By Victoria Martin Updated Aug 30, 2022
The nucleus is the atom’s core, the densest region composed of protons and neutrons. Protons carry a positive charge, defining the element’s identity and its position on the periodic table. Neutrons are neutral, adding mass without influencing charge.
Electrons, the third subatomic particle, surround the nucleus. They possess a negative charge and negligible mass compared to protons and neutrons. Electrons occupy an electron cloud—an energy‑based distribution of probability rather than fixed orbits—making their exact positions indeterminate.
Electrons reside in discrete energy shells. When an atom absorbs energy, electrons transition to higher shells until the energy is released. Each shell can accommodate a limited number of electrons: the first holds up to two, the second up to eight, and subsequent shells follow the 2‑8‑8… pattern.
The atomic number equals the proton count, ensuring electrical neutrality by matching proton and electron numbers. Variations arise when the neutron count changes—producing isotopes—or when electrons are gained or lost, forming ions with net positive or negative charge and altered reactivity.
