Chalcogens and their side subgroup
General properties of chalcogenes and their similarity
Chalcogens are elements of the 16ᵗʰ group of the main subgroup in the Periodic Table. Chalcogens include oxygen, sulfur, selenium, tellurium and polonium. All elements in the group have similar features and properties (as the configuration of their external electron level is similar).
Main subgroup of chalcogens
The change of chemical properties in the group of chalcogens can be seen by the change of position in the Periodic system and the electron configuration of the outer layer of atoms of elements:
Oxygen - 2s² 2p⁴ (possible oxidation states - -2,-1, 0, +1, +2);
Sulfur - 3s² 3p⁴ (possible oxidation states - -2, 0, +2, +4, +6);
Selenium - 4s² 4p⁴ (possible oxidation states - -2, 0, +2, +4, +6);
Tellurium - 5s² 5p⁴ (possible oxidation states - -2, 0, +2, +4, +6);
Polonium - 6s² 6s⁴ (radioactive).
Change of properties of elements in the group of chalcogens
The properties of chalcogens change regularly from top to bottom:
by the addition of new levels into the structure, the atomic radius increases;
the electrical negativity of elements decreases;
oxidation ability decreases, reduction properties increase;
non-metallic properties weaken and are replaced by semi-metallic/metallic properties.
Therefore, we can say that oxygen, which opens the subgroup of chalcogens, is the strongest oxidizer among them.
Oxygen, sulfur and selenium are typical non-metals, while tellurium is a metalloid (semi-metal). Polonium, depending on classification, is either a semi-metallic or metallic element (the metallic properties of polonium nevertheless predominate significantly).
Only the first two simple substances of the group are dielectrics (substances which do not conduct an electric current). Selenium and tellurium are semi-conductors.
Similarities of chalcogens
For chalcogens, the phenomenon of allotropy is characteristics (the existence of several simple substances of one chemical element differing by properties or structure). For example, oxygen has the allotropic modification O₃ – ozone, and sulfur can be crystalline, rhombic and monoclinic.
Selenium and tellurium are stable in a solid state (they have an atomic crystalline lattice with metallic elements). Sulfur and oxygen (in a solid state) have a molecular crystalline lattice.
All elements of the oxygen subgroup have a covalent non-polar bond between atoms (selenium and tellurium have features of a metallic bond – for this reason semi-conductivity arises). All simple substances apart from oxygen are in solid form under normal conditions.
Chalcogens can display oxidation-reduction duality – to act as oxidizers and reducers in reactions depending on conditions (this is less characteristic for oxygen, as oxygen is a typical oxidizer, and only in a compound with fluorine it is in oxidized form).
The side subgroup of chromium
The 16ᵗʰ group of the side subgroup has the elements chromium, molybdenum, wolfram and seaborgium. The subgroup of chromium usually contains the first three metals. They are similar by physical properties – they have a high melting point (in the transition from chromium to wolfram, the melting point rises – wolfram is the metal with the highest melting point) and a silvery-gray color.
Metals of the side subgroup, like those in the main subgroup of the 16ᵗʰ group, have a similar outer electron layer: Cr — 3d⁵ 4s¹, Mo — 4d⁵ 5s¹, W — 5d⁴ 6s².
The properties of molybdenum and wolfram are very similar (while certain differences may be distinguished between chromium and these two metals). For example, for molybdenum and wolfram the most stable oxidation state is the highest - +6, while chromium in its most stable compounds has an oxidation state of +3.
In an oxidation state of +6, chromium, molybdenum and wolfram are the most similar to one another.
Methods for obtaining them
All metals of the subgroup can be obtained by reduction from oxides:
- 2Al + Cr₂О₃ = Al₂О₃ + 2Cr (this is the aluminothermic reduction of chromium from its oxide, wolfram can be obtained by the same reaction; the mixture is ignited in aluminothermy);
WO₃ + 3H₂ = W + 3H₂O;
MoO₃ + 3H₂ = Mo + 3H₂O.
Click here for other experiments with hydrogen.
A low reactive ability at room temperature is common for the entire subgroup. Metals of the subgroup of chromium are even quite resistant to the effect of oxygen in the air and water under normal condition. The reaction of metals with fluorine is possible in normal conditions, but heating is required to carry out reactions with other non-metals.
The reactivity of metals drops further down the subgroup – for example, in diluted acids, chromium dissolves (in concentrated acids it passivates), while molybdenum dissolves in hot strong sulfuric acid, and wolfram only in a mixture of nitric and hydrofluoric acids:
W + 4HF + 2HNO₃ = H₂[WO₂F₄]+ 2NO + 2H₂O (in this reaction the mixtures H₂[WF₈] can also be found);
Mo + 4H₂SO₄ = H₄[Mo(SO₄)O₄] + 3SO₂ + 2H₂O.
Metals of the side subgroup are usually added to steel to increase its durability and resistance to wear and tear. Chromates, molybdates and wolframates are used as oxidizers. Members of the chalcogen group (for example oxygen) can also be used as oxidizing agents in carrying out reactions.