The remaining 18 electrons will be placed as lone pairs on the terminal atoms (the oxygen atoms). The nitrogen will be connected to each oxygen by at least one single bond each using 6 electrons. Nitrogen will be the central atom since it's the least electronegative of the two. This means we have 24 electrons to place in the Lewis structure 5 from nitrogen, 18 from the oxygens, and an extra electron due to the negative charge. In the case of nitrate we have the formula NO3^(-). So the best Lewis structure will be one where the octets of the atoms are completed and the formal charges are as little as possible. Atoms also generally want to have as little formal charge as possible since this is more stable. Nitrogen and oxygen are both second period elements and ideally want to follow the octet rule by having eight valence electrons. So when we correctly identify the bond orders (single and double bonds) then these bonds are indeed covalent bonds as you stated where the electrons are being shared between the bonding atoms. And remember the most important point is that it's not us creating these bonds, it's us trying to determine what type of bonding is actually occurring the molecule. So when we take that lone pair from oxygen, it's not really us taking oxygen's electrons since they never completely belonged to it in the first place. And before the double bond is formed each oxygen has 7 electrons and the nitrogen has only 3 electrons which means we've allocated some of the nitrogen's electrons to the oxygens already.īasically it's more correct to think about these valence electrons being more fluid and being able to move around the molecule more easily than being static and belonging to a single atom. Each oxygen brings six valence electrons as individual atoms and the nitrogen atom brings five electrons. If we think of each atom's electron belonging to it then even before we form the double there's some unequal distribution in a sense. Well Sal only put those lone pairs on each of the oxygen atoms in the first place to use up all the valence electrons according to the method used to create Lewis structures. So any unfavorable energetics nitrogen incurs from adding electrons to a half-filled p subshell are inconsequential compared to the stability achieved by a complete octet. Having a complete octet is very stable and a favorable state to be in for an atom like nitrogen. When nitrogen forms four bonds to the oxygen atoms, it will have eight valence electrons and complete it's octet (or fill its second electron shell). However the nitrogen in nitrate isn't a neutral atom of nitrogen becoming an anion instead it is bonding with three oxygen atoms to form a polyatomic anion. This means it has a half-filled p subshell (a full p subshell holds 6 electrons) and is more stable as opposed to having an extra electron. And this means it is an unfavorable process because of neutral nitrogen's electron configuration which is: 2s^(2)2p^(3). Meaning that adding an electron to a neutral nitrogen atom requires adding 6.8 kJ of energy per mole of nitrogen atoms. And these values are energy measurements so they need energy units like eV or kJ just saying 0 isn't meaningful.Īt any case, nitrogen's electron affinity isn't 0, instead it's -6.8 kJ/mol. Since it's defined as a release of energy, positive values indicate energy being released and negative values as energy being absorbed. It's the amount of energy released when a neutral atom (or molecule) gains an electron. So it's important to remember what electron affinity is.
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