Cryolite is an ionic compound with the formula Na3AlF6. Of these elements, fluorine is by far the most electronegative. In fact, it is the most electronegative element. In ionic compounds it will accept one additional electron to fill its octet and achieve a noble gas configuration. This extra electron will give fluorine a -1 charge. This is equivalent to saying that fluorine has a -1 oxidation state.
Sodium on the other hand is a metal in the first column of the periodic table. It loses an electron to form a Na +1 cation. This is equivalent to saying that Na has a +1 oxidation state.
Aluminum belongs to group IIIA or group 13 of the periodic table. It has 5 valence electrons. This means that aluminum is capable of accepting 3 more electrons for a complete octet. Therefore it has an oxidation state of +3.
The molecular formula as a whole is uncharged or neutral. It has an overall charge of zero. We also know that it contains three positive charges from its sodium cations and six negative charges from its fluorine anions. This means that aluminum must have a +3 charge to make the molecule neutral.
Answering this question:
Sodium, aluminum and fluorine are all representative elements. This means that they belong to the s-block and/or p-block of the periodic table. The oxidation states of the representative elements can be predicted through their group numbers. Elements under the group IA or 1 of the periodic table has an oxidation state of +1. Group IIA or 2 has +2. Group IIIA or 13 has +3. Group IVA or 14 has +4 or -4. Group VA or 15 has -3. Group VIA or 16 has -2. Group VIIA or 17 has -1.
Sodium is under group 1 so it has an oxidation state of +1. There are 3 sodium atoms in Cryolite so it would be a total of a +3 oxidation state for all the sodium atoms. Fluorine is under group 17 so it has an oxidation state of -1. There are 6 fluorine atoms, so it would be a total of -6 oxidation state for fluorine. Aluminum is under group 13 so it has an oxidation state of +3. There is only one aluminum atom in Cryolite. To verify, we add all the oxidation state. Since Cryolite is a neutral compound, the sum of the oxidations states must be zero.
So the correct answer is Na = +1, F = -1, and Al = +3.
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