What is a neutral atom? I was told that an atom's atomic number is defined as follows:


  The number of electrons or protons present in a neutral atom is called atomic number. It is represented by Z.


What does neutral mean here? Why isn't it just "..present in an atom..."?
Electrons and protons are charged particles. The electrons have negative charge, while protons have positive charge. A neutral atom is an atom where the charges of the electrons and the protons balance. Luckily, one electron has the same charge (with opposite sign) as a proton.

Example: Carbon has 6 protons. The neutral Carbon atom has 6 electrons. The atomic number is 6 since there are 6 protons.