Atoms: Nuclear Interactions The Nature of Atoms

The Atom The idea of the atom is usually credited with Leucippus and particularly, his student, Democritus around 400 BC. All matter is made up of atoms. Atoms are physically indivisible. Empty space lies between atoms. Atoms have been and always will be in motion. There are an infinite number and kind of atoms, that differ in shape, size and temperature.

The Atom Aristotle (384-322 BC) believed that all matter was made up of four distinct elements.

The Atom 1803-1805 – John Dalton was able to measure weights of atoms. He developed a new atomic theory based upon his experiments. 1. Atoms cannot be created or separated into smaller particles 2. Each element has its own atom, different from other elements 3. Atoms can combine with other atoms to form compounds 4. All atoms of a given element are identical 5. Elements are made up of tiny, indivisible units called atoms

A Great Discovery Modern atomic investigations comes from studies of radiation – the emission of energetic particles or electromagnetic waves. 1895 – Wilhelm Roentgen was working with cathode rays and fluorescence.

A Great Discovery Roentgen covered a cathode ray tube and noticed fluorescence even without an apparent radiation source. Roentgen called this unknown radiation source, X-rays – high energy electromagnetic radiation

A Great Discovery 1896 – Henri Becquerel wanted to see if fluorescent minerals gave off x-rays. Stored a piece of uranium on a photographic plate in a closed desk drawer. Discovered radioactivity – the spontaneous emission of particles and energy from atomic nuclei.

A Great Discovery 1897 – J.J. Thomson discovered the electron using a cathode ray tube. Noticed that the cathode ray was deflected by the negative end of a magnet and bent towards the positive end of a magnet. Came up with the plum pudding model of the atom. Later experiments proved the notion of isotopes.

A Great Discovery 1898 – Marie Curie, a student working with Henri Becquerel, started working on the isolation of radioactive components of the uranium mineral pitchblende. With help from her husband, Pierre Curie, she discovered the elements Polonium (named after her native Poland) and Radium.

A Great Discovery 1899 – Ernest Rutherford was studying uranium and discovered two different emissions from the radiation. He named them the alpha particle (+) and the beta particle (-). 1900 – Paul Villard discovered a third type of radiation from Radium, named gamma rays.

A Great Discovery 1904 – Hantaro Nagaoka creates the Saturnian model of the atom. 1. The atom contained a very massive nucleus. 2. The electrons revolved around the nucleus bound by electrostatic forces. These predictions were proven by Rutherford and the gold foil experiment.

The Gold-Foil Experiment 1910 – Ernest Rutherford, a student of Thomson, conducted the gold-foil experiment with help from his students Hans Geiger (creator of the Geiger counter) and Ernst Marsden.

The Gold-Foil Experiment The alpha particles would be detected when they struck a fluorescent screen. Most particles went straight through the gold foil. Rutherford expected to see the particles deflect only slightly through the plum pudding atom. Rutherford concluded that the atom must be mostly empty space with electrons orbiting around the positive nucleus.

The Atom 1913 – Niels Bohr, while working with Rutherford, comes up with an atomic theory describing the electrons in specific energy levels. Electrons have specific energy associated with each level.

The Atom 1913 – Henry Moseley showed that the positive charge of atoms is related to their atomic number. 1918 – Rutherford discovers the proton while experimenting with alpha particles passing through nitrogen gas and thus providing evidence that the atomic number is based on the proton. Gave proof to the proposal by Eugen Goldstein, following the work by Thomson, that since electrons existed in an atom, a positive particle must also exist in the neutral atom.

Electron Cloud Model The “Electron Cloud Model” is the wave mechanics interpretation of the Bohr Model of the atom. Based on the uncertainty principle of Werner Heisenberg from 1926 which gave rise to the statistical nature of the location of electrons in the atom as described in 1930 by Irwin Schroedinger.

Atomic Particles Atoms are made of three particles: Protons – positive and found in the nucleus Neutrons – neutral and in the nucleus Electrons – negative and surrounds the nucleus The number of protons and electrons are the same in atoms Particle Location Electrical Charge Molar Mass (g/mol) Proton Nucleus +1 1 Neutron Electron Outside Nucleus -1 0.0005 ~ 0

Atomic Particles The diameter of a typical atom is 10-10 m and the average nuclear diameter is 10-14 m. This makes the nucleus 10,000 times smaller than the atom. If the nucleus were a billiard ball, the electrons would be about 1/3 of a mile away. Imagine the Metrodome. If you are walking around the concourse, you would be the electron while a marble placed at the 50 yard line would be the nucleus.

Atomic Particles Atoms have the same number of protons and electrons (Z) Neutrons are determined by the mass (A) - the atomic number (Z) 39 K 19 protons 19 electrons 20 neutrons 19

Isotopes From 1900-1902, Frederick Soddy was working with Rutherford, and together they discovered isotopes – atoms of the same element with different numbers of neutrons. Name Atomic Number Neutrons Mass Number Electrons Carbon-12 6 12 Carbon-13 7 13 Carbon-14 8 14

The End

Plum Pudding Model of Atom Developed by Thomson around 1903/04 Imagine chocolate chip cookie dough. Where the dough is a ball of positive charge and the chocolate chips are the negative electrons just sitting in the dough.

Electromagnetic Radiation Radiation is the emission of energy from a source. It has no mass and travels at the speed of light (186,265 mph) Two types: non-ionizing and ionizing radiation.

Electromagnetic Radiation Non-ionizing Radiation – low energy radiation causing matter to vibrate, move to higher energy levels or to heat up. Usually non-fatal (microwaves and sunburns) Ionizing Radiation – high energy radiation can eject electrons from atoms or molecules. Can disrupt cellular chemistry causing cell damage. Can be fatal. Nuclear radiation is a form of ionizing radiation that emits energy and particles.