Manual How Did We Find Out About Atoms?

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Atoms are made up of smaller particles (Credit: Science Photo Funnily enough, the first particle that was discovered was actually the smallest.
Table of contents

What is an atom?

Ancient Greek thinkers believed that all matter was made of a combination of earth, air, fire and water. These substances were called 'elements' but they were not the same as modern elements. Later Greek thinkers suggested that matter could be made up of invisible particles. They called these particles atoms but they had no experimental evidence for their model. The scientist John Dalton carried out a series of experiments.

Rutherford at Manchester, 1907–1919

He concluded that all matter was made of tiny particles called atoms. He suggested that an atom was a tiny solid ball. He published his ideas in This atomic model has changed over time. Finally, in the Royal Society of Chemistry oversaw the placing a blue commemorative plaque on the wall of his birthplace, recognising his discovery at last. So the two scientists would certainly have known each other although neither was aware of all the work done by the other.

Meyer's roots, however, were firmly in Germany.

Atoms - What are they? What's inside them? - Explain that Stuff

Meyer was just four years older than Mendeleev, and produced several Periodic Tables between His first table contained just 28 elements, organised by their valency how many other atoms they can combine with. These elements were almost entirely main group elements, but in he incorporated the transition metals in a much more developed table. Meyer did contribute to the development of the periodic table in another way though. He was the first person to recognise the periodic trends in the properties of elements, and the graph shows the pattern he saw in the atomic volume of an element plotted against its atomic weight.

As we have seen, Mendeleev was not the first to attempt to find order within the elements, but it is his attempt that was so successful that it now forms the basis of the modern periodic table. Mendeleev did not have the easiest of starts in life. He was born at Tobolsk in , the youngest child of a large Siberian family. His father died while he was young, and so his mother moved the family km to St.

In his adult life he was a brilliant scientist, rising quickly in academic circles. Mendeleev discovered the periodic table or Periodic System, as he called it while attempting to organise the elements in February of He did so by writing the properties of the elements on pieces of card and arranging and rearranging them until he realised that, by putting them in order of increasing atomic weight, certain types of element regularly occurred.

For example, a reactive non-metal was directly followed by a very reactive light metal and then a less reactive light metal. Initially, the table had similar elements in horizontal rows, but he soon changed them to fit in vertical columns, as we see today. Not only did Mendeleev arrange the elements in the correct way, but if an element appeared to be in the wrong place due to its atomic weight, he moved it to where it fitted with the pattern he had discovered.

For example, iodine and tellurium should be the other way around, based on atomic weights, but Mendeleev saw that iodine was very similar to the rest of the halogens fluorine, chlorine, bromine , and tellurium similar to the group 6 elements oxygen, sulphur, selenium , so he swapped them over. He even predicted the properties of five of these elements and their compounds.

The table below shows the example of Gallium, which Mendeleev called eka-aluminium, because it was the element after aluminium. This gave the table the periodicity of 8 which we know, rather than 7 as it had previously been. Mendeleev never received a Nobel Prize for his work, but element was named Mendelevium after him, an even rarer distinction.

Formula Ea 2 O 3 , density 5. Soluble in both acids and alkalis. Formula Ga 2 O 3 , density 5. So he needed a new line of attack. The new line was very simple, a chemical procedure mixed with physics. For this work Rutherford recruited Thomas Royds — , who had earned his Physics Honours degree in They studied the emitted light in a spectroscope and found it to be identical to the spectrum of helium.

Within a few months, Rutherford was awarded the Nobel Prize for Chemistry, "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances. Rutherford next turned his attention to using them to probe the atom. The autumn of began an important series of researches.

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Geiger thought Ernest Marsden — , a year-old student in Honours Physics, was ready to help on these experiments and suggested it to Rutherford. Since Rutherford often pushed third-year students into research, saying this was the best way to learn about physics, he readily agreed. Geiger and Marsden began with small-angle dispersion and tried various thicknesses of foils, seeking mathematical relationships between dispersion and thickness of foil or number of atoms traversed. Marsden later recalled that Rutherford said to him amidst these experiments: "See if you can get some effect of alpha-particles directly reflected from a metal surface.

Rutherford was ever ready to meet the unexpected and exploit it, where favourable, but he also knew when to stop on such excursions. Birks, , p. This was Rutherford's playful approach in action.

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His students and others tried out his ideas, many of which were dead-ends. Rutherford wrote:.

Experiment, directed by the disciplined imagination either of an individual or, still better, of a group of individuals of varied mental outlook, is able to achieve results which far transcend the imagination alone of the greatest philosopher. Quoted in Eve, , Frontmatter. Sometime later in or , Marsden said, he reported his results to Rutherford. Rutherford recalled this a little differently:. I remember It was almost incredible as if you fired a inch shell at a piece of tissue paper and it came back and hit you. Rutherford , , p. Human memory is fallible.

Whether Marsden or Geiger told Rutherford, the effect was the same. Rutherford said they should prepare a publication from this research, which they submitted in May Moreover, this started Rutherford thinking toward what ultimately, almost two years later, he published as a theory of the atom. What was Rutherford doing for the rest of and all of ? For one thing, his close friend Boltwood was in Manchester for the academic year working with Rutherford on radioactive decay products of radium. He was also reviewing and speaking on earlier ideas about atomic structure. Rutherford did not have his bold idea — the nuclear atom — instantly, but he came to it gradually by considering the problem from many sides.

In the autumn of he brought Marsden back to Manchester to complete rigorous experimental testing of his ideas with Geiger. Rutherford tried to reconcile scattering results with different atomic models, especially that of J.

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Thomson, in which the positive electricity was considered as dispersed evenly throughout the whole sphere of the atom. At some point in the winter of —, Rutherford worked out the basic idea of an atom with a "charged center. To produce a similar effect by a magnetic field, the enormous field of absolute units would be required.

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Birks, p. Rutherford concluded in his May paper that such a remarkable deviation in the path of a massive charged particle could only be achieved if most of the mass of, say, an atom of gold and most of its charge were concentrated in a very small central body.

Note: at this point in , Rutherford did not call this a "nucleus.

click The first public announcement of the nuclear theory by Rutherford was made at a meeting of the Manchester Literary and Philosophical Society, and he invited us young boys to go to the meeting. The older people in the laboratory did, of course Geiger and Marsden knew because they were already doing the experiments. In fact, unless they had done some which were sufficient to be decisive, Rutherford never mentioned it publicly. And, of course, Darwin knew about it much earlier.