Topic 7: Atomic and nuclear physics 7.1 The atom

2 hours

Dalton Thompson Rutherford Bohr

John Dalton’s Atom~1808

– Each element is made up of tiny particles called atoms.

– The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.

– Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.

– Chemical reactions involve reorganization of the atoms--changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

J.J. Thompson’s Atom~1904

– The atom is composed of electrons (which Thomson still called "corpuscles", though G. J. Stoney had proposed that atoms of electricity be called electrons in 1894) surrounded by a soup of positive charge to balance the electron's negative charge, like negatively-charged "plums" surrounded by positively-charged "pudding".

Geiger and Marsden (and Rutherford)~1909

The majority of alpha (a) particles are slightly deflected, however, some are scattered at very large angles. Small deflections can be explained by positively charged a-particles passing the nucleus at large distances and being repelled. The large deflections are explained by the a-particles passing the nucleus at small distances. These large scattering angles were surprising to Geiger and Marsden as they did not know the nucleus existed.

Rutherford’s Interpretation

The large deflections indicated an enormous force of repulsion between the positive a-particles and the positive charge component of the atom. This means that the positive charge in an atom must be concentrated in one area and not spread out like a positive soup as Thompson had suggested. The fact that gold atoms do not recoil when the a-particles hit them means that they body holding the positive charge is very massive and yet small enough for the a-particle to get very close to.

Rutherford’s Interpretation

The Rutherford Model of the AtomAt atom (according to Rutherford) consists of a massive positively charged nucleus at its centre and electrons orbiting this nucleus like planets orbit the sun.

Problems with Rutherford’s Model:

1. Electrons that are accelerating (and here we have centripetal acceleration) are known to radiate light energy.The electrons in Rutherford’s model should, therefore lose energy and spiral into the nucleus in a matter of nanoseconds. Rutherford’s model, then, cannot explain why stable atoms exist.

2. Rutherford’s model did not explain the observed emission spectra

The Bohr Model of the AtomNeils Bohr examined the simplest atom (Hydrogen) and realized that its electron could exist in certain specific states of definite energy without radiating any energy away. The electrons energy is discrete (or quantized) rather than continuous and it can only lose energy by making a transition from one allowed state to another allowed state of lower energy. The emitted energy is equal to the energy between the initial and final states and is given off as light.

Excited States

Emission Spectra

Consider hydrogen (H). Under normal conditions the electron in each hydrogen atom occupies the lowest energy state (known as the ground state). If the atom is somehow excited, then the electrons will leave the ground state and move to a higher energy state. As soon as this happens they transition back to lower energy states radiating energy in the process.

The set of wavelengths of light emitted by the atoms of the element is called its emission spectrum. The absorbed wavelengths (which equal the emitted wavelengths) make up the absorption spectrum.

The Balmer Series• The Balmer series or Balmer lines in atomic physics, is the

designation of a set of the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirical equation discovered by Johann Balmer in 1885.

• The visible spectrum of light from hydrogen displays four wavelengths, 410 nm, 434 nm, 486 nm, and 656 nm, that correspond to emissions of photons by electrons in excited states transitioning to the quantum level described by the principal quantum number n equals 2. There are also a number of ultraviolet Balmer lines with wavelengths shorter than 400 nm.

Nuclear Structure

• The word nucleon is used to describe a proton or a neutron.

• The number of protons in the nucleus is denoted by Z and is called the atomic number of the element.

• The total number of nucleons (protons + neutrons) is called the mass number, and is denoted by A.

• A nucleus with a specific number of protons and neutrons is known as a nuclide.

Special ParticlesParticle Symbol

Alpha Particle (Helium-4 Nucleus) or

Electron or

Proton or or

Neutron or

Photon or

He42 a42

e01 e

p11p

n10 n

00

H11

The Forces Within The Nucleus• Consider an a-particle (a Helium-4 Nucleus)

• From our studies of Coulomb’s Law we know that there will be a strong repulsive force between the two protons and neither an attractive or repulsive force between the two neutrons or between the neutrons and the protons. We also know that all of nucleons will be very weakly attracted to each other by Newton’s Law of Universal Gravitation.

So what holds it together?• Shortly after the discovery of the neutron, Hideki Yukawa, a

Japanese physicist, postulated a strong force of attraction between nucleons that overcomes the Coulomb repulsion between protons. The existence of the force postulated by Yukawa is now well established and is known as the strong nuclear interaction.

• The force is independent of whether the particles involved are protons or neutrons and at nucleon separations of about 1.3 fm, the force is some 100 times stronger than the Coulomb force between protons. At separation greater than 1.3 fm, the force falls rapidly to zero. At smaller separations the force is strongly repulsive thereby keeping the nucleons at an average separation of about 1.3 fm. (1 femtometre = 10-15 m)

The Fundamental Forces

Interaction Current Theory Relative Strength Range (m)

Strong Quantum chromodynamics(QCD) 1038 10−15

Electromagnetic Quantum electrodynamics(QED) 1036 (Infinite)

Weak Electroweak Theory 1025 10−18

Gravitation General Relativity(GR) 1 (Infinite)

Read Tsokos, Page 367 – 372 and Answer Questions 1 to 12 on Page 372