Hello, I'm Stealthdevil, and welcome to my first guide on Mobafire
I bet a lot of you are thinking, why lux? She is outclassed by so many other mid-laners.
My answer? Because she is so much fun to play, nothing is quite as satisfying as firing a massive laser through the enemy team and chunking there adc for half his health from the back of the fight. She is the Ezreal of the Mid-Lane.

Pros:

High Range
Best Laugh
Skill shot that goes through one unit
Lucent Singularity is an AOE skill-shot that gives vision
Final Spark is on an extremely short cooldown
Shield that re-applies itself

Cons:

Squishy (requires good positioning)
100% skill-shots
No sustain
Can't CS under turret

Runes are very standard for an AP mid
Magic penetration/ ability power marks have always been the best option for an AP mid.
Flat mana regen seals are slightly unconventional, Armour runes seem to be much more common on all champions. However Lux has such great range, and she should never be getting hit. If you get a bruiser in your face, your going to die, your so squishy that armour seals aren't going to help. Armour seals are still viable if you are going to be playing super aggressive because you will be drawing minion aggro, but playing aggressive leads to mana problems, and the solution is.....Mana regen. Flat mana regen is much stronger than scaling mana regen because lux doesn't have many mana problems later on, and you should be getting blue buffs before scaling mana regen balances with flat mana regen
Magic resist seals are vital as a mid-laner. They help you win trades, though ideally you should out-range your opponent and not take any damage (poking).
AP quints are pretty self explanatory.

Athene's has been one of the most popular items for quite a while because it is ideal for a mid-laner, it gives tons of mana-regen, allowing for spells to be spammed, it gives CDR (more spamming) and magic resist, to win trades/survive burst and of course AP,
Morellonomicon, gives similar stats, Less mana regen, and more AP, it also has a very situational passive.

The difference is the magic resist, AP, and price (mana regen doesn't matter since you should be getting blue buff): 15AP, 400 gold and a situational passive VS. 40 magic resist According to the league of legends wiki, 15 AP is worth about 400 gold, while 40 MR is worth 800, making them practically equal without including passives.
Morellonomicon has a passive that is often useless, while Athene's Unholy Grail's passive is useless while you have blue buff (around 7 minute, so ages before you get Athene's).
It is completely your choice (however against people like Mundo, Fidddle, Aatrox, Swain and Sion, it is wise to get Morellonomicon