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Electron shells are distinct areas or energy levels where electrons reside around the nucleus. The first shell holds a maximum of two electrons, while the second shell holds eight. Electrons in shells closer to the nucleus have less energy than those farther away. These shells are often visualized as rings in the Bohr model, though they actually have complex shapes.

Thomson used a cathode ray tube with two charged metal plates to deflect a beam of particles. When the ray passed between a positive and negative plate, it bent toward the positive plate, indicating the particles were negatively charged. Calculating the mass-to-charge ratio showed each particle was about 1/2000 the mass of an atom, leading Thomson to conclude atoms contain many electrons.

Valence electrons are the electrons in an atom's outermost occupied shell. They determine an element's chemical properties and reactivity. Valence electrons can be shared between atoms to form covalent bonds or transferred to form ionic bonds, making them essential for understanding how atoms interact and combine.

Electrons have increasing energy as they move farther from the nucleus. Those closest to the nucleus carry the least energy, while outer electrons carry more energy. This energy difference explains why outer shells can hold more electrons than inner shells—there is more space available at greater distances from the nucleus.

Shells indicate an electron's general energy level and distance from the nucleus. Subshells describe the electron's location and energy more precisely within a shell. Orbitals describe the shape of the probability area where an electron orbits the nucleus. Together, these three concepts provide increasingly detailed information about electron location and behavior.

Thomson's cathode ray experiments revealed that negatively charged particles had a mass about 1/2000 that of any known atom. Since individual particles were so light, Thomson reasoned that many of these particles must exist within each atom to account for the atom's total mass. This discovery established electrons as fundamental subatomic particles present in all matter.

The Bohr or planetary model visualizes electron shells as rings orbiting around the nucleus, similar to planets around the sun. While this simplified representation helps students understand shell organization and energy levels, the actual shapes of electron shells are much more complex than simple rings.