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10.1 Atom

Atomic model

Dalton’s model

Matter was made up of very small particles called atoms.

Atoms can neither created or destroyed.

All atoms of the same element are identical; different elements have different types of atom.

Thomson’s model – plum-pudding model

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Atom is a solid sphere of positive charge with electrons distributed uniformly over it.

Rutherford model

An atom has a positively charged core or nucleus, which contains most of the mass the atom and which surrounded by orbiting electrons.

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Geiger and Marsden planned and carried out an experiment proposed by Sir Ernest Rutherford and found evidence for Rutherford Model.

They fired a stream of alpha particles at a very thin gold foil and counted how many alpha particles were scattered at a number of different angles. The results agreed well with the theory.

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The result and the conclusion of the experiment is simplified in the table below:

Result

Conclusion

Most of the alpha particles passed straight through the foil in their original direction.

Most of the space taken up by an atom must be completely empty. A very small nucleus is placed at the centre of the atom.

A few alphas particles were deflected through

very small angles

The nucleus are positively charged. The alpha particles also positively charged are repelled by the nucleus because repulsion force is produced between the like electric charges.

A very small number of alpha particles were bounced back by the gold foil.

When the alpha particles approach very close to the nucleus , they were exerted by a very large repulsion force because the repulsion obeys the inverse square law of the force between two charged objects

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Bohr’s model

Bohr’s model state that an atom has electrons in stable orbits around the nucleus , like planets round the sun.

Chadwick’s model

The neutron as a neutral particle found in the nucleus of atoms , along with the protons.

Today’s model of the atom

The atom with its very small and very dense nucleus surrounded by a cloud of negative electrons. The electrons may be represented as ‘waves’ or ‘clouds’ around the nucleus. The nucleus is made up of protons and neutrons or called nucleons.

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Protons, neutrons and electrons are the fundamental particles of an atom.

The properties of these particles are given in the table below.

Fundamental particle

Proton

Electron

Neutron

Charge

positive

negative

Neutral

Relative charge

+1

-1

Neutral

Value of charge

+1.6 x 10-19 C

-1.6 x 10-19 C

0

Relative mass

1

1

1840

1

Actual mass

1.67 x 10-27 kg

9.1 x 10-31kg

1.67 x 10-27 kg

Symbol

1p

1

0e

-1

0 n

1

Proton Number , Z and Nucleon Number, A

An element with a chemical symbol X, nucleon number , A and proton number , Z is represented by the symbol :

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A = Nucleon number

Z = Atom number

X = Element

Nucleon Number = number of proton + number of neutron

A = N + Z

The table below shows the examples of three elements :

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Nuclide

A type of atom characterized by its proton number and its neutron number.

Isotope

One of two or more atoms of the same element that have the same proton number but different nucleon number are called isotopes.

Since the chemical properties of an element are determined by the number of electrons, isotopes of the same element have similar chemical properties.

For example , the hydrogen element have three nuclides to form isotopes.

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Radioisotope

Isotopes with unstable nucleus tend to decay.

For example:

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One Response

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