Doc Brown's Chemistry Revision

Extra Notes on chemical bonding for advanced A level chemistry students

 Extra A level Notes on Ionic Bonding and Ionic Compounds

All the advanced A level 'basics' with lots of examples of dot and cross diagrams of ionic bonding, Lewis diagrams, properties of ionic compounds etc. is on a separate page.

Ionic bonding and advanced electron notation

I'm not going to repeat the diagrams, there's no point, so I'm just outlining the ionic bond formation in s, p and d terms.

Most involve the 'octet rule' to form a noble gas electron configuration.

Examples

You need to be able to write the electron configuration of ions in terms of s, p and d orbital notation.

e.g. sodium ion Na⁺ is 1s²2s²p⁶, [Ne], and the chloride ion Cl^- is 1s²2s²p⁶3s²3p⁶, [Ar]

More on electron configuration of ions and oxidation states

Which elements form ionic compounds? and how to work out and write an ionic formula?

numerical ion charges = the valency of A and B to deduce the formula A_xB_y

i.e. the valence or ionic charge = the combining power of the ion

'molecular' or ionic style of formula and compound name are shown

In the electrically balanced formula for a potentially stable compound, the total positive ionic charge must equal the total negative ionic charge.

number of positive ion 'A' x positive charge of ion 'A' = number of negative ion 'B' x negative charge of ion 'B' (you ignore charge sign)

Example: A moderately difficult example to work out!

Aluminium oxide consists of aluminium ions Al³⁺ and oxide ions O^2– 

number of Al³⁺ x charge on Al³⁺ = number of  O^2– x charge on O^2– 

the simplest whole number ratios are 2 of Al³⁺ x 3 = 3 of  O^2– x 2 (total 6+ balances total 6–)

so the simplest whole number formula for aluminium oxide is Al₂O₃ which is its empirical formula

More examples of how to work out ionic formulae

2 of Na⁺ balances 1 of O^2– because 2 x 1 = 1 x 2 gives Na₂O or (Na⁺)₂O^2–    sodium oxide

1 of Fe³⁺ balances 3 of F^– because 1 x 3 = 3 x 1 gives FeF₃ or   Fe³⁺(F^–)₃    iron(III) fluoride

2 of Fe³⁺ balances 3 of SO₄^2– because 2 x 3 = 3 x 2 gives Fe₂(SO₄)₃ or (Fe³⁺)₂(SO₄^2–)₃    iron(III) sulfate

LINK to Table of lots of common formulae of ionic compounds

(includes common oxides, hydroxides, carbonates, hydrogencarbonates, halides, sulfates, nitrates)

At Advanced A level you would be expected to work out any ionic formulae from given ions and you should know many of them anyway, or work out the charge on the ion from the position of the element in the periodic table and using the octet electron rule eg for groups 1, 2, 3/13, 4/14, 5/15, 6/16, 7/17, 0/18.

Naming ionic inorganic compounds

When combined with other elements in simple compounds the name of the non-metallic element changes slightly from ...??? to ...ide.

Sulfur forms a sulfide (ion S^2-), oxygen forms an oxide (ion O^2-), fluorine forms a fluoride (ion F^-), chlorine forms a chloride (ion Cl^-), bromine a bromide (ion Br^-) and iodine an iodide (ion I^-).

The other element at the start of the compound name e.g. hydrogen or a metal like sodium, potassium, magnesium, calcium, etc. remains unchanged because there is only one oxidation state.

So typical compound names are, sodium sulfide, hydrogen sulfide, magnesium oxide, potassium fluoride, hydrogen chloride, sodium chloride, calcium bromide, magnesium iodide etc.

However, with different oxidation states the complications will arise e.g.

(i) Where an element can form two different compounds with different formulae with the same  element there needs to be a way of expressing it in the name as well as in the formula e.g.

iron(II) chloride, FeCl₂ and iron(III) chloride, FeCl₃

copper(I) oxide, Cu₂O and copper(II) oxide, CuO

Hear chlorine has a combining power of 1 (valence 1) and oxygen 2 in both compounds.

However, iron can have a valence of 2 or 3 and copper 1 or 2 and these also correspond numerically to the charge on the metal ions in such compounds e.g. Fe²⁺ and Fe³⁺, Cu⁺ and Cu²⁺.

Therefore the 'Roman numerals' number in (brackets) gives the valence of the element in that particular compound. At a higher academic level this is known as the oxidation state.

(ii) When the non-metal is combined with oxygen to form a negative ion (anion) ion which combines with a positive ion (cation) from hydrogen or a metal, then the end of the 2nd part of the name ends in ...ate or ...ite e.g.

NO₃ in a compound formula is nitrate e.g. KNO₃, potassium nitrate.

SO₃ in a formula is sulphite, e.g. Na₂SO₃, sodium sulphite,

SO₄ is sulfate, e.g. MgSO₄, magnesium sulfate,

PO₄ is phosphate, e.g. Na₂HPO₄, disodium hydrogen phosphate

• Other examples

  For metallic or non–metallic elements the name of the element is used if NOT in an anion

  • Some of the old names are still in common use, but try to use the correct systematic name e.g.

  • copper(I) oxide Cu₂O and copper(II) oxide CuO

    • (once called cuprous oxide and cupric oxide)

  • iron(II) chloride FeCl₂ and iron(III) chloride FeCl₃

    • (once called ferrous chloride and ferric chloride)

  • iron(II) oxide FeO, iron(III) oxide Fe₂O₃ and diiron(II) iron(III) oxide, Fe₃O₄

    • (once called ferrous oxide and ferric oxide and tri–iron tetroxide)

    • Historic note: ...ous was the lower oxidation state, ...ic the higher.

  • vanadium(II) sulfate for VSO₄ or V²⁺SO₄^2– and vanadium(III) sulfate V₂(SO₄)₃

  • sulfur(IV) oxide SO₂ (sulfur dioxide) and sulfur(VI) oxide, SO₃ (sulfur trioxide)

  • nitrogen(I) oxide N₂O (dinitrogen oxide) nitrogen(II) oxide NO (nitrogen monoxide), nitrogen(IV) oxide NO₂ (nitrogen dioxide) and nitrogen(V) oxide N₂O₅ (nitrogen pentoxide).

  • transition metal complex cations e.g.

    • diaquatetraamminecopper(II) ion, [Cu(H₂O)₂(NH₃)₄]²⁺

      • (water and ammonia are electrically neutral ligands attached to the central Cu²⁺ ion)

• For elements (metal or non–metal) combined with oxygen or other more electronegative element, giving an anion, the ion name ends in ...ate with the prefix derived from the elements name. In such cases the oxygen carries the negative oxidation state of (–2) or chlorine (–1) e.g.

  • vanadate(V) ion, VO₄^3–,

  • manganate(VI) ion, MnO₄^2–, manganate(VII) ion, MnO₄^–, (was called the permanganate ion)

  • sulfate(IV) ion, SO₃^2– (sulphite) and sulfate(VI) ion,SO₄^2– (sulfate)

  • nitrate(III), NO₂^– (nitrite) and nitrate(V), NO₃^– (nitrate)

    • Historic note: ...ite was the lower oxidation state, ...ate the higher.

  • chlorate(I), ClO^–, chlorate(VII), ClO₄^– etc. oxygen is more electronegative than chlorine.

    • (once called the hypochlorite ion and the perchlorate ion respectively)

  • transition metal complex anions e.g.

    • tetrachlorocuprate(II) ion, [CuCl₄]^2– (oxidation states Cu +2, Cl –1)

      [SEARCH BOX]

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