Structure of Atom · Lesson 2 of 12
From Atomic Models to the Nucleus
“Use scattering evidence to explain the nucleus, atomic identity, and the limits of early models.”
• Compare Thomson’s and Rutherford’s descriptions of charge and mass in an atom. • Connect each gold-foil observation to the conclusion it supports. • Determine proton, neutron, and electron counts in atoms and ions. • Distinguish isotopes from isobars and explain why isotopes have similar chemistry. • Explain the classical stability problem in Rutherford’s model.
What must an atomic model explain?
Discovering particles does not tell us how they are arranged. A useful atomic model must explain why an atom is stable, how positive and negative charges are distributed, and how the arrangement relates to physical and chemical behaviour. It should also help explain why atoms absorb and emit particular radiation and why they combine into molecules.
Thomson pictured a sphere of spread-out positive charge with electrons embedded within it. The total negative charge balances the positive charge, so the whole atom is neutral. The model is often compared with seeds in a watermelon. That comparison represents a distribution of charge; it is not a literal description of atomic material.
In Thomson’s picture, mass is spread through the atom. A fast, heavy positive projectile should therefore pass through with only modest deflection. Testing that prediction is more powerful than deciding whether a model looks plausible.
Radiation becomes a probe of atoms
X-rays were discovered when energetic electrons struck dense metal targets. They cause fluorescence, penetrate matter strongly, and are not deflected by electric or magnetic fields. They are electromagnetic radiation, rather than a charged particle beam. Their short wavelengths make them useful for investigating structures inside objects.
Radioactivity is the spontaneous emission of radiation by certain substances. Historical investigations distinguished alpha, beta, and gamma radiation. Alpha particles are helium nuclei with charge and mass about ; the beta radiation discussed here consists of fast electrons; gamma radiation is uncharged electromagnetic radiation. Under comparable introductory comparisons, alpha has the least penetration, beta more, and gamma the greatest. Actual penetration also depends on energy and the material.
These distinctions matter because Rutherford used alpha particles as positive projectiles. Their deflection reveals where the atom’s positive charge is located. The experiment does not involve firing light at a solid ball.
Reading the gold-foil experiment
Rutherford’s group directed a narrow alpha-particle beam at an extremely thin gold foil, surrounded by a zinc sulphide screen. Each impact produced a tiny flash. Most particles went through essentially undeflected; a smaller number changed direction; very few were scattered backwards.
The first observation means that most of the atomic volume offers little obstruction to the incoming alpha particles. The rare large deflections show that a positive projectile can sometimes encounter a very strong repulsion in a very small region. A diffuse positive sphere could not account for that pattern. Positive charge and almost all atomic mass must be concentrated in a small nucleus.
In Rutherford’s model, the tiny positive nucleus is surrounded by electrons. The typical atomic radius is about , whereas a nuclear radius is of order . These are radius scales, not a claim that every atom or nucleus has exactly the same size.
Problem
If an atom has radius and its nucleus has radius , compare their radii and approximate volumes.
- 1.The radius ratio is . The atomic radius is about 100 000 times the nuclear radius.
- 2.For spheres, volume scales as the cube of radius. The volume ratio is .
- 3.Thus the nucleus occupies roughly one part in of this model atom’s volume. A tiny nucleus can still contain nearly all the mass.
Atomic identity and nuclear composition
The number of protons fixes an element’s identity. We call it the atomic number, . The total number of protons and neutrons is the mass number, ; these nuclear particles are collectively nucleons. Mass number is a count and is not the same as a measured atomic mass.
Write a species as , with any ionic charge at the upper right. A neutral atom has as many electrons as protons. A positive ion has lost electrons; a negative ion has gained them. Changing electrons changes charge but not the element or its mass number.
Problem
Find the particle counts in .
- 1.The lower-left number gives , so the nucleus has 35 protons.
- 2.The mass number is 80, so the neutron count is .
- 3.No ionic charge is shown, so the atom is neutral and contains 35 electrons.
Problem
A species has 18 electrons, 16 protons, and 16 neutrons. Write its symbol.
- 1.Sixteen protons identify sulphur: .
- 2.The mass number is . There are two more electrons than protons, so the charge is .
- 3.The complete symbol is . Its neutron count does not change when it forms this ion.
Isotopes and isobars
Isotopes belong to the same element: their proton counts agree, but their neutron counts, and hence their mass numbers, differ. Hydrogen’s isotopes are protium, deuterium, and tritium, containing zero, one, and two neutrons respectively. Protium is overwhelmingly the most abundant; deuterium is much less abundant and tritium occurs only in traces.
Carbon-12, carbon-13, and carbon-14 all have six protons. Chlorine-35 and chlorine-37 both have 17. Neutral isotopes of an element have the same electron count and broadly the same electronic arrangement, explaining their very similar chemical behaviour. Their masses and nuclear properties differ; “same chemistry” is an introductory approximation, not a claim that every physical property is identical.
Isobars instead share a mass number but have different atomic numbers. Carbon-14 and nitrogen-14 each contain 14 nucleons, but six and seven protons respectively, so they are different elements.
Why the nuclear model is incomplete
A planet-like drawing is tempting, but the force holding an electron is electrostatic, whereas planets are held by gravity. An electron moving in a circle accelerates because its direction changes even if its speed stays constant. Classical electromagnetic theory predicts radiation from an accelerating charge.
The radiated energy would come from the electron’s motion, so its orbit would shrink and it would spiral into the nucleus. Calculations give a collapse time of roughly , contrary to stable matter. Making the electron stationary does not solve the problem: electrostatic attraction would pull it towards the nucleus. The model also gives no satisfactory distribution or allowed energies for electrons. Understanding stability therefore requires new physical ideas.
“Mostly empty space” describes the tiny volume occupied by the nucleus; it does not mean an atom has no electrical influence outside the nucleus. Also, do not identify an element from its mass number alone: atomic number determines identity.
Quiz
Which result most strongly contradicts Thomson’s diffuse positive charge?
How many neutrons are in ?
Which pair represents isotopes?
Why is a circularly moving electron accelerated?
What changes when a neutral atom forms a singly positive ion?
Practice Problems
- Explain each major gold-foil observation and the deduction it supports.
- Compare alpha particles, beta particles, and gamma radiation in charge and nature.
- Write complete symbols for atoms with .
- Find the electron, proton, and neutron counts in .
- Explain why chlorine isotopes have similar chemistry despite different masses.
- Discuss why a foil of lighter atoms would generally give less large-angle alpha scattering than a heavy-element foil under comparable conditions.
- Explain why replacing moving electrons with stationary electrons cannot repair Rutherford’s model.
Key Takeaways
Rutherford’s observations locate positive charge and almost all mass in a tiny nucleus. Atomic number counts protons; mass number counts protons plus neutrons. Ions differ in electron count; isotopes differ in neutron count. Classical orbiting electrons would radiate and collapse, so the nuclear model needs a new account of electron behaviour.