Electrons are negatively charged particles that move around the nucleus of an atom in defined regions called energy levels or shells. These shells represent different energy states, with electrons in outer shells having higher energy than those closer to the nucleus.
Each shell can hold a certain maximum number of electrons:
- 1st shell (closest to the nucleus): up to 2 electrons
- 2nd shell: up to 8 electrons
- 3rd shell: up to 18 electrons, and so on
Electrons fill the lowest available energy levels first (the Aufbau principle), and the arrangement of electrons in these shells determines an atom’s chemical behavior. For example, atoms with full outer shells are stable (like noble gases), while those with incomplete outer shells tend to form chemical bonds to achieve stability.
Understanding electron shells is crucial for explaining periodic trends, reactivity, and the formation of molecules and compounds.