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CHAPTER 10

10.1 UNDERSTAND THE NUCLEUS OF AN

ATOM

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1. Describe the composition of thenucleus of an atoms in terms of

protons and neutrons.

2. Define proton numbers and nucleonnumber.

3. Explain the terms nuclide.

4. Use the nuclide notationX

A

Z

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5. Define the term isotope.

6. Describe the experiment to prove theatomic model

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NUCLEUS OF AN ATOM

Matter is made up of very small

particles called atoms

Each atom has a very small and very dense

core called nucleus. Most of the mass of atomis contained in the nucleus

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Nucleus

proton

neutron

electronNucleus

Structure of an Atom

The positively charged

mass within an atom,composed of neutrons and

protons, and possessingmost of the mass butoccupying only a smallfraction of the volume of theatom.//

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The electrons move in orbits around the nucleus.

There are a lot of empty space within atom A nucleus consists of a number of proton and

neutron.

Protons and neutrons also known as nuclide.

A proton has a positive charge.

A neutron is a neutral particle of about the same

mass as the proton.

An atom is neutral because it contains an equal

number of negatively charged electrons. So the net

charge is zero.

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Define proton number, Z Proton number, Z, is defined as the number

of protons in a nucleus.

The number of protons = the number of

electrons

An element is identified by its proton number

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Define nucleon number (A)

Nucleon number, A is defined as the total

number of protons and neutrons in anucleus. It is also known as mass number.

Nucleon number,

A = number of proton Z + number of neutrons

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What is nuclide?

A nuclide is an atom of a particular structure.Each element has nucleus with a specific

number ofprotons.

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Nuclide Notation

AX A = nucleon number

Z Z = proton number

X = chemical symbol of theelement

Example

Proton number of carbon = ___, Number of proton = _____ Number of electron = ___ Number of neutron = ______

C12

6

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DefineIsotope

Isotopes of an element contain the samenumber of protons and the same number ofelectrons. So isotopes have the samechemical properties chemical reactionsinvolve the electrons in an atom.

However they have different physicalproperties because their mass is different.

Some isotopes exist naturally. Isotopes canalso be made artificially.

Isotopes are atoms with the

same proton number butdifferent nucleon number.

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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.

History of Rutherford

Atomic Model

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Geiger and Marsdenexperiment

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The result and the conclusion of the

experiment is simplified in the table below:

Result Conclusion

Most of the alphaparticles passedstraight through thefoil in their originaldirection.

Most of the space takenup by an atom must becompletely empty. Avery small nucleus isplaced at the centre of

the atom.

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Result Conclusion

A few alphasparticles were

deflected throughvery small angles

The nucleus arepositively charged. The

alpha particles alsopositively charged arerepelled by the nucleusbecause repulsion force

is produced between thelike electric charges.

The result and the conclusion of the

experiment is simplified in the table below:

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Result Conclusion

A very small

number of alphaparticles werebounced backby the gold foil.

The alpha particles collides

head on with the nucleus

The result and the conclusion of the

experiment is simplified in the table below:

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