Determining oxidation states in magnesium and beryllium hydroxides

In mag­ne­sium hy­drox­ide Mg(OH)₂ there are two hy­dro­gen atoms with an ox­i­da­tion state of +1 and two oxy­gen atoms with ox­i­da­tion states of -2. If these ox­i­da­tion states are added tak­ing the in­dices into ac­count, we may re­ceive the val­ue of -2: (+1)*2+(-2)*2= -2.

The ox­i­da­tion state of mag­ne­sium in the com­pound is +2 (as mag­ne­sium is a mem­ber of the sec­ond group of the pe­ri­od­ic ta­ble).

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When we add the val­ues, we get zero: +2+(-2)=0.

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This means that the ox­i­da­tion states have been cal­cu­lat­ed cor­rect­ly: for mag­ne­sium the val­ue is +2, for oxy­gen -2 and for hy­dro­gen +1.

All atoms in mag­ne­sium hy­drox­ide Mg(OH)₂ have fixed val­ues of ox­i­da­tion states, so this com­pound is a rather sim­ple case for de­ter­min­ing con­di­tion­al charges in atoms.

The sit­u­a­tion with beryl­li­um hy­drox­ide Be(OH)₂ is sim­i­lar: the ox­i­da­tion state of beryl­li­um al­ways cor­re­sponds to its charge and is +2, the ox­i­da­tion state of oxy­gen of com­pounds is -2, and of hy­dro­gen +1. If these val­ues are added tak­ing into ac­count the in­dices, we get zero:

+2 + (-2 + (+1))*2 = 0.

How the ox­i­da­tion state dif­fers from va­lence and charge

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The ox­i­da­tion state, va­lence and charge of an el­e­ment are of­ten iden­ti­cal in val­ue. Nev­er­the­less, these con­cepts have a dif­fer­ent mean­ing. The ox­i­da­tion state is the con­di­tion­al charge on each atom in the com­pound (it is writ­ten above each atom, and first its al­ge­bra­ic sign must be in­di­cat­ed, and then the nu­mer­i­cal val­ue). The ion charge is writ­ten dif­fer­ent­ly: for sim­ple ions it is also writ­ten above the el­e­ment sym­bol, but first its val­ue is in­di­cat­ed, and then the al­ge­bra­ic sign (for ex­am­ple, 2+). For com­plex ions (such as the sul­fate ion SO₄²⁻), the charge is not in­di­cat­ed above the spe­cif­ic el­e­ment, as the ox­i­da­tion state, but above the en­tire com­plex ion. Click here to find out more about ox­i­da­tion states.

The charge is con­nect­ed with its ox­i­da­tion states: for ex­am­ple in Mg(OH)₂ two hy­drox­yl groups are present. The charge of the OH group is al­ways (1-). Ac­cord­ing to the rules, the sum of the ox­i­da­tion states of atoms in this group should be equal to its charge (for the OH group, which con­sists of oxy­gen and hy­dro­gen, this rule is ob­served, as -2+1=-1).

Giv­en that there are two OH groups in mag­ne­sium hy­drox­ide, we may say that their sum­ma­ry charge is (2-). The ox­i­da­tion state of mag­ne­sium (+2) co­in­cides with its charge (2+).

Va­lence is the abil­i­ty of atoms to form a cer­tain num­ber of chem­i­cal bonds. It can only have a pos­i­tive val­ue. Of­ten va­lence co­in­cides with the ox­i­da­tion lev­el in its nu­mer­i­cal val­ue, but there are also cer­tain ex­cep­tions – in ni­tric acid HNO₃ the va­lence of ni­tro­gen is IV, but the ox­i­da­tion state is +5.

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