Atomic models through time
Everyday matter is made up of A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. called The smallest part of an element that can exist.. The Greek word 'atomos' means 'indivisible' - in the past, atoms were thought to be the smallest particles possible.
Developing models
Dalton's model (1803)
John Dalton imagined atoms as tiny solid balls. Dalton's model included these ideas:
- atoms cannot be broken down into anything simpler
- the atoms of a given A substance made of one type of atom only. are identical to each other
- the atoms of different elements are different from one another
- during chemical reactions atoms rearrange to make different substances
In Dalton's time, it was not possible to investigate the structure of atoms.
Thomson's model (1897)
In 1897, J J Thomson discovered the Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons., which are negatively charged Objects found in atoms, including protons, neutrons and electrons. that are smaller than atoms. Atoms are neutral overall, so in Thomson's 'plum pudding model':
- atoms are spheres of positive charge
- electrons are dotted around inside
Rutherford's model and the Geiger-Marsden experiment (1909 - 1911)
Hans Geiger and Ernest Marsden tested the plum pudding model. They aimed beams of positively charged Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus). 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':
- atoms have a central, positively charged The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. with most of the mass
- electrons orbit the nucleus, like planets around a star
Bohr's model (1913)
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.
How big is an atom?
A A collection of two or more atoms held together by chemical bonds. consists of two or more atoms chemically joined together. The typical size of an atom or simple molecule is: 1 Ă— 10 -10 m (0.1 nm or 0.000,000,000,1 m).
Example
A steel pin head is about 1.5 mm in diameter. How many iron atoms, 3.0 Ă— 10-10 m in diameter, will fit across the pin head?
1.5 mm = 1.5 Ă— 10-3 m
Number of iron atoms = (1.5 Ă— 10-3) Ă· (3.0 Ă— 10-10)
= 5,000,000