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selenium, tellurium and polonium.
 Group 7, the halogens, includes fluorine, chlorine, bromine, iodine and astatine.
 Group 8, the noble gases, contains helium, neon, argon, krypton, xenon and radon.
The properties of the groups change markedly from group 0 to group 8. For example:
 The elements of group 0 and 8 are gases that are difficult to condense.
 The alkali metals, in group 1, are soft, metallic solids with low melting points.
 The alkaline-earth metals, in group 2, are harder and have higher melting points.
 The hardness and melting points increase through groups 3 and 4.
 Hardness decreases again through groups 5, 6 and 7.
H
Hafnium (Hf)
Element 72 on the periodic table. A silvery, soft, bendable metal belonging to the transition metals.
It was discovered by Dirk Coster and George Charles de Hevesy in 1923 and was named after the Latin word for the city of Copenhagen (Hafnia).
Hafnium absorbs neutrons and is used for control rods in nuclear reactors. An alloy of tantalum, hafnium and carbon has a very high melting point (4,215°C) and can hold other materials for melting.
Half-life
The time it takes for the radiation coming from a sample of a radioactive element to decrease by half.
Halide
A salt of one of the halogens (fluorine, chlorine, bromine and iodine), group 7 on the periodic table.
Halogen, halogen group
Fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At) are the five halogens and members of group 7 on the periodic table. Fluorine is the most reactive.
 Halogen – The halogens are all very reactive. Here chlorine reacts with copper to produce copper chloride. This experiment was conducted in a fume chamber since chlorine is very poisonous.
 Halide – Halides readily dissolve in water. They are only precipitated when water is evaporated, such as when desert lakes dry up or in coastal lagoons. The precipitates form white crystals, most commonly seen as the white ‘ghost lakes’ or playas in deserts, as shown here.
Hassium (Hs)
Element 108 on the periodic table. An artificial, radioactive element, also called unniloctium (Uno), belonging to the transition metals.
Hassium was discovered in 1984 by West German researchers at the Institute for Heavy Ion Research. It has properties similar to osmium, which is just above
it on the periodic table. It has
no practical uses. Hassium is a transuranium element.
Hassium (Hs)
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