Video: Identifying the Element Whose Ions Would Most Probably Have a Given Oxidation Number in a Set of Elements on an Abbreviated Periodic Table

Which of the letters corresponds to the element whose ions would most probably have an oxidation number of +2?

04:10

Video Transcript

Which of the letters corresponds to the element whose ions would most probably have an oxidation number of plus two?

We’re given an abbreviated periodic table that has most of the elements missing but some filled in. Assigning oxidation numbers to an element is a way for chemists to track the number of electrons in an atom or compound that it’s undergoing a redox or oxidation–reduction reaction. In this question, we’re being asked to identify the element whose ions would most probably have an oxidation number of plus two. The oxidation number of an ion is always equal to the charge of the ion. So, what this question is really asking us is to determine which letter will correspond to an element that is most likely to form two plus ions.

When atoms form ions, they gain or lose electrons. The most probable ion that they will form will be the one where they gain or lose electrons so that they have a full outer shell. For example, chlorine, which is found in group 17 of the periodic table, has seven outer electrons. Since a full outer shell has eight electrons, chlorine is most likely to gain an electron to form the one minus ion. Lithium, who’s found in group one of the periodic table, just has one outer electron. So, the most probable ion that lithium will form will be one where it loses an electron to form the lithium plus ion.

So, now we need to figure out, out of the letters that we’re given, which one will correspond to an element that’s most likely to lose two electrons to form a two plus ion. If we take a look at the abbreviated periodic table we’re given, the first column on the left contains lithium, which is in group one of the periodic table. So, this must be group one. The one next to it must be group two, which we can confirm because it contains magnesium which is in group two of the periodic table. All the way to the right, we see that this group contains krypton, which is a noble gas located in group 18 of the periodic table. So, this column must be group 18.

That means the one next to it must be group 17, which corresponds to the group containing the halogens like fluorine and chlorine. Which means that this group must be group 16, the group that contains oxygen and sulfur. As we’ve discussed with lithium, elements in group one only have one outer electron. So, when they form an ion, they’re most likely to lose that electron to form an ion with a charge of one plus. Elements in group two of the periodic table have two outer electrons. So, when these elements form ions, they’re most likely to lose those two outer electrons and form ions with a charge of two plus.

If we look at the right of the periodic table to group 18, this is the group that contains the noble gases, which are known for being unreactive. In group 18, all of the elements have a full outer shell of electrons, so they’re unlikely to form ions. Next to group 18 is the group that contains halogens like chlorine which, we’ve discussed, has seven outer electrons. So, it’s most likely to gain an electron to form an ion with a one minus charge. Elements in group 16 have six outer electrons, which means that they’re two electrons away from having a full outer shell of electrons. So, when elements in group 16 form ions, they’re most likely to gain two electrons to form ions with a two minus charge.

We’re looking to identify an element with a two plus charge, which would be an element in group two of the periodic table. So, the element that we’re looking for must be the one that corresponds to letter B. This letter corresponds to the element calcium which usually forms two plus ions. And so, it would most probably have an oxidation number of plus two.

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