Periodic Properties of the Elements

Group Trends: Selected Nonmetals

Hydrogen has a 1s¹ electron configuration and is placed above the alkali metal group.

Hydrogen is a non-metal, which occurs as a gas (H₂) under normal conditions.

• Its ionization energy is considerably higher (due to lack of shielding, and thus higher Z_eff) than the metals and is more like the nonmetals
• Hydrogen generally reacts with other nonmetals to form molecular compounds (typically highly exothermic)
• Hydrogen reacts with active metals to form metal hydrides which contain the H^- hydride ion:

• Hydrogen can also lose an electron to yield the aqueous H⁺(aq) ion

As we proceed down group 6A the elements become more metallic in nature:

• Oxygen is a gas, the rest are solids
• Oxygen, sulfur and selenium are nonmetals
• Tellurium is a metalloid with some metal properties
• Polonium is a metal

Oxygen can be found in two molecular forms, O₂ and O₃ (ozone). These two forms of oxygen are called allotropes (different forms of the same element in the same state)

3O₂(g) -> 2O₃(g) DH = 284.6 kJ

• Oxygen in combination with metals is almost always present as the O^2- ion (which has noble gas electronic configuration and is particularly stable)
• Two other oxygen anions are observed: peroxide (O₂^2-) and superoxide (O₂^-)

Sulfur also exists in several allotropic forms, the most common stable allotrope is the yellow solid S₈ (an 8 member ring of sulfur atoms)

Like oxygen, sulfur has a tendency to gain electrons from other elements, and to form sulfides (which contain the S^2- ion). This is particular true for the active metals:

Note: most sulfur in nature is present as a metal-sulfur compound

Sulfur chemistry is more complex than that of oxygen

• "Halogen" is derived from the Greek meaning "salt formers"
• Astatine is radioactive and rare, and some of its properties are unknown
• All the halogens are nonmetals
• Each element consists of diatomic molecules under standard conditions

Colors of diatomic halogens: (not flame colors)

Fluorine: pale yellow

Chlorine: yellow green

Bromine: reddish brown

Iodine: violet vapor

The halogens have some of the most negative electron affinities (i.e. large exothermic process in gaining an electron from another element)

• Fluorine and chlorine are the most reactive halogens (highest electron affinities). Fluorine will remove electrons from almost any substance

In 1992 22.3 billion pounds of chlorine was produced. Both chlorine and sodium can be produced by electrolysis of molten sodium chloride (table salt). The electricity is used to strip electrons from chloride ions and tranfer them to sodium ions to produce chlorine gas and solid sodium metal

Chlorine reacts slowly with water to produce hydrochloric acid and hypochlorous acid:

Hypochlorous acid is a disinfectant, thus chlorine is a useful addition to swimming pool water

The halogens react with most metals to form ionic halides:

• Nonmetals
• Gases at room temperature
• monoatomic
• completely filled 's' and 'p' subshells
• large first ionization energy, but this decreases somewhat as we move down the group

Rn is highly radioactive and some of its properties are unknown

They are exceptionally unreactive. It was reasoned that if any of these were reactive, they would most likely be Rn, Xe or Kr where the first ionization energies were lower.

Compounds of noble gases to date:

only one compound with Kr has been made

No compounds observed with He, Ne, or Ar (truly inert gases)

1996 Michael Blaber