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I was studying the periodic table. My teacher said that

Group 1 elements are called alkali metals because their oxides are soluble in water and group 2 elements are called alkaline earth metals as their oxides form in the earth and are water soluble.

  1. What did she mean by "oxides"? Aren't alkali metals bases which are soluble in water? Then what is the term "oxides" doing here?

  2. What is the difference between "alkali" and "alkaline"? Why do we call one of them "alkali metals" and the other "alkaline earth metals" when the concept is the same?

  3. Why are the bases of group 1 and group 2 elements soluble in water? What is the reason?

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    $\begingroup$ I really don't know where your teacher has this definition from, but the word "alkali" has Arabic origins and means ashes, which is where most of them initially were found. By oxides your teacher probably meant $$\ce{4M + O2 -> 2M2O; \hspace{2em} M2O + H2O -> 2MOH}.$$ $\endgroup$ – Martin - マーチン Jan 18 '17 at 8:36
  • $\begingroup$ alkalis are bases that dissolve in water so my first question is now answered. Thanks!! I just needed to know what she mean by oxides. If she mean pure compounds with oxygen or bases. $\endgroup$ – MartianCactus Jan 18 '17 at 8:57
  • $\begingroup$ I think she probably meant what @Martin-マーチン said she probably meant ;) $\endgroup$ – airhuff Jan 18 '17 at 9:10
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The description of the reasoning of the naming is … let’s call it simplified.

First, concerning your first point. Alkali metals are not bases by themselves, they are metals. Metals typically cannot be classified in the original Arrhenius or extended Brønsted-Lowry acid/base classification. The cannot dissociate to liberate either $\ce{H+}$ or $\ce{OH-}$ because they only consist of metal atoms. And they cannot act as a donor or an acceptor for protons — one because of the lack of protons and two because of the lack of favourable lone pairs.

It is only when we oxidise these metals in some way or another that they form a species that can interact with water in the acid-base formalities. One possibility for oxidation is simple burning, i.e. reacting with atmospheric oxygen. Unfortunately, only lithium is nice enough to ‘do what you might expect’, hence why I’m using that as an example in the following equations:

$$\begin{align}\ce{4Li + O2 &-> 2 Li2O}\tag{1}\\[0.4em] \ce{Li2O + H2O &-> 2 LiOH}\tag{2}\end{align}$$

As we can see, the oxide we generated, lithium oxide, can react with water to form a hydroxide which dissociates into $\ce{OH-}$ ions — an Arrhenius base. We can also say that the oxide anion is capable of accepting protons donated by water — a Brønsted-Lowry base. Therefore, lithium oxide (and also lithium hydroxide) are the bases while lithium metal is not.

The second question is easily answered. Alkali is a noun and alkaline is an adjective. Thus, alkali metals is a compound word formed by two nouns while alkaline earth metals is also a compound word formed by two nouns where the first noun is further specified by an adjective.

The third question is difficult to answer. There is not really an easy or understandable explanation; it all boils down to churning numbers and finding out that the dissolution is favourable, i.e. the corresponding equation represents a decrease in Gibbs free energy: $\Delta G < 0$.

We can break dissolution of solids into two general processes:

  1. breakdown of the ionic lattice, i.e. liberation of individual ions from a solid

    $$\ce{NaOH(s) -> Na+(s) + OH- (s)}\tag{3}$$

  2. dissolution of the individual ions

    $$\begin{align}\ce{Na+(s) &->[H2O] Na+(aq)}\\[0.3em] \ce{OH-(s) &->[H2O] OH-(aq)}\end{align}\tag{4}$$

The first of these steps requires energy because you are breaking bonds (this is always unfavourable). It is typically represented by $\Delta_\text{latt} H^0$ — the standard lattice enthalpy.

The second process is exothermic because the individual ions are now again somewhat balanced by the polar water molecules. It is typically represented by $\Delta_\text{solv} H^0$.

Both processes are favourable from an entropic point of view.

Whether the entire reaction $(5)$ happens, depends on the difference between lattice and solvation enthalpies. If the lattice enthalpy is too high to be offset by solvation enthalpies and the entropy effects, an ionic compound will not dissolve. For alkali metal oxides and alkaline earth metal oxides, the lattice enthalpy is typically low and thus they dissolve.

$$\ce{NaOH (s) -> Na+ (aq) + OH- (aq)}\tag{$3+4=5$}$$

The actual reason for their names is in another castle beyond the scope of this answer. At first glance, I would forward you to the corresponding Wikipedia articles.

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  • $\begingroup$ amazing answer!! But I need to get some things straight. First, check if my understanding of question 3) is correct- $\endgroup$ – MartianCactus Jan 18 '17 at 18:07
  • $\begingroup$ so firstly, ionic compounds can either be broken down when they are in a lattice form by water (which is endothermic as the ions are already being attracted by other ions) and 2nd when water breaks down free floating ionic compounds(which is exothermic as the bond energy in them is released). Now if the breaking of lattice is less endothermic than the breaking of single molecules is exothermic then there would be enough energy for reaction with water and thus the base will be soluble. Otherwise the base wont be soluble as there simply isn't enough energy(heat) there. Am I right? $\endgroup$ – MartianCactus Jan 18 '17 at 18:13
  • $\begingroup$ now, if the understanding is correct, the question about 3) is, how can you even find free floating ions when ions always stay in lattices? $\endgroup$ – MartianCactus Jan 18 '17 at 18:14
  • $\begingroup$ also about 1) I still don't understand..a metal oxide is not a metal base. Then how can we call it an alkali? $\endgroup$ – MartianCactus Jan 18 '17 at 18:15
  • $\begingroup$ @MartianCactus On 1): What is a ‘metal base’ in the first place? The term is not commonly defined. ‘Metal oxide’ is. $\endgroup$ – Jan Jan 18 '17 at 22:04
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Alkali metals when come in contact with water form alkali (base) and form metal oxides when reacts with water. Alkali metal oxides are also bases and form alkaline solution when dissolved in water. Group 2 metals also form bases during reaction with water and their oxides are basic and found in earth crust. Be in group 2 is not known as alkaline earth metal because its oxide is amphoteric and are rare.

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  • $\begingroup$ IUPAC includes Be as an alkaline earth metal. It also includes Sr and Ba, which are certainly "alkaline" but not much "earth" because their oxides/hydroxide are fairly soluble in water. $\endgroup$ – Oscar Lanzi Jun 8 '17 at 2:44

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