Chalcogens and their side subgroup

General properties of chalcogenes and their similarity

[Wikimedia]

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);

Selenium [Wikimedia]
  • 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:

  1. by the ad­di­tion of new lev­els into the struc­ture, the atom­ic ra­dius in­creas­es;

  2. the elec­tri­cal neg­a­tiv­i­ty of el­e­ments de­creas­es;

  3. ox­i­da­tion abil­i­ty de­creas­es, re­duc­tion prop­er­ties in­crease;

  4. 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.

[Wikimedia]

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.

Wolfram [Wikimedia]

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);
Chromium crystals [Wikimedia]
  • WO₃ + 3H₂ = W + 3H₂O;

  • MoO₃ + 3H₂ = Mo + 3H₂O.

Click here for oth­er ex­per­i­ments with hy­dro­gen.

Chem­i­cal prop­er­ties

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.