#4. The Chemist

#4. The Chemist:
A famous chemist was murdered in his own lab.
There was no evidence of any crime except for a piece of paper with the names of 5 chemical substances on it:
Carbon
Nickel
Oxygen
Sulphur
Lantanhum
On the day he was murdered, the chemist had only 3 visitors: his wife Mary, his nephew Nicolas, and his friend Johnathan. Who did the police arrest?
Did you solve the riddle?!