Atomic StructureAtomic Structure Principle protagonists• Empedocles (Greek)• Leucippe of Milet (Greek)• Democritus (Greek)• Robert Boyle• John Dalton) Henri Becquerel Sir J J Thomson• Ernest Rutherford• Niels Bohr• James Chadwick

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The Ancient Greek PhilosophersThe Ancient Greek Philosophers

Empedocles (c.492-432 BC). Divided matter into four elements: Earth, air, fire, water. Developed the idea that these elements were bound together by love and forced apart by conflict

Leucippe of Milet (420 BC). Had the idea that matter is composed of particles.

Democritus (460–370 BC). Follower of Leucippe. Explained that matter is composed of particles in perpetual motion.

Democritus’ view of the atomDemocritus’ view of the atom Atoms are invisible because of

their small size. Indivisible. (Greek word,

‘atomos’ means indivisible). Solid (no void inside) Eternal because they’re perfect. Surrounded by empty space

(explains movement and changes in density).

Different atoms have different shapes, (to explain the diversity observed in nature)Surprisingly modern view!

Robert Boyle (1627-1691)Robert Boyle (1627-1691)

In 1661 he published his Sceptical Chymist in which he distinguished between chemical elements and compounds, and adopted an atomic theory to explain chemical changes.

John Dalton (1766-1844)John Dalton (1766-1844) Manchester schoolmaster. Between 1802 and 1808 put forward his ideas in a

series of essays and papers: A few basic kinds of atom. that… Maintain their identity through all physical and

chemical changes. All atoms of a given element are identical in mass Atoms of different elements have different masses. Atoms only combine together in certain small, whole-

number ratios. Atoms can be represented as solid spheres (‘Billiard

ball model’).

He was the first to work out the masses of atoms from chemical experiments.

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Henri Becquerel (1852-1908)Henri Becquerel (1852-1908)

In 1896, discovers radioactivity. This showed that particles could be emitted from within the atom. Therefore the atom was not indivisible.

Sir J J Thomson (1856-1940)Sir J J Thomson (1856-1940)

In 1897, showed that cathode rays produced in a discharge tube consist of identical negatively-charged, very small particles…

He had discovered the electron. i.e. the atom is not indivisible – it

is made up of smaller parts. In 1904, he proposed a model in

which the atom is a sphere of electrically positive substance mixed with negative electrons – the so-called ‘plum pudding model’.

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Ernest Rutherford (1871-1937)Ernest Rutherford (1871-1937)

• In 1912, fired alpha particles (helium nuclei – positively-charged) into a thin film of gold.

• Expected most of the particles to pass straight through the foil.

• Perhaps a few might be deflected as a result of repulsion or attraction within the ‘plum pudding’ model.

• Astounded that a small number came straight back in the direction from which they’d come.

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Rutherford’s CommentRutherford’s Comment

“ It was quite the most incredible thing that has ever happened to me in my life. It was almost as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you”.

InterpretationInterpretation

• The helium nuclei must have rebounded from something inside the atom that was small, dense and positively charged.

• Most of the particles had passed through the outer part of the atom.

• Rutherford had discovered the atomic nucleus.

• Developed the ‘planetary model’…• The positive electrical charge and most

of the mass are concentrated in an almost point-sized nucleus .

• Proposed that the electrons move around the nucleus like planets around the Sun.

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Sir James Chadwick (1891-Sir James Chadwick (1891- Working at Cambridge Noticed that when a sheet of beryllium was placed

between an alpha source and a charged particle counter, no reading was registered…

But when a piece of paraffin wax was inserted between the beryllium and the detector, a signal was observed.

Surmised that alpha particles were stopped by beryllium foil…

But charged particles were leaving the paraffin wax. Attributed this to alpha particles displacing neutral

particles from the beryllium, which were not detectable. When these neutral particles hit the paraffin wax they

displaced positively-charged protons that were detected.

Chadwick had discovered that the nucleus of the atom also contains neutral particles. He had discovered the neutron.

Niels Bohr (1885-1962)Niels Bohr (1885-1962) Danish physicist. 1913. The modern picture of the atom. Electron orbits are grouped into definite

‘shells’. Each shell lies at a definite radius – the

Bohr radius. The electrons in each shell are at the

same energy. i.e. Only certain specific energies are

allowed – the energies are quantised. Each shell can contain only a fixed

maximum number of electrons… The inner-most shell can hold only 2… The next 8, and so on. The total number of electrons allowed

in a shell is given by 2n2,where n = ‘shell number’.

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IsotopesIsotopes

That is not the end of the story…

Even atoms of the same element can have different numbers of neutrons.

Such atoms have the same atomic number but…

Different atomic mass. Called isotopes.

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