is pale yellow.
  • Chlorine is yellowish-green.
  • Bromine is reddish-brown.
  • Iodine is black.
• Density increases down Group 17.
• Reactivity decreases down Group 17.

Group 17 - The Halogens

• Halogens exist as diatomic molecules, meaning they have 2 atoms.
• Each fluorine atom has seven valence electrons and shares electrons to achieve a fully-filled valence shell of eight electrons.

Physical Properties of Halogens (non-metals)

• Have relatively low melting and boiling points.

• They are poor conductors of electricity because they are uncharged and have no mobile ions or electrons.

• Poor conductors of heat that are colored with the color darkening down the group

• Halogens are generally reactive non-metals with 7 valence shell electrons.

• They need only 1 more electron to achieve a fully-filled valence shell.

• Halogens react readily with most metallic atoms to form salts called halides, with halide ion examples: Fluoride (F-), chloride (Cl-), bromide (Br-), iodide (I-).

Trends in Going Down Group 1

• Alkali metals become more reactive as you move down the group.

• The melting point of the alkali metals decreases as you move down the group.

• Elements X and Y in the same period, X is a metal and Y is a non-metal, Y has a higher atomic number than X.

• Two elements are in the same group and have the same number of valence electrons.

• X, Y, and Z are elements in the same period; X forms an acidic oxide, Y forms a basic oxide, and Z forms an amphoteric oxide.

• Increasing atomic number order: Y, Z, X.

Trends Across the Period

• A change from metallic to non-metallic character happens across each period.

• Metallic elements tend to lose valence electrons during chemical reactions, for example, Lithium loses one valence electron (2.1) to achieve a fully-filled valence shell (2).

• Non-metal atoms tend to gain or share valence electrons during a chemical reaction.

• The number of valence electrons for each element increases as you across each period and the tendency to gain valence electrons increases.

• The characteristic changes from metallic to non-metallic across each period.

Period Number

• The number of electron shells of an atom matches the period number.
• Elements in Period 2 have 2 electron shells each.
• Group and period numbers tell information about electronic structures of elements.

Periodic Trends

• Elements are arranged in increasing atomic (proton) number.
• Number of valence electrons of an atom = group number
• For group 13 and onwards, follow the last digit number.
• Valency refers to number of valence electrons involved in the formation of bonds with other atoms.
• Elements in the same group undergo similar chemical reactions.
• Alkali metals react with water to form metal hydroxides and hydrogen gas.
• Metal atoms lose electrons to form positive ions and non-metal atoms gain electrons to form negative ions.