Atomic and Nuclear PhysicsTopic 7

Atomic models• Videos :D

Properties of protons, neutrons and electrons

Position in the atom

Relative mass

Relative electric charge

PROTON

NEUTRON

ELECTRON

nucleus

nucleus

outside nucleus

1

10.0005

+ 1

- 1

0

Nuclear notation

Nucleon number = mass numberA = Z + N

Atomic number = number of protons

Terms • Page 150 in purple book.• Nucleon• Nuclide• Isotopes• Nucleon number (A)• Proton number (Z)• Neutron number

A nuclide is a type of atom whose nuclei have specific numbers of protons and neutrons (both are called nucleons). Therefore, nuclides are composite particles of nucleons.

Nuclear notation

C14

6Number of protons (Atomic number)

Chemical symbol

An isotope of carbon consists of 6 protons and 8 neutrons. This can be written as:

OR:

carbon 14 Number of protons PLUS neutrons (Mass number or nucleon number)

Isotopes Isotopes are atoms of the same element (same atomic

number) with different numbers of neutrons.

The three isotopes of hydrogen neutrons

hydrogen 1 hydrogen 3 (tritium)

hydrogen 2 (deuterium)

Note: The number after ‘hydrogen’ is the mass number of the isotope.

Question 1Determine the number of protons and neutrons in the isotopes notated below:

N13

7

(a)Co

60

27

(b)

Au197

79

(c)Pu

239

94

(d)

Question 2

Question 3

Question 4

Question 5

The Plum Pudding Atomic Model

Before about 1910 many scientists believed that an atom consisted of:Positively charged matter spread out like a pudding embedded by negatively charged electrons (like plums in a pudding). The ‘Plum Pudding’ Model

Rutherford’s Atomic ModelIn 1909 Ernest Rutherford suggested that an atom consists of a a tiny positively charged nucleus surrounded by negatively charged electrons.

Lord Rutherford 1871 - 1937

Geiger & Marsden’s alpha particle scattering experiment

In 1909 Hans Geiger and Ernest Marsden performed an experiment using alpha particles to determine which of the two models was the better in describing the structure of an atom. Geiger and Marsden

The apparatus

What was observedthin metal foil

1. Virtually all of the alpha particles went straight through the metal foil.

2. A few alpha particles were deflected through a small angle.

3. About 1 in 8000 were deflected backwards.

alpha source

nucleus (highly enlarged)

atom

How the results can be explained

1. Deflections occur because there is a force between the charged nucleus and the positively charged alpha particles.

2. Most of the alpha particles do not go near enough to the nucleus to be deflected.

3. Backwards deflections occur when the alpha particles make near head on collisions with the positively charged nucleus.

nucleus (highly enlarged)

atom

How the results can be explained

why isn’t the electron’s deflecting the alpha’s path?

How their results supported Rutherford’s atomic model

1. The relatively small number of deflections indicates that most of the atom is empty space with only a very small nucleus.

2. The backward deflections can only occur if the nucleus is positively charged and contains most of the atom’s mass.

3. The ‘plum pudding’ model would not produce backward deflections.

Bohr’s Model• Suggested electrons’ orbit the nucleus like planets orbiting

the sun• If its circling the nucleus, what should then be between

them?• What provides it?

Bohr’s Model• Suggested electrons’ orbit the nucleus like planets orbiting

the sun• If its circling the nucleus, what should then be between

them?• Electrostatic force

Bohr’s Model• Made a connecting between atom and light.• According to this model, electron’s only exist in certain orbits.• More radius (large orbit) more energy in electron• When electrons drop orbits, this energy is released as light

Bohr’s Model• Frequency of emitted light prop. to the change in energy• ΔE = E2 – E1 = hf • h = Plank’s constant = 6.6 x 10-34 Js

Bohr’s Model• Remember the colors of light?• Which has the highest frequency? Energy?

Bohr’s Model

Spectra

Emission Spectrum of Hydrogen

Absorption Spectrum of Hydrogen

Absorption Spectrum of Hydrogen

Sodium emission spectrum

Photons • What is light?

Photons • What is light? A wave• Electrons can only occupy given energy levels. • Energy of the electron is said to be quantized.• When electrons move between orbitals they must emit or

absorb energy.• Amount of energy depends on gap.• This energy is emitted or absorbed in “packets” of light called

photons.• ΔE = E2 – E1 = hf • h = Plank’s constant = 6.6 x 10-34 Js• Each photon has a frequency that is proportional to the

change of energy of the electron

Aurora Borealis

Question• Calculate the energy change required to produce a photon of

red light of wavelength 700 nm.• Convert answer to eV.