Periodic Properties of the Elements

Group Trends: The Active Metals


Group Trends: The Active Metals

Group 1A: The Alkali Metals

1A

3
Li

11
Na

19
K

37
Rb

55
Cs

87
Fr

The word "alkali" is derived from an Arabic word meaning "ashes". Many sodium and postassium compounds were isolated from wood ashes (Na2CO3 and K2CO3 are still occasionally referred to as "soda ash" and "potash").

As we move down the group (from Li to Fr) we find the following trends:

The alkali metals have the lowest I1 values of the elements

This represents the relative ease with which the lone electron in the outer 's' orbital can be removed.

The alkali metals are very reactive, readily losing 1 electron to form an ion with a 1+ charge:

M -> M+ + e-

Due to this reactivity, the alkali metals are found in nature only as compounds. The alkali metals combine directly with most nonmetals:

2M(s) + H2(g) -> 2MH(s)

(Note: hydrogen is present in the metal hydride as the hydride H- ion)

2M(s) + S(s) -> M2S(s)

2M(s) + Cl2(g) -> 2MCl(s)

2M(s) + 2H2O(l) -> 2MOH(aq) + H2(g)

The reaction between alkali metals and oxygen is more complex:

4Li(s) + O2 (g) -> 2Li2O(s) (lithium oxide)

2Na(s) + O2 (g) -> Na2O2(s) (sodium peroxide)

K(s) + O2 (g) -> KO2(s) (potassium superoxide)

Note:

Flame colors:

Group 2A: The Alkaline Earth Metals

2A

4
Be

12
Mg

20
Ca

38
Sr

56
Ba

88
Ra

Compared with the alkali metals, the alkaline earth metals are typically:

The first ionization values (I1) of the alkaline earth metals are not as low as the alkali metals:

the alkaline earth metals are therefore less reactive than the alkali metals (Be and Mg are the least reactive of the alkaline earth metals)

Calcium, and elements below it, react readily with water at room temperature:

Ca(s) + 2H2O(l) -> Ca(OH)2(aq) + H2(g)

The tendency of the alkaline earths to lose their two valence electrons is demonstrated in the reactivity of Mg towards chlorine gas and oxygen:

Mg(s) + Cl2(g) -> MgCl2(s)

2Mg(s) + O2(g) -> 2MgO(s)

The 2+ ions of the alkaline earth metals have a noble gas like electron configuration and are thus form colorless or white compounds (unless the anion is itself colored).

Flame colors:


1996 Michael Blaber