Models of the atom change with new evidence
Definition
Atomic model
A scientific description of what an atom contains and how its parts are arranged.
- Atoms are far too small to see, so scientists build models to explain how atoms behave in experiments.
- A model is kept for as long as it explains all the experimental evidence.
- When a new experiment gives a result the model cannot explain, the model is changed or replaced.
- The main steps ran from Dalton in the early 1800s, through Thomson, Rutherford and Bohr, to Chadwick in 1932.
- Each new model kept the ideas that still fitted the evidence and changed only the parts that did not.
Key Idea
Every change to the atomic model followed a new experimental result, so the reason for each change is the evidence behind it.
Thomson's electron discovery replaced the solid sphere model
Definition
Electron
A tiny, negatively charged particle found in shells around the nucleus of an atom.
Definition
Plum pudding model
Thomson's model of the atom as a ball of spread-out positive charge with negative electrons embedded through it.
- In the early 1800s, John Dalton proposed that elements are made of tiny atoms that cannot be divided.
- Dalton pictured each atom as a solid sphere, with every atom of one element identical and different from the atoms of other elements.
- In 1897, J. J. Thomson found that cathode rays are streams of tiny negative particles, far lighter than any atom.
- These particles came from inside atoms, so atoms had to contain smaller parts.
- Atoms have no overall charge, so Thomson reasoned that positive charge must balance the negative electrons, and in 1904 he proposed the plum pudding model.
Example
- In a currant bun, the dough stands for the spread-out positive charge and the currants stand for the electrons.
- Nothing in the bun is concentrated in one place, which is the idea the next experiment tested.
Rutherford's gold foil experiment revealed a tiny nucleus
Definition
Alpha particle
A small, fast-moving, positively charged particle given out by some radioactive substances.
Definition
Nucleus
The tiny, dense centre of an atom, which carries its positive charge and almost all of its mass.
- In 1909, Geiger and Marsden, working with Ernest Rutherford, fired alpha particles at a very thin sheet of gold foil.
- The plum pudding model predicted that every alpha particle would pass through with at most a tiny deflection, because its positive charge was spread thinly.
- Most alpha particles did pass straight through, which showed that most of the atom is empty space.
- A few were deflected through large angles, and about 1 in 8000 bounced back, which only a small, concentrated positive charge could cause.
- In 1911, Rutherford replaced the plum pudding model with the nuclear model, in which electrons surround a central nucleus.
Common Mistake
- Do not say that most alpha particles were repelled or bounced back, because most passed straight through the foil.
- Do not credit the large deflections to electrons, because electrons are far too light to turn back a fast alpha particle.
Bohr placed electrons in shells at fixed distances
Definition
Electron shell
A fixed distance from the nucleus at which electrons orbit, with each shell at its own energy level.
- Rutherford's nuclear model placed the electrons around the nucleus but did not describe the electron arrangement.
- In 1913, Niels Bohr suggested that electrons are not free to sit anywhere around the nucleus.
- Bohr's theoretical calculations agreed with measurements of the light given out by heated atoms, which strongly supported the shell idea.
- His model kept Rutherford's nucleus and added a definite structure for the electrons around it.
- Shell diagrams, with electrons drawn on rings around a central nucleus, are still used to show how electrons are arranged in atoms.
Key Idea
Bohr's shells give each element a definite electron arrangement, which later links an element's electrons to its place in the Periodic Table.
Chadwick's discovery of the neutron completed the model
Definition
Neutron
A particle in the nucleus with no charge and a relative mass of 1.
- By about 1920, Rutherford had shown that the positive charge of the nucleus is carried by particles called protons.
- Nuclei turned out to be heavier than their protons alone could explain, which suggested that the nucleus held another particle.
- In 1932, James Chadwick provided experimental evidence for the neutron.
- The resulting model has a tiny nucleus of protons and neutrons, with electrons in shells around it.
Exam technique
- When a question asks why the model changed, name the experiment or discovery first.
- Then say what the new evidence showed that the earlier model could not explain, such as large deflections that a spread-out positive charge could not produce.
Self review
- What did Dalton think atoms were like?
- Which discovery showed that atoms contain smaller particles?
- What did most alpha particles passing straight through the gold foil show?
- Why could the plum pudding model not explain the alpha particles that bounced back?
- What did Bohr and then Chadwick add to the nuclear model?
