Electrons and Other Discoveries in Atomic Physics Presentation

Topic target

Atoms are not indivisible: electrons changed the model

Dalton’s model explained chemical laws, but later experiments showed that atoms contain smaller particles. Cathode ray tubes, oil drops, and radioactivity revealed a deeper structure inside matter.

\[ \text{atom} \rightarrow \text{electrons, protons, and neutrons} \]

Learning target: connect experimental evidence to the discovery of subatomic particles and the development of atomic structure.

Why it matters

Subatomic particles explain modern chemistry

Bonding

Electrons determine how atoms form ionic and covalent bonds, such as Na⁺ with Cl⁻ or H₂O molecules.

Identity

Protons determine the element. Carbon has 6 protons, while oxygen has 8 protons.

Isotopes

Neutrons explain why atoms of the same element can have different masses, such as carbon-12 and carbon-14.

Core shift: chemistry moved from “atoms are solid spheres” to “atoms have internal structure that controls properties.”

Core concept

Cathode rays showed that atoms contain negative particles

In a cathode ray tube, a beam travels from the negative electrode toward the positive electrode. Thomson found that the beam bends in electric and magnetic fields.

The bending showed that cathode rays are made of charged particles. Their attraction toward a positive plate revealed that those particles are negatively charged electrons.

Key vocabulary

Particles and measurements used in early atomic physics

Term	Meaning	Role in atomic structure
Electron	Negatively charged subatomic particle.	Controls bonding, ions, and many chemical properties.
Proton	Positively charged particle in the nucleus.	Determines the atomic number and element identity.
Neutron	Neutral particle in the nucleus.	Changes isotope mass without changing element identity.
Charge-to-mass ratio	The ratio \(q/m\), measured from particle deflection.	Helped Thomson identify electrons as very light charged particles.

Main relationships

Deflection connects charge, mass, and fields

A charged particle in an electric field experiences a force. A stronger field or larger charge causes more deflection. A larger mass resists acceleration.

\[ F = qE \] \[ a = \frac{F}{m} = \frac{qE}{m} \]

Thomson measured the electron’s charge-to-mass ratio, \(q/m\). Millikan later measured the electron charge, allowing the electron mass to be calculated.

Higher charge magnitudemore force

Higher field strengthmore bending

Higher massless acceleration

The key evidence was not only that the beam moved, but that its motion could be measured quantitatively.

Interactive simulation

Predict how a cathode ray bends

Change the electric field and particle type. A negative particle bends toward the positive plate. A neutral particle does not bend.

Electric field direction and strength: +4

Particle charge negative

Prediction Bends upward

Model comparison

Each experiment answered a different question

Thomson

Cathode ray deflection showed the electron has negative charge and a very large \(q/m\) value.

charge-to-mass

Millikan

Oil-drop measurements showed electric charge comes in multiples of \(1.60 \times 10^{-19}\ \mathrm{C}\).

electron charge

Radioactivity

Alpha, beta, and gamma emissions showed atoms can release particles and energy from inside their structure.

nuclear evidence

\[ \text{deflection data} + \text{charge data} + \text{radioactivity} \Rightarrow \text{subatomic model} \]

Worked example

Use Millikan’s result to count electrons on a droplet

An oil droplet has charge \(-4.80 \times 10^{-19}\ \mathrm{C}\). The charge of one electron is \(-1.60 \times 10^{-19}\ \mathrm{C}\).

Question: How many excess electrons are on the droplet?

\[ n = \frac{\text{total droplet charge}}{\text{charge of one electron}} \]

Use magnitudes because the negative sign only shows electron charge direction.
Substitute: \(n = \frac{4.80 \times 10^{-19}\ \mathrm{C}}{1.60 \times 10^{-19}\ \mathrm{C}}\).
Cancel units and powers: \(n = 3.00\).
Final answer: the droplet has 3 excess electrons.

Common mistake

Do not confuse charge, mass, and identity

The mistake

“Electrons determine which element an atom is because electrons are involved in chemistry.”

The correction

Electrons control charge and bonding behavior, but protons determine the element. Changing electrons makes ions. Changing protons makes a different element.

Change	Result	Example
Gain or lose electrons	Ion forms	Na → Na⁺ + e⁻
Change protons	New element forms	6 protons = C, 7 protons = N
Change neutrons	Isotope forms	carbon-12 and carbon-14

Practice check

Interpret evidence from a cathode ray experiment

A beam in a cathode ray tube bends toward a positively charged plate. What does this tell you about the particles in the beam?

Show answer

The beam particles must be negatively charged because opposite charges attract. This supports the conclusion that cathode rays are streams of electrons.

If the beam were neutral, it would not bend in an electric field. If the beam were positive, it would bend toward the negative plate instead.

Reasoning pattern: observe deflection → infer charge sign → connect charge to particle identity.

Application

How to use this topic in future problems

Atomic structure

Use protons, neutrons, and electrons to identify atoms, ions, and isotopes.

Periodic trends

Electron arrangement helps explain atomic size, ionization energy, and reactivity.

Nuclear chemistry

Alpha, beta, and gamma radiation introduce changes involving nuclei and energy.

\[ \text{subatomic particles} \rightarrow \text{atomic identity, charge, isotopes, and bonding} \]

Final summary

Most important takeaways

Cathode rays

Thomson’s experiments showed that atoms contain negatively charged electrons.

Oil drops

Millikan showed that electron charge is quantized, with magnitude \(1.60 \times 10^{-19}\ \mathrm{C}\).

Subatomic structure

Electrons, protons, and neutrons replaced the idea that atoms are indivisible solid spheres.

\[ \text{experiments changed the atomic model} \]