Which of the following is the family of elements whose oxides form acids in water? (A) Alkali metals [B) alkaline earth metals [C) transition elements [D) halogens, or (E) noble gases.
Elements on the periodic table are arranged into specific columns and rows. The rows are called periods and the columns groups. Elements are grouped into families according mostly to the manner in which the element atoms react. The families can be classified into metals, nonmetals, and metalloids or into another type of categorization as follows.
Group one is the alkali metals. Note that hydrogen is in the same group as the alkali metals even though it is not a metal. And this is because it reacts in a similar manner. Group two is the alkaline earth metals. Groups three to 12, the transition elements or transition metals, which includes the lanthanides and actinides. Groups 13 to 16, which show changing properties as we move down the groups from nonmetal to metalloid to metal. Group 17, which is the family of halogens. And group 18, the noble gas family.
We have said that elements are grouped in specific groups or families because they react in a similar manner. Let’s now look at options (A) to (E) to see how each of these families react. We will specifically look at their oxides to see if they react with water to form acids.
The oxides of the alkali metals are all of the form E2O, where E is the symbol of the alkali metal. For example, Li2O, lithium oxide; Na2O, sodium oxide; and K2O, potassium oxide. All of these oxides are white and are very soluble and reactive with water. Let’s take lithium oxide as an example to see how this oxide reacts with water. The reaction produces LiOH, or lithium hydroxide, which is soluble in water. The other alkali metals also produce hydroxides in water of the form EOH. But is lithium hydroxide acidic or basic?
The pH scale, which is really a measure of the concentration of hydronium ions, is measured from zero to 14, where seven, the middle point, shows a neutral pH. Anything below seven is acidic and anything above seven basic. The smaller the value below seven, the more acidic is the solution. And the higher the value above seven, the more basic is the solution.
Water itself can dissociate into hydronium and hydroxide or OH− ions. An alkali metal hydroxide such as a lithium hydroxide adds more hydroxide ions to a water solution. This will decrease the relative concentration of hydronium ions present and increase the basicity of the solution. So a lithium hydroxide solution or any alkali metal hydroxide solution is basic, and so this is not a possible answer.
Alkaline earth metal oxides of the form EO, where E is the symbol of the alkaline earth metal. And in this case, we’ll take the example magnesium oxide, MgO. Partially dissolves and reacts with water to form magnesium hydroxide, Mg(OH)2. Note that beryllium oxide, which is also an alkaline earth metal oxide, does not react with water. Because magnesium hydroxide contains hydroxide ions, it forms a basic solution in water. And so option (B) is not a possible answer.
Only a few of the many transition metal oxides are soluble with and react directly with water. The general oxide formulae are very complicated and come in many different forms. We can consider transition metal oxides not to commonly form acids in water.
The family of halogens all form oxides by a variety of types of reactions. And there are many forms of halogen oxides. For example, chlorine can form Cl2O, ClO2, or Cl2O7. Many halogen oxides are unstable. Let’s look at Cl2O7 and its reaction with water to see if it produces an acid. Liquid Cl2O7 can react with water to produce HClO4, or perchloric acid, which is a colorless liquid. And if this dissolves in water will then be aqueous. Perchloric acid is a very strong acid. In general, the halogen oxides if they react with water will produce acids. Most nonmetal oxides when reacting with water will also produce acids.
The noble gases are unreactive or inert. So they do not react with oxygen to form oxides. And thus, there are no oxides to react with water to form acids. So we can rule out noble gases as a possible answer.
And so the family of elements whose oxides form acids in water is option (D) the halogens.