John Dalton imagined atoms as tiny solid balls. Dalton's model included these ideas:
In Dalton's time, it was not possible to investigate the structure of atoms.
Hans Geiger and Ernest Marsden tested the plum pudding model. They aimed beams of positively charged alpha particles at very thin gold foil. According to the plum pudding model, these particles should have passed straight through. However, many of them changed direction instead.
Ernest Rutherford explained these results in his 'planetary model':
There is a problem with Rutherford's model - the electrons would eventually fall into the nucleus because they are negatively charged and so attracted to the positive nucleus. Niels Bohr improved Rutherford's model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus.
A molecule consists of two or more atoms chemically joined together. The typical size of an atom or simple molecule is: 1 × 10 -10 m (0.1 nm or 0.000,000,000,1 m).
A steel pin head is about 1.5 mm in diameter. How many iron atoms, 3.0 × 10-10 m in diameter, will fit across the pin head?
1.5 mm = 1.5 × 10-3 m
Number of iron atoms = (1.5 × 10-3) ÷ (3.0 × 10-10)