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Periodic table - Period 4 elements potassium to krypton - explaining bonding, formula, oxidation states and chemical reaction trends

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Doc Brown's Chemistry  Advanced Level Inorganic Chemistry Periodic Table Revision Notes

Part 6.3 Period 4 element trends in bonding, formulae and oxidation state

[Author © Dr Phil Brown PhD: Doc Brown's advanced level inorganic chemistry exam revision notes suitable for students of UK Advanced A level inorganic chemistry courses & US K12 grade 11, grade 12 and AP honors inorganic chemistry courses: [periodic table page updated April 5th 2026 *]

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All my periodic table advanced A level chemistry revision notes

All my advanced A level inorganic chemistry revision study notes

GCSE Level periodic table revision notes

6.3 Period 4 trends in bonding and  formulae

A survey of all the elements of period 4 in terms of metallic/non–metallic character, electron configuration, atomic radii, ionic radii, oxidation states, nature and formulae of oxides, nature and formulae of chlorides, nature and formulae of hydrides.

There is also an extra section tabulating a series of isoelectronic species.

Krypton is omitted since there is no relevant or comparable chemistry and the table is more likely to fit on the page!

Abbreviations: ampho = amphoteric, cov = covalent

old/latest  Group no. 1 2 3 4 5 6 7 8 9 10 11 12 3/13 4/14 5/15 6/16 7/17
ZSymbol 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br
element potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine
electron configuration [Ar]4s1 [Ar]4s2 [Ar]3d14s2 [Ar]3d24s2 [Ar]3d34s2 [Ar]3d54s1 [Ar]3d54s2 [Ar]3d64s2 [Ar]3d74s2 [Ar]3d84s2 [Ar]3d104s1 [Ar]3d104s2 [Ar]3d14s24p1 [Ar]3d14s24p2 [Ar]3d14s24p3 [Ar]3d14s24p4 [Ar]3d14s24p5
ox. states +1 +2 +3 +2, +3, +4 +2, +3, +4, +5 +2, +3, +6 +2, +4, +6, +7 +2, +3 +2, +3 +2, +3 +1, +2 +2 +1, +3 +2, +4 +3, +5 –2, +2, +4, +6 –1, +1, +3, +5, +7
oxides K2O, K2O2, KO2 CaO Sc2O3 TiO, TiO2 VO, V2O3, VO2, V2O5 CrO, Cr2O3, CrO3 MnO, MnO2, Mn2O7 Fe2O3 CoO NiO Cu2O, CuO ZnO Ga2O3 GeO, GeO2  As4O6, As4O10 SeO2, SeO3 Br2O, BrO2, BrO3
nature of oxides basic basic basic basic, ampho basic, ampho, ampho, weakly acidic basic, ampho, acidic basic, ampho, acidic ampho? basic ampho basic, basic ampho ampho ampho, weakly acidic weakly acidic? both weakly acidic all acidic
chlorides KCl CaCl2 ScCl3 TiCl3, TiCl4 VCl3 CrCl3 MnCl2 FeCl2, FeCl3 CoCl2 NiCl2 CuCl, CuCl2 ZnCl2 GaCl3 GeCl2, GeCl4 AsCl3 Se2Cl2, SeCl4 BrCl
nature of chlorides ionic ionic ionic ionic, covalent ionic? ionic? ionic ionic, covalent ionic ionic ionic ionic ? ionic, covalent covalent both cov cov
hydride KH CaH2 ? ? ? ? ? ? ? ? ? ? GaH3 GeH4 AsH3 H2Se HBr
nature of hydride ionic ionic ? ? ? ? ? ? ? ? ? ? cov covalent covalent cov cov

  • The patterns across Period 4 are not as clear cut as for periods 2 and 3 – one reason being the interjection of the 3d block of metals

  • Metallic or non–metallic character of period 4 elements

    • From far left (metallic) to the far right (non–metallic) the non–metallic character increases as the electronegativity and ionisation energies increase.

    • This trend shows up as the electronegativity difference in M–X decreases (M to the left of X)

      • (i) the ionic bonding of the giant ionic lattice ==> covalent bonding character of small molecules of the oxides and chlorides

      • (ii) the oxide changes from basic ==> amphoteric ==> acidic

      • BUT within the 3d block the patterns are quite complicated and the oxidation state of the metal very much determines the structural and chemical character of the compound.

  • Period 4 element oxidation states

    • From potassium (+1) to manganese (+7) the maximum oxidation state is determined by the maximum number of outer valency electrons.

    • After Mn, there is a tendency to fall to a stable +2 state e.g. cobalt, nickel, copper and zinc.

    • Beyond zinc, the last element in the 3d block, the maximum oxidation state is governed by the maximum number of s + p electrons beyond the full 3d sub–shell i.e. from gallium to bromine the maximum oxidation state rises from +3 to +7.

  • Period 4 element oxides – formulae, bonding and chemical character

    • Ionic lattice  ==> covalent character of the oxides and chlorides

    • and in chemical character the oxide changes from basic ==> amphoteric ==> acidic.

    • The oxide of maximum oxidation state for potassium to manganese is determined by the maximum number of outer valency electrons (from 1 to 5).

    • Again the patterns within the 3d block are complicated.

  • Period 4 element chlorides – formulae, bonding and chemical character

    • the ionic ==> covalent character of the oxides and chlorides as the electronegativity difference in M–X decreases (M to the left of X)

  • Radii of isoelectronic ions

  • Isoelectronic means species having the same total number of electrons.

  • The table below considers the isoelectronic ions associated with Periods 2, 3 and 4.

isoelectronic system Group 4/14 Group 5/15 Group 6/16 Group 7/17 (Group 0/18) Group 1 Group 2 Group 3/13
Period Period 2 Period 3
[Ne] 10e 1s22s22p6 C4– N3– O2– F– (Ne) Na+ Mg2+ Al3+
total nuclear charge +6 +7 +8 +9 (+10) +11 +12 +13
radius in picometre (pm) 260 171 140 136 (38–112*) 95 65 50
name of ion carbide nitride oxide fluoride (neon) sodium magnesium aluminium
Period Period 3 Period 4
[Ar] 18e 1s22s22p63s23p6 Si4– P3– S2– Cl– (Ar) K+ Ca2+ Sc3+
nuclear charge +14 +15 +16 +17 (+18) +19 +20 +21
radius  in picometre (pm) 271 212 184 181 (71–154*) 133 99 81
name of ion silicide phosphide sulfide chloride (argon) potassium calcium scandium

  • Excluding the noble gases themselves, there is a clear pattern of decreasing ionic radius with increase in nuclear charge (+ atomic/proton number) for the two isoelectronic series tabulated above.

  • From left to right the proton/electron ratio is steadily increasing so that the electrons are experiencing an increasingly greater attractive force of the nucleus, hence the steady decrease in radii for an isoelectronic series.

  • * all sorts of values are quoted for noble gas radii e.g. atomic, covalent and ionic, but most don't fit in the pattern above which is quite clear for all the cations and anions listed.

WHAT NEXT?

PLEASE NOTE GCSE Level periodic table notes are on separate webpages

Period 2-4 survey sub-index: 4.1 Period 2 Survey of the individual elements, 4.2 Period 2 element trends and explanations of physical properties * 4.3 Period 2 element trends in bonding, structure, oxidation state, formulae & reactions, 5.1 Period 3 survey of elements, 5.2 Period 3 element trends & explanations of physical properties, 5.3 Period 3 element trends in bonding, structure, oxidation state, formulae & reactions, 6.1 Survey of Period 4 elements, 6.2 Period 4 trends in physical properties, 6.3 Period 4 trends in bonding, formulae and oxidation state, 6.4 Important element trends down a Group

Advanced Level Inorganic Chemistry Periodic Table Index: Part 1 Periodic Table history Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr AND important trends down a group * Part 7 s–block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p–block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots All 11 Parts have their own sub-indexes near the top of the pages

Group numbering and the modern periodic table

The original group numbers of the periodic table ran from group 1 alkali metals to group 0 noble gases. To account for the d block elements and their 'vertical' similarities, in the modern periodic table, groups 3 to group 0/8 are numbered 13 to 18. So, the p block elements are referred to as groups 13 to group 18 at a higher academic level, though the group 3 to 0 notation is still used, but usually at a lower academic level. The 3d block elements (Sc to Zn) are now considered the head (top) elements of groups 3 to 12.

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