In which one of the following species is the central atom (the first atom in the formula) an exception to the octet rule? 17) a) bh4 – b) nh3 c) i2 d) nh4 + e) sf6

The specie in which the central atom does not obey the octet rule is SF6.

The octet rule states that atoms are stable when they have eight electrons around their outermost shell. It then follows that compounds are formed in obedience to this octet rule.

However, some atoms are capable of expanding their octet. This means that they are capable of having more than eight electrons on their outermost shell. This is common in p -  block elements that have accessible d orbitals which could participate in bonding such as sulfur. Hence, SF6 has twelve instead of eight electrons on its outermost shell. It does not obey the octet rule.

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E) SF6

Explanation:

sulfur hexafluoride (SF6) is an example of molecule that deviate from the octet rule by having more than 8 electrons around the central atom. It is referred to as an hypervalent molecule.

A hypervalent molecule is a molecule that contains one or more main group elements (Elements that are not part of the transition metal block in the periodic table) that bear more than eight electrons in their valence levels as a result of bonding.

In the SF6 molecule, the central sulfur atom is bonded to six fluorine atoms, so sulfur has 12 bonding electrons around it.

In the options given;

A) Boron is bonded to 4 hydrogen atoms, however it has a velency of 5 hence the negative sign present.

B) Nitogen is bonded to 3 hydrogen atoms, it belongs to group 5 and hence needs 3 more electrons to complete its octet.

C) I2 is a covalent molecule formed due to the sharing of electrons. Molecules are known generally to have a complete octet configuration.

D) NH4, Nitrogen (N) belongs to group 5 and hence needs 3 more electrons to complete its octet. The Hydrogen is in excess of one, hence the positive charge. the bond here is coordinate covalent so it satisfies the octet rule.

the answer is option a.