When building an atom, pay close attention to the particles' electrical charges. Electrons have a charge of -1.
Neutrons are neutral, as their name implies.
A stable atom has a net charge of 0. In other words, it has an equal number of protons and electrons. The positive protons cancel out the negative electrons. When the number of electrons does not equal the number of protons, the atom is ionized. Do not allow your atom to become too ionized.