An atom consist of electron, protons and neutrons. Protons and neutrons are present with in nucleus while the electrons are present out side the nucleus.
All these three subatomic particles construct an atom. A neutral atom have equal number of proton and electron. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other.
In given statement there are 8 electrons it means the atomic number of given atom is 8 so it would oxygen. The atomic mass of oxygen is 16 while in given statement mass is 17 it means it is the isotope of oxygen that have one more neutron than oxygen 16.
Isotope of an element have same atomic number but different atomic mass due to different number of neutrons.
You should always study. you can be above your class. thats what i do.. search google for ur course. they can tell u some key points for it. u should even study what you know and want to know as well.
