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Electrons are negatively charged subatomic particles that orbit the nucleus of an atom. Because the nucleus is positively charged, it attracts electrons through opposite electrical charges. This attraction keeps electrons in orbit around the nucleus at specific energy levels called shells.

Electrons occupy energy levels, or shells, around the nucleus, with each shell containing smaller regions called orbitals where electrons are most likely found. The first energy level has one orbital holding up to two electrons. Higher energy levels contain more orbitals and can hold more electrons, with electrons farther from the nucleus carrying more energy.

Valence electrons are electrons in the outermost energy level of an atom. They shape an element's chemical properties because they can be transferred or shared with other atoms, allowing atoms to form ionic or covalent bonds and participate in chemical reactions.

Thomson used cathode ray tubes in the late 1890s, directing beams between charged metal plates. He observed the beam deflected toward the positive plate, indicating the particles were negatively charged. Calculating the mass-to-charge ratio revealed these particles were tiny, about 1/2000 the mass of any known atom, leading to the electron's discovery.

Electrons closest to the nucleus experience stronger attraction to the positive charge and occupy the lowest energy level. As distance from the nucleus increases, the attraction weakens and electrons carry more energy. The outer shells have more space available, allowing them to hold more electrons at higher energy levels.

Thomson's experiments revealed that atoms contain many electrons, each with tiny mass. The discovery of protons and neutrons later explained how atoms maintain overall neutral charge despite containing numerous negatively charged electrons. These subatomic particles account for an atom's mass and electrical neutrality.

The number of orbitals in each energy level determines electron capacity. The first energy level contains one orbital holding up to two electrons. Higher energy levels contain progressively more orbitals, allowing outer shells to accommodate more electrons than inner shells, creating the characteristic electron distribution in atoms.