The correct answer is They revolve around the nucleus in fixed orbits.. Explanation & Key Takeaways: • Electrons are subatomic particles that revolve around the central nucleus of an atom in specific circular paths known as orbits or shells. These orbits represent discrete energy levels. Hence, statement 1 is correct. • The nucleus at the center of the atom contains protons and neutrons, which are collectively called nucleons. Electrons are located in the extranuclear region outside the nucleus. Hence, statement 2 is incorrect. • The mass of an electron is approximately 9.11 x 10^-31 kg, which is about 1/1836 times the mass of a proton or a hydrogen atom. Because it is so small, its mass is usually considered negligible. Hence, statement 3 is incorrect. • Electrons carry a negative electric charge of magnitude 1.602 x 10^-19 Coulombs. In contrast, protons are positively charged. Hence, statement 4 is incorrect. • Hence, the correct answer is They revolve around the nucleus in fixed orbits. Related Concepts: • Discovery of the Electron: • The electron was discovered by Sir J.J. Thomson in 1897 during his experiments with cathode ray tubes. • He observed that the rays were deflected by electric and magnetic fields, suggesting they consisted of negatively charged particles. • Bohr's Model of an Atom: • Proposed by Niels Bohr in 1913, this model refined the idea of electron orbits. • He stated that electrons move in fixed circular orbits without radiating energy. These orbits are designated by letters K, L, M, N or principal quantum numbers n = 1, 2, 3, 4. • Charge and Mass: • The charge-to-mass ratio (e/m) was first calculated by J.J. Thomson. • The precise negative charge of the electron was later determined by Robert Millikan through the Oil Drop Experiment in 1909. • Valence Electrons: • Electrons in the outermost shell are called valence electrons. They are responsible for the chemical properties and valency of an element. • The maximum number of electrons that can be accommodated in a shell is given by the formula 2n². Important Notes: • Atomic Neutrality: An atom is electrically neutral because the number of electrons (negative charge) is equal to the number of protons (positive charge) in the nucleus. • Subatomic Mass: The atomic mass of an element is almost entirely due to the nucleus (protons + neutrons), as the electron mass is so tiny (0.0005485 amu). • Quantum Nature: In modern Quantum Mechanics (developed by Schrödinger in 1926), electrons are described as existing in orbitals, which are regions of high probability rather than strict circular tracks.