Which groups form cations


There are some general rules for the main groups

1. Physical state of the element

Gases are found on the right (noble gases, F.2, Cl2, O2, N2) and H2
1 liquid on the right (Br2)
The other elements are solids


{In the subgroups, Hg is liquid, all other elements are solid.}

2. Metallic or non-metallic character

If you draw a diagonal from top left to bottom right,
are metals to the left of the diagonal and non-metals to the right of it.
In the area of ​​the diagonal are the "semi-metals" -
better known as the "semiconductors" (e.g. Si, Ge)


{The subgroups only contain metals.}

Chemically, elements with a metallic character form cations. Elements with a non-metallic character form anions; this also includes composite anions. In the case of a metallic character, salts are preferred; in the case of a non-metallic character, covalently bound molecules are used.



Example of an element

chemical behavior


N / A - metal

Cations Na+


Approx - metal

Cations approx2+


Al - metal

Cations Al3+


C. - non-metal

Ge - semiconductors
Pb - metal

no cations; Anions C4-; compound anion CO32-;
covalent compounds (organic chemistry)
no element cations; no element anions; covalent bonds
Cations Pb2+


N - non-metal

Bi - metal

Anions N3-; compound anions, e.g. NO3-;
covalent compounds e.g. NH3
Cations Bi3+, but more often covalently built connections


O - non-metal

Anion O2-, compound anions, e.g. OH-;
covalent compounds, e.g.2O


Cl - non-metal

Anions, e.g., Cl-; compound anions; e.g. ClO4-;
covalent compounds, e.g., ClO2

{In the 4th and 5th group it can also be seen that the character changes from top to bottom.
The upper elements are to the right and the lower elements to the left of the diagonal.}

3. Possible ions or possible oxidation numbers

  • For the 1st and 2nd main group the rule is simple:
    Elements of the 1st group form cations Li+, ...;
    Elements of group 2 cations Be2+, ...
  • In the 3rd main group there are preferably trivalent cations, e.g. Al3+.
    The trivalent ion is possible for the last element of the group, but the 1-valent ion Tl+ more stable.
  • In the 4th group, 2- and 4-valent cations are possible with Sn and Pb.
  • In the 5th group there are 3-valent cations in Bi.
  • Anions predominate in the 6th and 7th groups.
  • In compounds, the maximum possible oxidation number is always the same as the group number.
    Example for group VII: in the anion ClO4- the oxidation number of chlorine is +7.
  • There are "rules" in the subgroups, but these are only rough guidelines. The statement that "the maximum oxidation number is equal to the group number" is useful. The example of iron shows what caution is necessary here! Iron occurs in the 8th subgroup, but does not form compounds with an oxidation number of +8, but rather Fe2+ and Fe3+. The following statement is partially applicable to manganese: In the permanganation MnO4- the oxidation number of manganese is +7, a Mn7+-Cation does not exist, but Mn2+-Cations.

4. Size of atoms

The atomic radius (and also the ionic radius) decreases continuously within a period. This can be justified qualitatively with the shell model: more and more electrons are added to a shell; this also increases the number of protons in the nucleus accordingly. "+" and "-" attract each other; the shell is more and more attracted to the nucleus and thus the atom becomes smaller.
When changing to the next period, electrons are built into a shell further out. With the transition to the next period there is a sudden increase in the atomic radius.



157 64 pm

N / A


191 99 pm



235 114 pm



250 133 pm

In addition, the values ​​show that the atom size naturally increases from top to bottom within a group because the outermost valence electron shell is always further out.

Questions for understanding (the attraction between core and shell must be observed!):
What's smaller? (Use a simple shell model, 1st, 2nd shell
- corresponding only to the principal quantum numbers.)
1. a Li atom or a Li+-Ion?
2. an O atom or an O2-Ion?
3. a na+-Ion or an Al3+-Ion?


Solutions to this:

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