The Atom (CIE IGCSE Physics)
Revision Note
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Atomic Structure
- Atoms are the building blocks of all matter
- They are incredibly small, with a radius of only 1 × 10-10 m
- This means that about one hundred million atoms could fit side by side across your thumbnail
- Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus
- The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom
- They consist of small dense positively charged nuclei, surrounded by negatively charged electrons
An atom: a small positive nucleus, surrounded by negative electrons
(Note: the atom is around 100,000 times larger than the nucleus!)
Rutherford's Experiment
EXTENDED
- In 1909 a group of scientists were investigating the Plum Pudding model
- Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the experiment
- This involved the scattering of alpha (α) particles by a sheet of thin metal supports the nuclear model of the atom
- A beam of alpha particles (He2+ ions) were directed at a thin gold foil
- They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
- Instead, they discovered that :
- Most of the alpha particles passed straight through the foil
- Some of the alpha particles changed direction but continued through the foil
- A few of the alpha particles bounced back off the gold foil
- The bouncing back could not be explained by the Plum Pudding model, so a new model had to be created
- This was the first evidence of the structure of the atom
When α-particles are fired at thin gold foil, most of them go straight through but a very small number bounce straight back
- When α-particles are fired at thin pieces of gold foil:
- The majority of them go straight through (A)
This happens because the atom is mainly empty space
- Some are deflected through small angles (B)
This happens because the positive α-particles are repelled by the positive nucleus which contains most of its mass
- A very small number are deflected straight back (C)
This is because the nucleus is extremely small
- The majority of them go straight through (A)
Atoms & Ions
- An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
- An atom will lose or gain electrons to become more stable
- A stable atom is normally electrically neutral
- This means it has the same number of protons (positive charge) and electrons (negative charge)
- Positive ions are therefore formed when atoms lose electrons
- There will be more protons than electrons
- Negative ions are therefore formed when atoms gain electrons
- There will be more electrons than protons
The difference between positive and negative ions
Exam Tip
You may hear the term 'net charge'. This just means the 'overall' charge of the atom. If an atom has 5 protons, 5 neutrons and 6 electrons, it has a net negative charge because it's a negative ion (more electrons than protons).
Remember which way around the charges are by proton being positive.
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