Structure of Atom · Lesson 1 of 12

Discovering the Particles Inside Atoms

“Follow the experiments that revealed electrons, protons, and neutrons.”

Learning Objectives

• Explain how discharge-tube observations establish the existence and negative charge of electrons. • Connect Thomson’s charge-to-mass measurement with Millikan’s measurement of charge. • Use charge quantisation and particle masses in simple calculations. • Compare electrons, protons, and neutrons and explain what canal rays reveal.

From an indivisible atom to experimental evidence

An atom is too small to inspect with an ordinary light microscope. To learn what it contains, scientists investigate how matter behaves and then ask what internal structure could produce those observations. Early Indian and Greek ideas about indivisible particles were philosophical proposals; Dalton later built an atomic theory that explained measured chemical laws.

Dalton’s theory successfully accounted for conservation of mass and fixed and multiple proportions in compounds. However, charging glass by rubbing it with silk suggested that matter could gain or lose something electrical. Faraday’s work on electrolysis linked electricity with chemical changes at electrodes. These clues encouraged scientists to test whether electricity, and therefore matter, involved smaller particles.

Remember the basic interaction throughout these experiments: like charges repel and unlike charges attract. A particle’s response to an electric field can therefore reveal the sign of its charge. An observation becomes useful evidence only when we distinguish what was seen from the explanation inferred from it.

Cathode rays reveal a universal negative particle

A discharge tube contains two metal electrodes in gas at very low pressure. A high voltage between the negative cathode and positive anode produces a stream travelling from cathode towards anode. With a perforated anode, the stream passes through the hole and makes a zinc sulphide coating glow. The rays themselves are not seen; the glowing screen shows where they arrive.

The stream travels straight when no field acts on it. Electric and magnetic fields deflect it as expected for negatively charged particles. Crucially, changing the electrode material or the gas does not change the characteristic particles in the rays. This is why electrons are understood as constituents common to atoms, rather than peculiar products of one gas.

A fluorescent screen converts impacts into visible light, so the apparatus lets us investigate an invisible beam indirectly. The same broad principle was used in older television picture tubes: a controlled electron beam made selected screen regions glow.

Seeing a cathode-ray beam— The beam passes through the anode hole; a field changes where it reaches the screen.

Measuring charge, mass, and their ratio

Thomson compared beam deflections in electric and magnetic fields. With suitable directions, the two deflections oppose one another. Adjusting the fields can restore the original straight path. The deflection depends on field strength, particle charge, and particle mass: for otherwise comparable conditions, a more highly charged or lighter particle is deflected more strongly.

The experiment gives a ratio, not a separate charge and mass. Here, $e$ means the positive magnitude of the elementary charge; the charge of an electron is $- e$ . Keeping that distinction prevents a sign error when using the measured ratio.

Electron charge-to-mass ratioLaTeX

\frac{e}{m _{e}} = 1.758820 \times 1 0^{11} C k g^{- 1}

The ratio compares the magnitude of electron charge with electron mass.

Millikan supplied the missing independent measurement. Tiny oil drops acquired charge from ions produced when X-rays ionised the air. By observing the drops through a telescope, measuring their falling motion, and changing an electric field, he could slow, speed up, or suspend them. Gravity, electric force, and, during motion, viscous drag were involved. The key result was that measured drop charges occurred in whole-number multiples of one smallest magnitude.

Charge quantisationLaTeX

∣ q ∣ = n e, n = 1, 2, 3, \dots

Here q is a drop’s charge, n counts elementary charges, and e is approximately 1.602176 × 10⁻¹⁹ C. A negatively charged drop has an excess of electrons.

Millikan’s oil-drop experiment— Adjusting the electric field can balance the weight of a charged drop.

Example — Counting excess electrons

Problem
An oil drop has charge $- 1.282 \times 1 0^{- 18} C$ . How many excess electrons does it carry?

1.Use the magnitude: $n = ∣ q ∣/ e$ . The negative sign tells us that electrons were gained; it does not make the count negative.
2. $n = (1.282 \times 1 0^{- 18}) / (1.602176 \times 1 0^{- 19}) \approx 8.00$ .
3.The drop has eight excess electrons. A measured result close to an integer is expected because the charge is quantised.

Example — Finding electron mass

Problem
Use $e = 1.602176 \times 1 0^{- 19} C$ and Thomson’s ratio to find $m_{e}$ .

1.Rearrange the measured ratio: $m_{e} = e / (e / m_{e})$ .
2. $m_{e} = (1.602176 \times 1 0^{- 19}) / (1.758820 \times 1 0^{11}) kg$ .
3.The result is approximately $9.1094 \times 1 0^{- 31} kg$ . The charge units cancel, leaving mass.

Positive particles and the missing neutral mass

In a modified discharge tube, canal rays carry positive charge and deflect oppositely to electrons. Their masses and charge-to-mass ratios depend on the gas. They are positive gaseous ions: unlike cathode rays, they are not all one universal kind of particle. Some carry more than one elementary positive charge.

The smallest and lightest positive ion came from hydrogen and was identified with the proton. Later, Chadwick bombarded beryllium with alpha particles and established the existence of neutral particles with mass slightly greater than a proton. These are neutrons. Their lack of electric charge explains why they do not behave like charged beams in an electric field.

An electron’s mass is tiny compared with either nuclear particle. A proton and neutron each have a mass close to one atomic mass unit, but their masses are not exactly identical. Approximate masses are useful for counting nucleons; measured masses matter for precise calculations.

Particle	Charge / C	Relative charge	Mass / kg	Approximate mass / u
Electron	$- 1.602176 \times 1 0^{- 19}$	$- 1$	$9.1094 \times 1 0^{- 31}$	$0.00054$
Proton	$+ 1.602176 \times 1 0^{- 19}$	$+ 1$	$1.6726 \times 1 0^{- 27}$	$1$
Neutron	$0$	$0$	$1.6749 \times 1 0^{- 27}$	$1$

Example — Comparing particle masses

Problem
Approximately how many electron masses equal one proton mass?

1.Compare like quantities in the same units: $m_{p} / m_{e} = (1.6726 \times 1 0^{- 27}) / (9.1094 \times 1 0^{- 31})$ .
2.The ratio is about $1836$ . A proton is therefore about 1836 times as massive as an electron.
3.This comparison concerns mass, not charge. Proton and electron charges have equal magnitudes despite their very different masses.

Common mistake

Cathode rays are electrons, but canal rays are positive ions whose identity depends on the gas. Do not treat all canal-ray particles as protons. Also, finding an electron mass by combining two measurements does not mean Thomson measured charge and mass separately.

Quick check

Which observation best shows that electrons are common constituents of atoms?

Quick check

A negative oil drop has charge magnitude $3 e$ . What does this mean?

Quick check

Why does Thomson’s ratio alone not determine electron mass?

Quick check

Which beam’s properties depend on the gas in the discharge tube?

Quick check

Which particle has zero electric charge and a mass slightly greater than a proton?

Practice Problems

Explain how a fluorescent screen reveals the path of an invisible electron beam.
Separate two cathode-ray observations from the conclusions drawn from them.
Find the number of excess electrons on a drop carrying $- 6.4087 \times 1 0^{- 19} C$ .
Calculate the mass of one mole of electrons using $N_{A} = 6.022 \times 1 0^{23} mo l^{- 1}$ .
Explain why canal rays from different gases need not have the same charge-to-mass ratio.
Compare the charges and masses of the three subatomic particles without confusing relative charge with relative mass.

Key Takeaways

Discharge experiments reveal a universal negative particle: the electron. Thomson measured e/m; Millikan measured elementary charge, allowing electron mass to be calculated. Charge occurs in whole-number multiples of the elementary charge. Protons are positive, neutrons are neutral, and both are much heavier than electrons.

Start of chapter

Next · Lesson 2

From Atomic Models to the Nucleus

From an indivisible atom to experimental evidence

Cathode rays reveal a universal negative particle

Measuring charge, mass, and their ratio

Positive particles and the missing neutral mass

Quiz

Practice Problems

Key Takeaways