ATOMIC PHYSICS

Early Atomic Theory1. Democritus 440BC The smallest indivisible particle of matter

is the atom.2. Aristotle 340BC criticised an alternative model based on

the four elements – fire, water, air and earth.

This model fitted well with religious beliefs and held for 1600 years.

Alchemy appeared 400 years after Aristotle.

End of the Dark Age

• Galileo (1564 – 1642)

Returned to the Democritus model with a world made of void and atoms.

• Lavoisier (1743 – 1794)

Law of Conservation of Mass during a chemical reaction.

“the sum of the mass of the reactants equals the sum of the masses of the products.”

Dalton – a revolutionary theory

In 1803 proposed that:• Elements are made up of tiny particles

called atoms.• Elements contain only one type of atom.• Atoms cannot be created or destroyed – a

chemical reaction being the re-arrangement of elements.• Compounds contain more than one type of

atom

J.J. Thomson (1856 -1940)

• 1897 measured charge to mass ratio of electrons e/m.

• Determined that cathode rays were electrons.

V

v

m

e

mveV

2

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1

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Thomson’s plum-pudding model (1897)

Rutherford’s model (1909)

Bohr’s model (1913)

Charge-cloud model (present)

1803 John Dalton pictures atoms astiny, indestructible particles, with no internal structure.

1897 J.J. Thomson, a Britishscientist, discovers the electron,leading to his "plum-pudding" model. He pictures electronsembedded in a sphere ofpositive electric charge.

1904 Hantaro Nagaoka, aJapanese physicist, suggests that an atom has a centralnucleus. Electrons move in orbits like the rings around Saturn.

1911 New Zealander Ernest Rutherford statesthat an atom has a dense,positively charged nucleus. Electrons move randomly in the space around the nucleus.

1913 In Niels Bohr'smodel, the electrons move in spherical orbits at fixed distances from the nucleus.

1924 Frenchman Louis de Broglie proposes thatmoving particles like electronshave some properties of waves. Within a few years evidence is collected to support his idea.

1926 Erwin Schrodinger develops mathematicalequations to describe the motion of electrons in atoms. His work leads to the electron cloud model.

1932 James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons.

+--

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e

e

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+

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e

ee

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ee

Thomson Model

In the nineteenth century, Thomson described the atom as a ball of positive charge containing a number of electrons uniformly distributed.

Plum-pudding model

meanwhile …….Spectra

1886 Balmer - when an element is ionized in a discharge tube the light emitted produces a line spectrum in a spectroscope. pic

Radioactivity

Bequerel and the Curies (1898) identified elements that produced radioactive rays and particles.

RutherfordScattering Experiment

• Tried to verify Thomson’s model of atom.

Aim: to prove that atoms in the gold foil were spheres of positive fluid with negative charges embedded.

Prediction: less than 1% of alpha particles would be deflected more than 3º.

The Experiment

• To test this he designed and experiment directing ‘alpha’ particles toward a thin metal foil.– The screen was coated with a substance that produced flashes

when it was hit by an alpha particle.

Experimental Observations

• most alpha particles passed through the gold foil undeflected.

• large numbers of α were deflected at angles >3º

• 1/10,000 α were backscattered through 180º

• pic

Rutherford’s proposed model

• most of the atom is space

• a very small dense and positive nucleus in the centre of the atom.

• electrons were around nucleus but at a distance.

Problems of the model• line spectra could not be explained• electrons could not be at rest – they would be

attracted to the nucleus and be neutralized.

• if electrons circled the nucleus, they would be undergoing centripetal acceleration – energy?

• accelerated charges emit electromagnetic radiation, so electrons should lose energy continuously and spiral into the nucleus.

• stability of atoms could not be explained

The Atom

Nucleus

• Positive protons and neutral neutrons each of atomic mass unit 1. Together they are the nucleons.

• held together by strong nuclear forces balanced by strong electric forces

• As the number of protons increases the nuclear forces decreases and the nucleus becomes more unstable

shorthand

Atomic number Z = number of protons

Mass number A = mass of nucleons

C

Cl

xaz

126

3517

Radioactive Isotopes and radioactivity

Radioisotopes - isotopes that have unstable nuclei and will spontaneously disintegrate and emit radiation.

Becquerel 1896 mineral pitchblende

Curie 1898 radium and polonium

Three types or radiation:

1. Alpha particles

2. Beta particles

3. Gamma radiation

Isotopes

• Isotopes are atoms of the same element with different numbers of neutrons

• C12 C13 C14

• Atoms which give out radioactivity are called radioactive isotopes.

• The nucleus of a radioactive isotope is unstable.

Characteristics of types of radiation

Alpha particles

• Nuclei of helium atoms He =2p+2n

• Heavy particle – low speeds 2x10 7 ms-1

• Strong ionizers – attracting e from other molecules

• Low penetration in air

• Deflected magnetic by a and electric field

• Emitted by elements>

• \

Half life of Isotopes

• The rate at which a nucleus decays is characteristic of that nucleus

• Rate is measure by half life t1/2 and is an exponential decay function

• The time it takes for half the given mass of an element to decay into a new element.

• It is constant but random process

• eg. at zero time if there is 10g of a radioactive element

• eg. at zero time if there is 10g of a radioactive element with a half life of 6hr then:

after 6 hrs there will be 5g left

12 2.5g

18 1.25g

Uses of Radioactivity

• Carbon-14 dating• Radiotherapy• Sterilisation – medical supplies • Nuclear medicine• Food irradiation• Smoke detectors• Industrial uses• Agriculture applications

ELECTROMAGNETIC SPECTRUM