va + singular noun beginning with a consonant: a boy; a car; a bike; a zoo; a dog
van + singular noun beginning with a vowel: an elephant; an egg; an apple; an idiot; an orphan
va + singular noun beginning with a consonant sound: a user (sounds like 'yoo-zer,' i.e. begins with a consonant 'y' sound, so 'a' is used); a university; a unicycle
van + nouns starting with silent "h": an hour
va + nouns starting with a pronounced "h": a horse
Nitrogen is the chemical element with the symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772.