Atomic Theory Ochran 2014
Evidence for nuclear atom using alpha particle scattering. 19/11/2018 A is for ATOM + Evidence for nuclear atom using alpha particle scattering. Next
History of the Atom - Democritus The Nuclear Atom 19/11/2018 History of the Atom - Democritus From the ancient Greeks through to the 19th centaury, there has been the question ‘what is matter made from?’ The idea of atoms was first proposed by Greek Philosopher Democritus in 530 B.C. The concept was that matter could only be split in half and half again until indivisible units were reached. ATOMS Back Next
History of the Atom – John Dalton The Nuclear Atom 19/11/2018 History of the Atom – John Dalton In 1808, John Dalton (a teacher!) proposed the modern ATOMIC THEORY. It simply states that all elements are made up of atoms and an element is only made up from one type of atom. Dalton's view of tiny indivisible spheres remained unchallenged until the end of the 19th Century. Dalton’s Atom Back Next
Discovering the Electron The Nuclear Atom 19/11/2018 Discovering the Electron In 1897, Joseph (JJ) Thomson (British Physicist) was experimenting with electrical currents through gases. The cathode rays he produced could be deflected or moved when in electromagnetic fields. Cathode rays were made up of tiny negatively charged particles – ELECTRONS. Back Next
Thomson’s Model of the Atom The Nuclear Atom 19/11/2018 Thomson’s Model of the Atom From his evidence, Thomson proposed that atoms were made up of just tiny electrons. He accounted for the neutrality of atoms by the stating the electrons existed in a ‘soup of positive charge’. Sometimes referred to the plum-pudding model. Back Next
19/11/2018 Radioactivity The Nuclear Atom Around the same time, Henri Becquerel discovered that some unstable elements gave off smaller particles – RADIOACTIVITY. Therefore atoms must be divisible and made up of smaller parts – SUBATOMIC PARTICLES. Marie and Pierre Curie and Ernest Rutherford confirmed this. Back Next
19/11/2018 Radiation DIG – The Dating Game Alpha Particle: Positively charge. Large in comparison. Essentially a Helium Nucleus (as proved by Rutherford) Beta Particle: Negatively charged. Light. (later to be shown as electrons) Gamma Rays: Neutrally charged. No mass – Energy. Back Next
Rutherford’s Experiment The Nuclear Atom 19/11/2018 Rutherford’s Experiment Rutherford and his colleagues bombarded a thin foil of gold with a beam of alpha particles and then onto a fluorescent screen. Small amounts were deflected. Fluorescent Screen 99.9% passed straight through unaffected. Thin Gold Foil Back Next
Alpha Particle Scattering The Nuclear Atom 19/11/2018 Alpha Particle Scattering Why were alpha particles scattered? To explain back scattering Rutherford proposed the Nuclear Model of the Atom. + Back Next
Alpha Particle Scattering The Nuclear Atom 19/11/2018 Alpha Particle Scattering Alpha particles are positive. High speed alpha particle bullet travels through atom. The electrons have little effect since they are very light and the electrons in the pudding model are very spread out. Very little deflection. Does not support observations. Plum-Pudding Model Back Next
Alpha Particle Scattering The Nuclear Atom 19/11/2018 Alpha Particle Scattering What is observed is that alpha particles in some instances are strongly deflected. With electrons practically dismissed, the only electrostatic force available could be a positive charge somewhere within the atom. Alpha Source An atom Back Next
19/11/2018 Rutherford’s Model + The Nuclear Atom He suggested that all of the atom’s positive charge, together with most of its mass, is concentrated in the centre. Alpha particles which travel close to the nucleus are strongly deflected. The degree of deflection depends on how close it approaches. + Back Next
19/11/2018 Rutherford’s Model The Nuclear Atom The nucleus must be very small in comparison to the atom. This will account for the vast majority making it through unaffected. Back Next
Rutherford’s Nuclear Model of an Atom The Nuclear Atom 19/11/2018 Rutherford’s Nuclear Model of an Atom In summary, He envisioned an atom that had a positively charged nucleus in the centre. The atom was mostly empty space. An he deemed it reasonable that electrons orbit this nucleus like planets orbit the Sun. + Nuclear Model of an Atom Back Next
Rutherford’s Nuclear Model of an Atom The Nuclear Atom 19/11/2018 Rutherford’s Nuclear Model of an Atom The model appeared flawless and convinced most of the scientific community. Rutherford and his colleagues (Hans Geiger and Ernest Marsden) were able to precisely predict the effects of: Alpha particle energy Thickness of sample Different metals However there was a problem… + Nuclear Model of an Atom Back Next
19/11/2018 The Problem… The Nuclear Atom As the electrons move in circles, they would lose energy. Losing energy would slow them down. Therefore they would be pulled into the positively charged nucleus. It has been calculated that a Rutherford atom would only exist for about 1 billionth of a second! The answer lies within QUANTUM MECHANICS – when things get really small!! Back Next
+ Rutherford’s model could not explain: Why the electrons did not lose energy as they orbited. What held the protons together in the nucleus. The origins of emission spectra of gases could not be explained. +
James Chadwick’s Protons The Nuclear Atom 19/11/2018 James Chadwick’s Protons The number of protons in a nucleus did not match the atomic weight of the atom. Therefore a third neutrally charged particle must exist! These he named NEUTRONS. Alpha Radiation Neutron Released Beryllium Foil Back Next
Atomic Spectra A glass prisim can be used to generate a colour spectrum. If this the light generated by a hot (glowing) gas is viewed through a prism specific colour lines are seen as AN EMISSION LINE SPECTRUM. If light is shone through a cold sample of the same gas, the same specific colour lines are absent and appear as an ABSORPTION LINE SPECTRUM.
Bohr’s Atom This model enabled Bohr to explain the hydrogen spectrum. Bohr - brought the concept of quantization into atomic theory. Electrons could only move in certain specific orbits corresponding to specific amounts of energy. These ENERGY LEVELS radiated out from the nucleus with higher energies being further away. Electrons do not radiate energy in these orbits. Energy is gained or lost when they move between orbits. This model enabled Bohr to explain the hydrogen spectrum.
On what basis could Bohr make these claims? In 1900, a physicist named Max Planck proposed an idea: Planck suggested that matter, at the atomic level, can absorb or emit only discrete quantities of energy. Each of these specific quantities.is called a quantum of energy. In other words, Planck said that the energy of an atom is quantized. (can exist only in certain discrete amounts). a quantum is an extremely small "packet" of energy.
Line Spectra Gases absorb certain frequencies of light. Each gas absorbs a unique combination of frequencies – each frequency corresponding to a unique colour. So each gas has a unique set colours which is known at its “line spectra” – because they are unique they can be used to identify a gas – similar to fingerprints.
Absorption & Emission spectrum ABSORBED LIGHT EMITTED LIGHT ABSORBED LIGHT - - - - In absorption spectrum radiation is again absorbed by electrons being excited to higher energy levels. The same frequencies (colours) are again emitted when the excited electrons drop to the ground state in an emission spectrum.
The Bohr-Rutherford model http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BohrModel/Flash/BohrModel.html
Emission Spectrum Excited electrons dropping from unstable energy levels radiate energy in the form of light. The frequency (colour) of the radiation is directly related to the energy gap between the energy levels. Since each element has its own unique series of energy levels, each element also has its own unique series of emission/absorption lines. The line spectrum can therefore be used to identify each element much like a fingerprint.
References Simon Ball Nelson 12 Mcgraw-Hill Ryerson