Which experiment and subsequent observation led to the discovery that atoms contain negatively charged particles, now known as electrons? (LO 2.10–2.12) (a) Oil is sprayed into a chamber and the speed at which the oil droplets fall is measured with and without an applied voltage. X rays in the chamber knock electrons out of air molecules. The electrons stick to the oil pro-ducing an overall negative charge on the drops. Adjusting the voltage changes the speed at which the negatively charged oil droplets fall. (b) When a high voltage is applied across metal electrodes at opposite ends of a sealed glass tube, a cathode ray is produced. The cathode ray is repelled by a negatively charged plate. (c) A radioactive substance emits alpha particles, which are directed at a thin gold foil. Most of the alpha par-ticles pass through the foil, but a few alpha particles are slightly deflected and some even bounce back toward the radioactive source. (d) The mass of different elements in a pure chemical com-pound are measured. Different samples of the compound always contains the same proportion of elements by mass.

Verified step by step guidance

1. First, let's understand the four experiments mentioned in the question.
2. (a) This experiment is known as the Millikan's Oil Drop Experiment. It was used to measure the charge of an electron, but it did not lead to the discovery of the electron itself.
3. (b) This experiment is known as the Cathode Ray Tube Experiment, conducted by J.J. Thomson. When a high voltage is applied across metal electrodes at opposite ends of a sealed glass tube, a cathode ray is produced. The cathode ray is repelled by a negatively charged plate. This experiment led to the discovery of electrons, as the rays were found to consist of negatively charged particles.
4. (c) This experiment is known as the Rutherford's Gold Foil Experiment. It led to the discovery of the atomic nucleus, not the electron.
5. (d) This statement describes the Law of Definite Proportions, which is a fundamental principle in chemistry but does not relate to the discovery of the electron.
6. From the above explanations, we can conclude that the experiment which led to the discovery of electrons is the Cathode Ray Tube Experiment conducted by J.J. Thomson. So, the correct answer is (b).

Key Concepts

Cathode Ray Experiment

The cathode ray experiment, conducted by J.J. Thomson, demonstrated that cathode rays are streams of negatively charged particles, later identified as electrons. When a high voltage is applied across electrodes in a vacuum tube, the rays produced travel from the cathode to the anode. Their deflection by electric and magnetic fields confirmed their negative charge, leading to the conclusion that atoms contain these subatomic particles.

Charge and Mass of Electrons

Electrons are fundamental subatomic particles with a negative charge and a very small mass compared to protons and neutrons. The charge of an electron is approximately -1.6 x 10^-19 coulombs, which is essential for understanding atomic structure and chemical bonding. This property allows electrons to interact with other charged particles, influencing chemical reactions and the behavior of atoms.

Atomic Structure

The atomic structure consists of a nucleus containing protons and neutrons, surrounded by electrons in various energy levels. The discovery of electrons as negatively charged particles was pivotal in developing the modern atomic model, which explains how atoms bond and interact. Understanding atomic structure is crucial for grasping concepts in chemistry, such as chemical reactions, bonding, and the periodic table.