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1.1.2 Structure of the atom

Parts of an atom

Definition

Atom

The smallest particle of an element, made of a positively charged nucleus surrounded by negatively charged electrons.

Definition

Nucleus

The tiny, positively charged centre of an atom that contains almost all of its mass.

Definition

Subatomic particle

A particle smaller than an atom, such as a proton, a neutron or an electron.

  1. Every atom has two regions: a central nucleus and the space around it in which the electrons move.
  2. The nucleus carries a positive charge and sits at the very centre of the atom.
  3. The nucleus is made of protons, which are positively charged, and neutrons, which have no charge.
  4. The electrons carry a negative charge and move around the nucleus in energy levels, also called shells.
  5. Protons, neutrons and electrons are all subatomic particles, because each is much smaller than the atom it belongs to.
  6. The positive nucleus and the negative electrons attract each other, and this electrostatic attraction holds the electrons in the atom.
  7. An atom has no overall charge, because the total negative charge of its electrons is equal in size to the total positive charge of its nucleus.

A diagram of an atom showing a central nucleus containing positively charged protons and neutral neutrons, with negatively charged electrons orbiting in shells.

Size of the nucleus

  1. The radius of the nucleus is much smaller than the radius of the whole atom.
  2. The radius of a nucleus is less than about one ten-thousandth of the radius of the atom it sits in.
  3. Almost all of the volume of an atom is therefore the region around the nucleus, where the electrons move through what is otherwise empty space.
  4. If an atom were scaled up to the size of a football stadium, the nucleus would be about the size of a pea on the centre spot.
  5. Diagrams of atoms, including the one above, are not drawn to scale, because a nucleus drawn at its true relative size would be too small to see.
Example

Scaling a model atom

  • A student makes a scale model of an atom using a bead of diameter 1 mm1\ \text{mm}1 mm as the nucleus.
  • The atom's radius is at least 10 00010\,00010000 times the nuclear radius, so the diameters are in the same ratio.
  • Diameter of the model atom =10 000×1 mm=10 000 mm= 10\,000 \times 1\ \text{mm} = 10\,000\ \text{mm}=10000×1 mm=10000 mm.
  • Converting to metres gives 10 000 mm÷1000=10 m10\,000\ \text{mm} \div 1000 = 10\ \text{m}10000 mm÷1000=10 m.
  • The model atom would need to be at least 10 m10\ \text{m}10 m across, larger than a classroom, to hold a nucleus the size of a bead.

Where the mass is

  1. Almost all of the mass of an atom is found in its nucleus.
  2. Protons and neutrons have very similar masses, and each is roughly 200020002000 times heavier than an electron.
  3. The electrons therefore add only a tiny amount to the mass of an atom, typically much less than 0.1%0.1\%0.1% of the total.
  4. This produces a striking contrast: the nucleus takes up almost none of the volume but holds almost all of the mass.
  5. The electron region is the opposite, taking up almost all of the volume but holding very little of the mass.
  6. Because so much mass is packed into such a small volume, the material of a nucleus is extremely dense.
Exam technique

Describing atomic structure

  • A full description of an atom needs three separate ideas: the charges and positions of the parts, the relative size of the nucleus and the location of the mass.
  • Write much smaller when comparing the nuclear radius with the atomic radius, because a bare smaller does not show the scale of the difference.
  • Write almost all of the mass is in the nucleus, because the electrons do have a small mass.
  • Name the charges with their signs, positive for the nucleus and negative for the electrons, rather than calling the parts simply charged.
Common Mistake
  • Do not write that the nucleus contains all of the mass of an atom, because the electrons contribute a very small amount.
  • Do not judge the size of a nucleus from a diagram, because diagrams exaggerate it so that it can be seen.
  • Do not describe the electrons as sitting inside the nucleus, because they move in energy levels well outside it.
  • Do not confuse the atom with its parts, because the nucleus and the electrons are pieces of one atom and are not atoms themselves.
Self review
  • State the charge of the nucleus and the charge of an electron.
  • Name the two types of particle found in the nucleus.
  • Explain why an atom has no overall charge.
  • Compare the radius of the nucleus with the radius of the atom.
  • State where almost all of the mass of an atom is found and explain why the electrons add so little.
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1.1.2 Structure of the atom Revision Guide

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  3. /1.1.2 Structure of the atom

Revision notes for OCR GCSE Physics 1.1.2 Structure of the atom: explanations and worked examples.

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