Which atomic particle is outside the nucleus?

The atomic particle outside the nucleus is the electron. Electrons carry a negative electric charge and occupy regions around the nucleus called atomic orbitals or the electron cloud.

The nucleus contains positively charged protons and electrically neutral neutrons. In a neutral atom, the number of electrons equals the number of protons, balancing the atom's overall charge. If an atom gains or loses electrons, it becomes an ion.

Electrons are not best pictured as tiny planets following fixed circular tracks. Quantum mechanics describes where an electron is likely to be found, with different orbitals corresponding to different energy levels and shapes. Electrons can move between these levels by absorbing or releasing specific amounts of energy.

A useful neighbouring fact is that nearly all of an atom's mass is concentrated in its tiny nucleus, even though the electron cloud occupies most of the atom's volume. Electrons are also responsible for chemical bonding, electricity and many properties of light and matter.