AS level Chemistry AQA: Atomic Models

8/7/2019 AS level Chemistry AQA: Atomic Models

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JJ Thompson made the

´plum pudding modelµ as

he knew from his

experiments that atomshad charge and mass. He

concluded from this that

atoms must contain

smaller, negatively charged

particles- electrons.



Rutherford then came up with the nuclear modelof the atom. In this, there·s a tiny, positively

charged nucleus at the centre, surrounded by acloud of negative electrons.

Most of the alpha particles went straight throughthe gold atoms and only a very small number

were deflected backwards. This meant the plumpudding model could not be correct.

Ernest Rutherford conducted the famous goldfoil experiment. He fired alpha particles at an

extremely thin sheet of gold.



Scientists realised that electrons in a cloud would spiral down to the nucleus, causing

the atom to collapse. Niels Bohr then proposed a new model of the atom with four

basic principles:

Electron can onlyexist in fixed

shells

Each shell has afixed energy

When an electronmoves between

shellselectromagnetic

radiation willeither be emitted

or absorbed

Because theenergy of shells is

fixed, theradiation willhave a fixedfrequency

Other scientists realised not all electrons in each shell had the same energy, so

they refined this to include sub-shells.

This model now explained why the noble gases are inert. Bohr said each shell can

only hold fixed numbers of electrons, so when an atom has full shells it·s stable

and does not react.



We now know that the refined Bohr model isnot perfect, but it is still widely used to

describe atoms as it is simple and explainsmany observations from experiments.

Scientists therefore use whichever model isrelevant to what they're investigating.

ther scientists realised not all electrons in each shell had the same energy,so they refined this to include sub-shells.

This model now explained why the noble gases are inert. Bohr said each shellcan only hold fixed numbers of electrons, so when an atom has full shells it·sstable and does not react.



1. Read the passage below and then answer the questions that follow

In 1911 Ernest Rutherford proposed a new model of the atom based on observations of the behaviour of atoms

made by his students two years earlier. He said that atoms consisted of small, positively charged nucleus around

which negative electrons orbited. Scientists predicted that as electrons orbited the nucleus they would emit

radiation with a continuous range of frequencies. This was tested and it was found that atoms emitted only certain

fixed frequencies of radiation. In 1915 Niels Bohr proposed a model of the atom in which electrons were

constrained to fixed orbits and could not exist anywhere in between.

a) Why did Rutherford think that a new model of the atom was needed?In order to explain the observations made during his student·s experiments.

b) Why is Bohr·s model thought to be a truer description of the atom than

Rutherford·s?(1) The Bohr model gives a better explanation of the observations

(2) Of the frequencies of radiation emitted by atoms.

c) More accurate models of the atom have been developed since the Bohr

model. Explain why the Bohr model is still used today.The more accurate models are very complicated/

The Bohr model is still useful for explaining most observations.



The Ca2+ has the same electronic configuration as an argon atom, whilst the

Ca+ ion has the electronic configuration identical to that of a potassium atom.

a) Which of these two ions is most stable?Ca2+ is more stable.

b) How do atomic models help to explain the relative stability of these ions?(1) Atomic models predict that full electron shells are most stable.

(2) Since Ca2+ has full electron shells (noble gas electronic configuration) it should

be more stable than Ca+ which does not.

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AS level Chemistry AQA: Atomic Models