Halogens and Noble Gases

The halogens (group 17: fluorine, chlorine, bromine, iodine and astatine) are highly reactive nonmetals with seven valence electrons that form 1− ions and diatomic molecules, while the noble gases (group 18: helium, neon, argon, krypton, xenon and radon) are colorless, monatomic gases with full outer shells that are almost completely unreactive. In brief: a halogen needs just one more electron to complete its octet, so it reacts readily with metals to make salts, and a noble gas already has a stable octet (two electrons for helium), so it rarely reacts.

These two groups lie next to each other on the right of the periodic table and show the contrast between maximum reactivity among nonmetals and minimum reactivity. This article describes their properties and trends, the displacement reactions of halogens, the uses of each group, explains why noble gases are inert and how some form compounds, includes data tables and calculations, and tells the story of their discovery.

What are the halogens?

The name halogen means “salt former”, since they react with metals to make salts such as sodium chloride. Their atoms have the outer configuration ns² np⁵. They gain one electron to form halide ions (F⁻, Cl⁻, Br⁻, I⁻) or share one electron in a covalent bond. As elements, they form diatomic molecules (F₂, Cl₂, Br₂, I₂) and are toxic and corrosive. Their reactivity decreases down the group, since the larger atoms attract an additional electron less strongly. Fluorine is the most reactive of all elements and reacts even with some noble gases and with glass.

Properties of the halogens

The melting and boiling points increase down the group because larger molecules have stronger London dispersion forces between them.

How do halogens displace each other?

A more reactive halogen displaces a less reactive one from a solution of its halide. For example, chlorine water added to potassium bromide solution turns the solution orange-brown, because bromine is formed: Cl₂ + 2KBr → 2KCl + Br₂. Chlorine also displaces iodine, giving a brown solution: Cl₂ + 2KI → 2KCl + I₂. Bromine displaces iodine but not chlorine: Br₂ + 2KI → 2KBr + I₂, and Br₂ + KCl gives no reaction.

What are the noble gases?

The noble gases have a complete outer shell: 1s² for helium and ns² np⁶ for the others. This arrangement is very stable, since they have the highest ionization energies in their periods and almost no tendency to gain electrons. They exist as single atoms, with only weak London forces between them, and have extremely low melting and boiling points. Helium does not solidify at normal pressure, even at absolute zero. They are colorless, odorless and nonflammable. They make up about 0.93% of dry air by volume, mostly argon.

Properties of the noble gases

Do noble gases form compounds?

For a long time the noble gases were called inert gases, because no compounds were known. In 1962, Neil Bartlett prepared a solid compound from xenon and platinum hexafluoride, since the ionization energy of xenon (1,170 kJ/mol) is close to that of oxygen, which also reacted with this fluoride. Soon after, xenon fluorides such as XeF₂, XeF₄ and XeF₆ were made. Krypton forms KrF₂, and some compounds of radon are also known. Helium, neon and argon form compounds only under extreme conditions, if at all.

Worked examples

1. Displacement with masses. How much bromine is produced when excess chlorine reacts with 0.100 mol of potassium bromide? From 2KBr + Cl₂ → 2KCl + Br₂, 0.100 mol of KBr gives 0.0500 mol of Br₂. The molar mass of Br₂ is 159.8 g/mol, so the mass is 0.0500 × 159.8 = 7.99 g. The chlorine consumed is 0.0500 mol, which is 0.0500 × 22.4 = 1.12 L at 0 °C and 1 atm.
2. Test for halide ions. Adding silver nitrate to a solution gives a cream precipitate. The ion is bromide, since AgBr is cream. For 0.020 mol of Br⁻, the mass of AgBr (187.8 g/mol) is 0.020 × 187.8 = 3.76 g.
3. Lifting power of a helium balloon. At 20 °C and 1 atm, a mole of gas has a volume of about 24.0 L. The density of helium (4.00 g/mol) is 4.00 ÷ 24.0 = 0.167 g/L, and that of air (29.0 g/mol) is 29.0 ÷ 24.0 = 1.21 g/L. The lift is 1.21 − 0.167 = 1.04 g per liter, which is 1.04 kg per cubic meter. A balloon with a volume of 10 L can lift about 10 g, apart from its own mass.
4. Predicting a formula. Aluminum (3+) and iodide (1−) form AlI₃. Sodium (1+) and fluoride (1−) form NaF.
5. Trend in boiling points. From fluorine (−188 °C) to iodine (184 °C) the boiling point rises by 184 − (−188) = 372 °C, because of the stronger dispersion forces between larger molecules.

What are they used for?

• Fluorine: fluoride in toothpaste, non-stick coatings (PTFE) and refrigerants.
• Chlorine: disinfection of drinking water and swimming pools, PVC plastic and bleach.
• Bromine: flame retardants and some pharmaceuticals.
• Iodine: antiseptic, and needed in the diet for thyroid hormones; table salt is often iodized.
• Helium: balloons, cooling of superconducting magnets in MRI scanners, breathing mixtures for deep diving.
• Neon: red-orange glow in lighting signs.
• Argon: inert atmosphere for welding and in filament light bulbs.
• Krypton and xenon: high-performance lamps, camera flashes and xenon ion engines for spacecraft.
• Radon: a radioactive gas that can accumulate in buildings and increases the risk of lung cancer.

How were they discovered?

Carl Wilhelm Scheele prepared chlorine in 1774, and Humphry Davy showed in 1810 that it is an element. Bernard Courtois found iodine in 1811 in seaweed ash, and Antoine Jérôme Balard discovered bromine in 1826. Henri Moissan isolated fluorine in 1886 by electrolysis and received the Nobel Prize in Chemistry in 1906. Astatine was first made in 1940. Helium was detected in the spectrum of the Sun in 1868, before it was found on Earth. William Ramsay and Lord Rayleigh discovered argon in 1894 when a small difference in density between nitrogen from air and from chemicals led to an unknown gas, and Ramsay and others discovered neon, krypton and xenon in 1898. Both received Nobel Prizes in 1904.

Which mistakes are common?

Mistake 1: Saying that halogens are metals or that they are found as free elements

They are reactive nonmetals, and in nature they are found as halide compounds such as salt.

Mistake 2: Thinking that reactivity increases down group 17

It decreases down the group, in contrast to the alkali metals.

Mistake 3: Calling the noble gases completely inert

Xenon, krypton and radon can form compounds, mainly with fluorine and oxygen.

Mistake 4: Forgetting that halogens are diatomic

Write Cl₂, not Cl, for chlorine gas in equations.