Video Transcript
Which of the following alkali metals is the most reactive? (A) Cesium, (B) potassium, (C) lithium, (D) rubidium, or (E) sodium.
The alkali metals can be found in group one of the periodic table. The alkali metals include lithium, sodium, potassium, rubidium, cesium, and francium. Hydrogen is in group one of the periodic table, but it is not an alkali metal. All of the alkali metals are in group one. So they have one valence or outer shell electron. And the size of the atoms increases as you descend the group.
Lithium has an atomic number of three. So a neutral atom of lithium will contain three electrons: two in the first shell and one in the second. Sodium has an atomic number of 11. So a neutral atom of sodium will have 11 electrons: two in the first shell, eight in the second shell, and one in the outer shell. So, as we go from lithium to sodium, the number of full electron shells increases by one. The same is true when we go from sodium to potassium. The number of full electron shells increases by one. This pattern is repeated all the way down the group so that francium will have the most full electron shells. So we know that the number of full electron shells increases as we descend the group. And as a result of this, the valence electron becomes further from the nucleus.
Let’s now look at how this impacts the atoms’ properties. As the nucleus of an atom has an overall positive charge and electrons have a negative charge, there is an electrostatic attraction between them. When the distance between these charged particles is larger, the electrostatic attraction between them becomes weaker. So the electrostatic attraction between the nucleus and the valence electron of larger atoms is weaker due to the larger distance between the nucleus and the valence electron. The number of full electron shells between the nucleus and the valence electron also contributes to this weaker attraction by what is known as the shielding effect, which is a reduction in attraction between the valence electrons and protons in the nucleus due to the presence of core electrons.
The core electrons essentially shield the nucleus from the valence electron. So there is a weaker electrostatic attraction between them. If there is a weaker electrostatic attraction between the valence electron and the nucleus, then the valence electron must be easier to remove. So the valence electron is easier to remove for larger atoms than it is for smaller atoms.
The question asks us which of the alkali metals is the most reactive. When alkali metals react, they lose their valence electron, forming cations. So the easier it is to remove the valence electron, the more reactive the metal will be. As we descend group one, the size of the atoms increases. Thus, it becomes easier to lose the valence electron, and in turn reactivity increases.
The question asks for the most reactive of the following alkali metals. The alkali metals listed in the answer choices are cesium, potassium, lithium, rubidium, and sodium. As the most reactive would be the largest and size increases as we descend the group, cesium must be the most reactive of the alkali metals given in the question. So the answer to the question “Which of the following alkali metals is the most reactive?” is (A) cesium.
