Video: Identifying the True Statement About Alkali Metals in a Set of Statements

Which of the following statements about alkaline metals is true? [A] They readily react to form salts. [B] They all form X⁻ ions. [C] Their atomic radius decreases going down the group. [D] They have the highest electronegativity of the elements in their respective periods. [E] Lithium and sodium are more reactive than potassium and rubidium.

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Video Transcript

Which of the following statements about alkaline metals is true? A) They readily react to form salts. B) They all form X⁻ ions. C) Their atomic radius decreases going down the group. D) They have the highest electronegativity of the elements in their respective periods. Or E) Lithium and sodium are more reactive than potassium and rubidium.

The alkaline metals are the elements that are found in the first group of the periodic table, excluding hydrogen. These elements include lithium, sodium, potassium, rubidium, and cesium. Francium is also in the first group of the periodic table, but it is radioactive and decays relatively quickly. So we don’t really consider it much when we’re talking about the alkaline metals in general.

The alkaline metals originally got their names because they all react to form hydroxides, which were originally called alkalies. Examples of these include sodium hydroxide and potassium hydroxide. The alkaline metals are all soft, shiny metals that must be stored under oil because they will react readily with the oxygen in air. In fact, this is a feature of alkaline metals in general that they are extremely reactive.

So now let’s take a look through our answer choices to see which of these statements is true regarding alkaline metals. Answer choice A says that alkaline metals react readily to form salts. As I’ve mentioned already, alkaline metals are indeed highly reactive. They react readily to form all kinds of salts. We’ve already mentioned one type of salt that they form, which are hydroxide, such as lithium hydroxide, sodium hydroxide, and potassium hydroxide.

Alkaline metals will also react readily with halogens to form salts. Halogens are the elements found in group 17 of the periodic table. For example, sodium can react with chlorine to form sodium chloride, which is commonly known as table salt. Other examples of these salts would include lithium fluoride, potassium iodide, or any combination of alkaline metal and halogen. Alkaline metals can also react readily with acids to form salts and hydrogen gas.

In fact, alkaline metals react so readily to form salts that it’s quite difficult to find them in their pure form. If we want to obtain an alkaline metal in its pure form, we usually have to use some kind of very energy-intensive reaction on an alkaline metal salt as it’s the case in the electrolytic reduction of molten lithium chloride. In this reaction, we can obtain pure lithium metal from lithium chloride that has been melted. So answer choice A is true. Alkaline metals most certainly react readily to form salts. But let’s take a look at our other answer choices.

Answer choice B says that they all form X⁻ ions. Since alkaline metals are in the first group of the periodic table, they all only have one electron in their valence shell. So the alkaline metals will easily lose this outermost electron to form a positively charged ion because this will leave them with a full shell. So answer choice B is not true. Alkaline metals all form X⁺ ions, not X⁻ ions.

Answer choice C says that their atomic radius decreases going down the group. In general, the trend for atomic radius is that it increases as we go down the periodic table. This is because as we go down the periodic table, we’re adding more electrons and therefore more electron shells to the atom. So the atom will be bigger. So answer choice C is false. The atomic radius increases going down the group, not decreases.

Answer choice D says that they have the highest electronegativity of the elements in their respective periods. The general trend for electronegativity is that it increases as we go up the periodic table and to the right, with fluorine being the most electronegative atom on the periodic table. So answer choice D is not correct either. Alkaline metals actually have the lowest electronegativity of the elements in their respective periods.

Our final answer choice says that lithium and sodium are more reactive than potassium and rubidium. Lithium and sodium are the first two of the alkaline metals. And potassium and rubidium follow. When alkaline metals react with something, they give up their outermost electron. If we compare lithium to rubidium, lithium only has two electron shells. So its outermost electron is in the second shell. Rubidium, on the other hand, has five electron shells. And its outermost electron is in that fifth shell.

This means that the outer electron, which is the electron that will be removed when the alkaline metal undergoes the reaction, is much further from the nucleus in rubidium than it is in lithium. Since it’s further from the nucleus, it will be easier to remove. This has the overall effect of rubidium being more reactive than lithium because it’s easier to remove the outermost electron in rubidium than it is in lithium. So answer choice E is not correct. Lithium and sodium would be less reactive than potassium and rubidium are.

So as we stated at the beginning, of our statements, the only one that is true about alkaline metals is answer choice A, that they react readily to form salts.

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