Comprehensive Notes on Halogens, Noble Gases, Bonding, and Transition Metals

Halogens (Group VII)

• Halogens are found in Group VII of the Periodic Table and exhibit clear similarities. These common properties include:
  • Being poisonous with a strong, similar smell.
  • Being non-metals.
  • Forming diatomic molecules (e.g., $Cl<em>2$, $Br</em>2$, $I_2$).
  • Having a valency of 1, forming compounds with similar formulae such as hydrogen chloride ($HCl$), hydrogen bromide ($HBr$), and hydrogen iodide ($HI$).
  • Forming strong acids when their compounds with hydrogen are dissolved in water (e.g., hydrochloric acid ($HCl$), hydrobromic acid ($HBr$), hydriodic acid ($HI$)).
  • Producing a series of compounds with other elements: chlorides, bromides, and iodides, collectively known as halides.
  • Reacting directly with metals to form metal halides (or salts).
  • Forming negative ions carrying a single charge, such as chloride ions ($Cl^−$), bromide ions ($Br^−$), and iodide ions ($I^−$).

Properties of Specific Halogens

Chlorine ($Cl_2$)

• A dense pale-green gas that is smelly and poisonous.
• Occurs as chlorides, especially sodium chloride in the sea.
• Relative atomic mass: $35.5$

Bromine ($Br_2$)

• A deep-red liquid with red-brown vapor, also smelly and poisonous.
• Occurs as bromides, especially magnesium bromide in the sea.
• Relative atomic mass: $80$

Iodine ($I_2$)

• A grey solid with purple vapor, smelly and poisonous.
• Occurs as iodides and iodates in some rocks and seaweed.
• Relative atomic mass: $127$

Trends in Halogen Properties

• Gradual changes in properties are observed down the group.
  • Boiling points increase.
  • The state changes from gas to liquid to solid.
  • The intensity of color increases from pale to dark.
  • Fluorine is a pale yellow gas at room temperature.

Reactivity of Halogens

• Fluorine and chlorine are very reactive.
• Chlorine dissolves in water to form chlorine water, an acidic solution containing hydrochloric acid ($HCl$) and hypochlorous acid ($HClO$).
• $Cl<em>2 + H</em>2O \rightarrow HCl + HClO$
• Chlorine water acts as an oxidizing agent due to hypochlorous acid giving up oxygen to other substances.
• It also acts as a bleach because colored substances lose their color when oxidized, serving as a chemical test for chlorine gas.
• Halogens become steadily less reactive down the group.

Displacement Reactions

• Chlorine displaces bromine from potassium bromide:
  • $Cl<em>2 + 2KBr \rightarrow 2KCl + Br</em>2$
  • Potassium bromide solution is colorless and turns orange when chlorine is bubbled through it.
• Chlorine displaces iodine from potassium iodide:
  • $Cl<em>2 + 2KI \rightarrow 2KCl + I</em>2$
  • The solution changes from colorless to yellow-brown.

Noble Gases (Group 0/VIII)

• Noble gases were discovered later due to their lack of reactivity.
• William Ramsay isolated all the elements in the group and was awarded the Nobel Prize for this.
• All noble gases are present in the Earth's atmosphere, making up about 1% of the total, with argon being the most common.
• They are particularly unreactive, formerly known as inert gases.
• Some compounds of xenon and krypton have been made since the 1960s.

Uses of Noble Gases

• Helium is used in airships and balloons because it is light and unreactive.
• Argon is used to fill light bulbs because it will not react with the filament at high temperatures.
• Noble gases are used in 'neon' lights, where different gases produce different colors when an electric discharge takes place.

Properties of Noble Gases

• Atoms of noble gases do not combine with each other to form molecules or any other form of structure.
• They have extremely low melting and boiling points.
• Helium has the lowest melting point of any element and cannot be solidified by cooling alone (pressure is needed).
• These properties indicate that the atoms of noble gases are particularly stable.

Chemical Bonding

• Chemical bonding involves the outer electrons of each atom.
• The diversity of the material world is produced by the different ways in which atoms can join together.
• Elements like oxygen ($O<em>2$) and hydrogen ($H</em>2$) consist of diatomic molecules.
• Noble gases (Group VIII/0) are made up of individual atoms moving almost independently of each other due to their stable electron arrangements.

Bonding in Metals

• Metal atoms have relatively few electrons in their outer shells.
• When packed together, each metal atom loses its outer electrons into a 'sea' of free electrons (or mobile electrons).
• Having lost electrons, the atoms become positive ions.
• The structure of a metal is made up of positive ions packed together, surrounded by electrons that can move freely between the ions.
• These free electrons are delocalized and form an electrostatic 'glue' holding the structure together.
• Metals can conduct electricity because the mobile electrons can move through the structure, carrying the current.
• This type of bonding (metallic bonding) is present in alloys as well.

Bonding in Non-metals

• Hydrogen normally exists as diatomic molecules ($H_2$).
• Two atoms bond together by sharing their electrons.
• The orbits overlap and a molecule is formed.

Transition Metals

• Transition metals are located in the center of the Periodic Table (Period 4).
• They are considered as a block or row rather than a vertical group.
• Transition metals are hard, strong, and have high density, as well as high melting and boiling points.
• Many of their compounds are colored.
• They often show more than one valency (variable oxidation state), forming more than one type of ion (e.g., iron can form $Fe^{2+}$ or $Fe^{3+}$ ions).
• They are less reactive than metals in Groups I and II.
• Many have excellent corrosion resistance.

Trends Across a Period

• Vertical groups show similar properties, but trends can be observed across a period.
• The change is from metallic to non-metallic properties.
• In Period 3 (sodium to argon), there appears to be a gradual change in physical properties.
• Elements before silicon behave as metals, while those after it behave as non-metals.
• Metalloids (silicon and germanium) are in the center of Group IV.