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24/05/2017 A is for ATOM + Evidence for nuclear atom using alpha particle scattering. Next The Nuclear Atom History of the Atom - Democritus 24/05/2017 From the ancient Greeks through to the 19th centaury, there has been the question ‘what is matter is 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 The Nuclear Atom History of the Atom – John Dalton 24/05/2017 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 The Nuclear Atom Discovering the Electron 24/05/2017 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 The Nuclear Atom Thomson’s Model of the Atom 24/05/2017 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 plumpudding model. Back Next The Nuclear Atom Radioactivity 24/05/2017 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 DIG – The Dating Game Radiation 24/05/2017 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 The Nuclear Atom Rutherford’s Experiment 24/05/2017 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 The Nuclear Atom 24/05/2017 Alpha Particle Scattering Why were alpha particles scattered? To explain back scattering Rutherford proposed the Nuclear Model of the Atom. + Back Next The Nuclear Atom Alpha Particle Scattering 24/05/2017 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 The Nuclear Atom Alpha Particle Scattering 24/05/2017 What is observed is that alpha particles in some instances are strongly deflected. Alpha Source With electrons practically dismissed, the only electrostatic force available could be a positive charge somewhere within the atom. An atom Back Next The Nuclear Atom 24/05/2017 Rutherford’s Model 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 The Nuclear Atom Rutherford’s Model 24/05/2017 The nucleus must be very small in comparison to the atom. This will account for the vast majority making it through unaffected. Back Next The Nuclear Atom 24/05/2017 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 The Nuclear Atom 24/05/2017 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 The Nuclear Atom The Problem… 24/05/2017 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. + The Nuclear Atom 24/05/2017 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! Alpha Radiation Neutron Released Beryllium Foil These he named NEUTRONS. Back Next 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. Bohr’s Atom 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. Atomic Spectra A glass prism can be used to generate a colour ………………….. If this the light generated by a hot (glowing) gas is viewed through a prism specific colour lines are seen as AN ……………… ………….SPECTRUM. If light is shone through a cold sample of the same gas, the same specific colour lines are absent and appear as an ……………………….. LINE SPECTRUM. 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. Absorption & Emission spectrum ……………………… - ………………………… • In absorption spectrum radiation is again absorbed by electrons being …………… to higher energy levels. • The same frequencies (colours) are again emitted when the excited electrons ……………………. to the ground state in an ………………………spectrum. Absorption & Emission spectrum ABSORBED LIGHT - EMITTED 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 down to the ground state in an emission spectrum. Emission Spectrum Excited electrons dropping down from unstable energy levels ……………………..in the form of light. The frequency (colour) of the radiation is directly related to the ……………………….. between the energy levels. Since each element has its own ……………………series of energy levels, each element also has its own unique series of …………………………… lines. The line spectrum can therefore be used to …………………each element much like a fingerprint. Emission Spectrum Excited electrons dropping down 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. THE NEUTRAL ATOM • The atom consists of a _____________________ ______________________________ surrounded by a __________________________. • Atomic Number Z: ___________________ in the Nucleus = _________________ in a ______ atom. • Mass number A - Number of ______ + ________ Notation ___________ Number (bigger) ______________ Number (smaller) A Z X symbol THE NEUTRAL ATOM • The atom consists of a nucleus containing protons and neutrons surrounded by a cloud of electrons. • Atomic Number Z - Number of protons in the Nucleus = number of electrons in a neutral atom. • Mass number A - Number of protons + neutrons. Notation Mass Number (bigger) Atomic Number (smaller) A Z X symbol Relative Masses • Relative atomic(Ar): The mass of the atom relative to ________________________________________. (Number of times heavier than…) Eg: O - 16 one atom of oxygen is ________________ than 1/12 of the mass of a C12 atom, Formula mass (Mr) - The _______________________ of the atoms in a molecule. Water H2O one molecule of water has a relative mass of _____________________________ that is the molecular or formula mass of water. Mr(H2O) = 18 (Times heavier than…) Relative Masses • Relative atomic mass (Ar): The average mass of an atom of an element relative to 1/12 of the mass of a C12 atom. (Number of times heavier than…) Eg: O - 16 one atom of oxygen is 16 times heavier than 1/12 of the mass of a C12 atom. • Formula mass (Mr) - The sum of all the atomic masses of the atoms in a molecule. Water H2O one molecule of water has a relative mass of (2x(1)+16) = 18 - that is the molecular or formula mass of water. Mr(H2O) = 18 (Times heavier than…) Relative Masses - examples Calculate the Formula masses of: • O2 (oxygen gas) Mr(O2) = • Cl2 (chlorine gas) • NaCl (sodium chloride - table salt) • CaCO3 (calcium carbonate) • (NH4)2Cr2O7 (ammonium dichromate) Relative Masses - examples Calculate the Formula masses of: • O2 (oxygen gas) Mr (O2) = 2x16 = 32 • Cl2 (chlorine gas) Mr (Cl2) = 2x35.5 = 71.0 • NaCl (sodium chloride - table salt) Mr (NaCl) = 23+35.5 = 58.5 • CaCO3 (calcium carbonate) Mr (CaCO3) = 40.1+12+(3x16) = 100.1 • (NH4)2Cr2O7 (ammonium dichromate) Mr ((NH4)2Cr2O7 ) = 2(14+4)+2(52)+7(16) = 252 Isotopes The two atoms below both belong to carbon but they are not identical – can you spot what is different? e- e e- e- - e- e- 6 C 13 e e- - e- e- e- e- 6 C 12 Isotopes Atoms of the same element which have different numbers of neutrons. Others – Boron 10 & 11, Hydrogen 1 & 2, Chlorine 35 & 37. Write notation and work out numbers of neutrons. Isotopes • Isotopes - Atoms of the same element which have different numbers of neutrons. Eg: 613C & 612C • 37Cl (25%) & 35Cl (75%) - ratio 1:3 Av Ar(Cl) = (37x25)+(35x75) = 35.50 100 Or Av Ar(Cl) = (37x1)+(35x3) = 35.50 4 Relative atomic mass is (actually) the average mass of an atom of an element relative to 1/12 of the mass of a carbon-twelve atom. Bohr’s Atom - problems • Only explain hydrogen spectrum. • Could not explain molecules (bonding of atoms) - formation or properties. • Why fixed orbits and no energy radiation in orbits. • At variance with Heisenberg’s uncertainty principle. Heisenberg: Not possible to know both the position and velocity of an electron at the same time with the same amount of accuracy. A Wave Model • De Broglie - light - ‘matter wave’ - theory. • Davisson & Germer - electron diffraction -proof of ‘matter waves’. • Shroedinger, Heisenberg et. al. - wave & quantum mechanical model. • Orbits ---> Orbitals standing electron waves - a region or space defining the standing wave pattern. Orbital: - a region where there is a high probability of finding an electron. Ionisation Energy (Ei) The ENERGY REQUIRED to REMOVE AN ELECTRON completely from an atom. Sodium atom Electronic structure Na: 1s22s22p63s1 Sodium ion Na+: 1s22s22p6 FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST electron. M M+ + 1eSECOND ionisation energy (Ei2): Energy required to remove SECOND OUTERMOST electron. M+ M2+ + 1e- Successive Ionization Energies Analyse this graph in light of your knowledge of atomic & electronic structure. This graph can be used to provide ‘evidence’ for some of the features of modern atomic theory. What can be inferred about the electronic structure of the atom? Successive Ionization Energies FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST electron. M M+ + 1eSECOND ionisation energy (Ei2): Energy required to remove SECOND OUTERMOST electron. M+ M2+ + 1eHard to remove close to Inner nucleus level Second energy level Easy to remove far from the nucleus Outer (valence) level What can be inferred about the electronic structure of the atom? This graph provides EVIDENCE for energy levels. Patterns in Energies FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST electron. M M+ + 1eWhat does this graph tell us about the electronic structure of the atom? Patterns in Energies FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST electron. M M+ + 1e- First Ionisation Energies Which of the following electrons would be easier to remove? H’s electron would be removed. 1s 1 proton H Electron Structure Bohr Orbits energy levels N=4 N=3 3s 2s N=2 N=1 Electron Structure Bohr Orbits energy levels N=4 Energy sub levels and orbitals 4p 4s 3p orbitals N=3 3s 2p orbitals N=2 N=1 2s 1s orbital (3d orbitals) Electron distribution Pauli: – Electrons occupy lowest vacant energy levels. – Max two electrons per orbital – spin paring occurs when two electrons sharing the same orbital. N S S N Energy levels (1,2,3 etc) are divided up into sub-levels (s, p, d, f) each of which has a specific number of orbitals (s-1, p-3, d-5). N=2 second energy leve N=1 first energy level Electron Structure 4s 4s 3p 3p 3s 3s 2p 2p 2s 1s 2s 1s Electron Structure 4s 4s 3p 3p 3s 3s 2p 2p 2s 1s 2s 1s Electron Structure 4s 4s 3p 3p 3s 3s 2p 2p 2s 1s 2s 1s Electron Structure Afbau Diagrams 4s 3p 3s 2p 2s 1s 1s H 1s1 He 1s2 Electron Structure 4s 4s 3p 3p 3s 3s 2p 2p 2s 2s 1s 1s Li 1s2 2s1 Be 1s2 2s2 Electron Structure 4s 4s 3p 3p 3s 3s 2p 2p 2s 1s B 2s 1s C 1s2 2s2 2p2 Electron Structure 4s 4s 3p 3p 3s 3s 2p 2p 2s 1s 2s 1s Electron Structure 4s 4s 3p 3p 3s 3s 2p 2p 2s 1s 2s 1s Patterns in Energies FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST electron. M M+ + 1e- The Periodic Table Atomic Radius Trends The Periodic Table Rows Periods VIII II 1 2 Columns-Groups I 1 <-- Group Numbers --> III 2 3 3 4 5 S Block IV V VI 4 5 O 6 VII O P block d Block 6 7 Energy Levels F block Oxygen is in group 6 and period 2. It therefore has an outer electronic structure of: 2s22p4 The Periodic Table - Blocks D-block – TRANSITION METALS METALS F – Block – Lanthanides and Actinides Noble (Inert gasses) – NON Halogens Metals SAlkali-earth - Block Alkali Metals P Block References • Simon Ball • Nelson 12 • Mcgraw-Hill Ryerson