Journey Inside the Atom · Lesson 2 of 14
A Short Historical Journey Through Atomic Models
“Radioactivity, cathode rays and Thomson's model reveal that atoms contain smaller particles”
• Explain why radiation challenged the idea of an indivisible atom. • Describe the basic cathode ray tube arrangement. • Explain how cathode rays led to identification of electrons. • State the relative charge of an electron. • Describe Thomson's model and explain neutrality within it.
For a long time the atom was treated as the smallest indivisible unit of matter. That picture began to change when certain elements were found to emit invisible energy and particles. If particles can come from atoms, atoms must have internal structure.
A Short Historical Journey Through Atomic Models
Evidence That Atoms Are Divisible
A phenomenon in which certain elements emit radiation in the form of energy and particles.
Radioactivity showed that the old picture of a permanently indivisible atom was incomplete. Scientists therefore began looking for smaller components and for experiments that could reveal their properties.
Discovery of the Electron
J. J. Thomson studied the conduction of electric current through gases at very low pressure. In a glass tube with a cathode and an anode, a high voltage produced rays travelling from the negative cathode toward the positive anode. These rays were called cathode rays.
Rays produced in a low-pressure discharge tube that travel from the cathode and were shown to consist of negatively charged particles.
Thomson observed how the rays behaved in electric and magnetic fields and concluded that they were streams of negatively charged particles with mass much smaller than the mass of an atom. These particles were later called electrons. Because the nature of the rays did not depend on the cathode material or the gas in the tube, electrons were inferred to be a common component of atoms of all elements.
A negatively charged subatomic particle. Its charge is approximately −1.602 × 10⁻¹⁹ C and its relative charge is taken as −1.
Thomson's Model of an Atom
Atoms are ordinarily neutral, so discovery of a negative particle required an explanation for positive charge. Thomson proposed that an atom is a sphere of positive charge with electrons embedded throughout it. The picture is often compared with plum pudding or with seeds distributed through the pulp of a watermelon.
A model in which negatively charged electrons are embedded throughout a positively charged sphere.
A neutral atom in this model requires total positive charge to equal total negative charge. If the positive charge were smaller than the electron charge, the atom would be negative overall. The food analogies are only visual aids; atoms do not literally have colours, pulp or seed-like electrons.
Quiz
Which description best matches Radioactivity?
Which description best matches Cathode Rays?
Which term matches this description: A phenomenon in which certain elements emit radiation in the form of energy and particles.
Which term matches this description: Rays produced in a low-pressure discharge tube that travel from the cathode and were shown to consist of negatively charged particles.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why did radioactivity challenge atomic indivisibility?
- Describe the direction of cathode rays.
- Why did Thomson conclude that electrons occur in all atoms?
- How does Thomson's model represent a neutral atom?
- Why is a watermelon only an analogy for the model?
Key Takeaways
• Experiments showed that atoms contain particles smaller than the atom itself. • The electron is a negatively charged subatomic particle. • Thomson proposed that electrons were embedded within a positively charged sphere. • A neutral atom must contain equal amounts of positive and negative charge.