There is no universal net charge for atoms. An atom's net charge is determined by comparing the number of protons and electrons that are in each atom.
There are three types of particles in an atom: protons, neutrons and electrons. Protons are positive, neutron are neutral (having no charge) and electrons are negative. Due to the fact that electrons can transfer from one atom to another, every atom has the possibility of becoming negatively or positively charged. If an atom loses an electron then it has more protons, which makes it positively charged. If an atom gains an electron then it has more electrons, which gives it a negative charge. If the number of electrons and protons are the same then the net charge is zero.