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The alkali metals are a highly reactive family of metals occupying Group 1 (1A) of the periodic table. In their elemental form, the alkali metals have a shiny, metallic appearance, but they are also very soft and can be easily cut by hand. Due to their high reactivity, however, the alkali metals do not exist in their elemental form in nature.Because the alkali metals possess only a single valence electron with a low first ionization energy, the alkali metals (M) readily react with nonmetals and lose this electron to form cations with a +1 charge (M⁺) that are incorporated into ionic compounds. Consequently, elemental samples of alkali metals will rapidly tarnish in oxygen by forming oxides (Reaction 1) and vigorously react with elemental halogens (X₂) to form halide salts (Reaction 2) .2 M(s) + O₂(g) → M₂O(s) Reaction 12 M(s) + X₂(g) → 2 MX(s) Reaction 2Alkali metals also react violently with water to form very basic metal hydroxide (alkali) solutions while releasing hydrogen gas in the process (Reaction 3) .2 M(s) + 2 H₂O(l) → 2 MOH(aq) + H₂(g) Reaction 3As demonstrated by these reactions, alkali metals possess a low electron affinity that makes accepting an additional electron to form an anion less favorable than losing the valence electron to form a cation.Of the alkali metals, francium is the rarest since it is a trace radioactive byproduct formed from the decay of uranium ores and has a very short half-life. As a result, any francium formed will quickly decay into radium or astatine by beta decay or by alpha decay, respectively.Whereas hydrogen is a part of Group 1 based on its number of valence electrons, hydrogen is not an alkali metal. Interestingly, some research has suggested that under extreme pressures and very low temperatures, hydrogen may form a solid with metallic properties.
-At higher concentrations, alkali metal ions close to the size of a potassium ion can displace potassium ions in biological systems and promote low potassium levels (hypokalemia) . Which cation is most likely to displace K⁺ ions?

Question

Quizplus · Accepted Answer

11ecba2d_11e0_eac7_a3bf_23e55a482882_MD0008_00 Ionic radii tend to decrease in size across a period (row) of the periodic table (left to right) and increase moving down a group (column). This trend occurs for metal cations, and then resets and repeats for anions beginning near the division between metals and nonmetals, past which anions tend to preferentially form. Losing electrons to form a cation causes the remaining electrons to experience a greater effective nuclear charge (Z_eff), pulling the electrons closer to the nucleus. Conversely, gaining electrons to form an anion produces greater electronic repulsion and nuclear shielding (lesser Z_eff), which pushes electrons farther from the nucleus. Alkali metal ions that are more similar in size to K⁺ ions are the most likely to displace a K⁺ ion. Because the size of the alkali metal ions increases moving down the column, the elements adjacent to potassium will form ions that are closer to the size of the K⁺ ion. As a result, out of the options given, a Rb⁺ ion is closer to the size of a K⁺ ion and will be more likely to displace K⁺ ions than Li⁺, Cs⁺, or Fr⁺ ions. (Choice A) Because alkali metal ions decrease in size moving up the column, Li⁺ is much smaller than K⁺. (Choices C and D) Because Cs⁺ and Fr⁺ are larger than Rb⁺ (and even larger than K⁺), these ions do not displace K⁺ as effectively as Rb⁺. Educational objective: For ions having the same charge, ionic radii tend to decrease across a period and increase down a group on the periodic table. Ions of adjacent elements tend to be closer in size than those farther away.