AQA AS Physical Chemistry Atomic Structure PDF

AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE EVOLUTION OF ATOMIC STRUCTURE The scientific model of the atom has been developed over the last 200 years. New discoveries have meant that scientists are better equipped to build more accurate models....

AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE EVOLUTION OF ATOMIC STRUCTURE The scientific model of the atom has been developed over the last 200 years. New discoveries have meant that scientists are better equipped to build more accurate models. 1803 | The Solid Sphere John Dalton proposed that atoms are solid spheres and that the atoms in any given element were identical and could not be divided. 1904 | The Plum Pudding + - J.J. Thompson discovered - + - + electrons. This lead him to the - + idea that atoms are made up - + + of a ball of positive charge with negative electrons 1911 | The Nuclear Model embedded in it. Ernest Rutherford fired alpha radiation particles at gold foil and discovered that most of an atom was empty space, with the 1913 | The Planetary Model positive charge centred in a nucleus. x Niels Bohr then discovered that x electrons existed in orbits x around the nucleus at fixed distances. We know these as x x x x x x energy levels or “shells”. He x backed this up with theoretical calculations. 1932 | Subatomic Particles Ernest Rutherford’s experiments proved that the nucleus contained two types of subatomic particles. Protons that have a positive charge and neutrons that had no charge. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE FUNDAMENTAL PARTICLES Atoms are very small, typically around 0.1 nanometers in size. That’s about 0.0000001 mm! It is made up of a central nucleus which contains protons and neutrons. Electrons orbit the nucleus. The radius of the nucleus is about 1/10,000th of that of the atom, so most of the atom is empty space! Almost all of the mass of an atom is in the nucleus. Electrons X Nucleus Protons X Neutrons X Relative Mass Charge Proton 1 +1 Neutron 1 0 Electron ≈ (very small) -1 HINTS | TIPS | HACKS Atoms always have the same number of electrons as they do protons. The charges cancel each other out! The number of neutrons in an atom of an element can vary and is not related to the number of protons or electrons. Atoms of the same element that have different numbers of neutrons are known as isotopes. Ions are atoms that have either gained or lost electrons. e.g. in theory, if an atom of “X” has 12 electrons.. Charge X3- X2- X- X X+ X2+ X3+ no. of 15 14 13 12 11 10 9 electrons AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE How to Calculate Mass Number Particle Numbers = total number of protons + neutrons 7 3 Li Atomic Number = number of protons RELATIVE ATOMIC MASS (Ar) You will see that some mass numbers in the periodic table are not whole numbers. That is because this is actually their relative atomic mass, which is an average mass of the isotopes of that element. Isotopes: Atoms that have the same number of protons, but a different number of neutrons. Isotopes all have very similar physical properties as they all have the same bonding. Isotopes all have very similar chemical properties as they all have the same electronic configuration. The Relative Atomic Mass (Ar) of an element is the average mass of these different isotopes. It takes into account both their mass and their relative abundance (how common they are). Relative Atomic Mass (Ar) = ∑ (isotope mass x % abundance) 100 Most elements have a 3 or 4 different isotopes. To calculate relative atomic mass: Multiply the mass of each isotope by its abundance Add up these values Divide by 100 Check your answer. It should be between the lowest and highest isotope mass you used. It is an average after all! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE TIME OF FLIGHT MASS SPECTROMETER 1 2 3 4 5 e- e- + + e- e- e- 1 Vaporisation The sample is “vaporised” so all so all atoms / molecules are completely separated. These are injected into the spectrometer. All done in a vacuum to prevent interference from other substances. 2 Ionisation The atoms/molecules are “ionised” to give a 1+ charge. This is so that they can be accelerated by the charged plates and create a current in the detector. Two ways this can be achieved: A) Electron Impact: Electrons are fired at the sample by an electron gun. These remove an electron from the atoms / molecules to create a 1+ ion. X(g) + e- → X+(g) + 2e- B) Electrospray Ionisation: Sample is dissolve din a polar solvent and is passed through a needle with a high voltage across it. This causes them to GAIN a proton (H+) to create a 1+ ion. This causes the m/z value to increase by 1! X(g) + H+ → XH+(g) 3 Acceleration The positive ions are accelerated by a negatively charged plate. All ions are given the same kinetic energy. This means that lighter ions travel faster, heavier ions travel slower! 4 Ion Drift The flight tube is very long, so causes the ions to drift apart. The lightest ions get to the detector first, then second lightest etc, and the heaviest ions get there last. Hence they are separated by mass so we can measure them. 5 Detection The 1+ ions hit the negatively charged plate. They attract electrons towards them to create a current. The current is measured so that we are able to quantify two things: A) The time it took to reach the detector (and therefore calculate the m/z value) B) The abundance (how many there were). The more there are, the greater the current. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE MASS SPECTRUM e.g. silicon 100 92.2% Si has 3 stable isotopes: 28Si, 29Si, 30Si Everything that comes out of Relative a mass spectrometer has a 50 Abundance 1+ charge. (%) So, m/z = isotopic mass of 4.7% 3.1% the isotope since 0 m/z = mass/charge. 20 21 22 23 24 25 26 27 28 29 30 31 32 33 m/z (Mass:charge ratio) CALCULATING RELATIVE ATOMIC MASS (Ar) Ar Si = (28 x 92.2) + (29 x 4.7) + (30 x 3.1) = 28.1 g.mol-1 100 This calculation finds the weighted average for a silicon atom. HINTS | TIPS | HACKS We divide by 100 as, most of the time, abundance is given as a percentage! Sometimes abundance is given as a relative value and NOT a percentage. In which case you replace the “100” in the equation for the sum total of the abundances. How To Tackle Ar Isotopic masses are always whole numbers (integers). Calculation Questions Ar values tend to give to 1d.p. In exam questions, you may be given the Ar value and asked to find the isotopic mass of missing isotopes. Be prepared to rearrange the equation to do so! When asked “which species” causes a peak, ALWAYS be sure to put a “+” sign on it! e.g. Which species causes a peak at m/z 28? …. answer 28Si+ AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE DIATOMIC ELEMENTS e.g. chlorine Diatomic elements such as chlorine show both 9 individual Cl+ ions: 3 35Cl+ & 37Cl+ 6 Relative and molecular Cl2+ ions that Abundance contain different 1 combinations of Cl isotopes: 1 (35Cl-35Cl)+ m/z 70 (35Cl-37Cl)+ m/z 72 10 20 30 40 50 60 70 80 90 (37Cl-37Cl)+ m/z 74 m/z (Mass:charge ratio) THE 9:6:1 RATIO The 35Cl isotope is approximately 3 x more abundant than the 37Cl isotope. i.e. they exist in a 3:1 ratio. Therefore, the chances of a 35Cl being in a Cl2 molecule is 3 in 4. the chances of a 37Cl being in a Cl molecule is 1 in 4. 2 The Statistics The chances of creating a 35Cl-35Cl molecule = 3 x 3 = 9 The chances of creating a 35Cl-37Cl or 37Cl-35Cl molecule = (3 x 1) + (1 x 3) = 6 The chances of creating a 37Cl-37Cl molecule = 1 x 1 = 1 Therefore the ratio of the peaks is 9:6:1 Watch out for this in your exam! Also…. bromine 79Br -81Br 2 79Br 81Br Relative 11 1 1 Abundance 79Br -79Br 81Br -81Br 80 90 100 110 120 130 140 150 160 170 m/z (Mass:charge ratio) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE VELOCITY CALCULATIONS When accelerated, ALL ions gain the SAME KINETIC ENERGY. This is key here. However, as they have different masses, their VELOCITY will be different. Where mass is low, velocity is high and vice versa. K.E. = ½ m.v2 K.E. = Kinetic Energy m = mass of a single ion (kg) v = velocity (m.s-1) Also…. v=d/t v = velocity (m.s-1) d = distance (length of flight tube) (m) t = time taken to reach detector (s) You may be asked to calculate any one of the following: Kinetic Energy, Mass of isotope (via m), Velocity, Length of flight tube (Distance) or Time! HINTS | TIPS | HACKS To find velocity… v2 = 2K.E. … then square root your answer m To find mass… m = 2K.E. v2 d If asked to find distance (length of flight tube) or time taken….. v t How To Tackle Velocity Calculations AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE ELECTRON CONFIGURATION Electrons are organised into ENERGY LEVELS (Shells). Within these energy levels they exist in ORBITALS. Each orbital can hold a maximum of 2 electrons (an electron pair) which have opposite spins. “s” orbital “p” orbital Exist in sets of three Spherical in shape Figure of 8 in shape (x, y, z) ELECTRONIC STRUCTURE You must be able to write the electronic structures of atoms and ions up to and including Kr. 36Kr 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 LEARN THIS SEQUENCE! LARGE NUMBERS = ENERGY LEVEL LETTERS = ORBITAL TYPE SMALL NUMBERS = NUMBER OF ELECTRONS IN THOSE ORBITALS ELECTRONS IN BOXES These show the pairings of the electrons in each orbital. The arrows represent the fact that pairs of electrons have opposite spins. 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 ↼↼ ↼ ↼ ↼↼ ↼ ↼ ↼↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼ ↼↼ Important! Hund’s Rule: When filling “p” and “d” orbital groups, the electrons occupy the orbitals individually first, then pair up! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE HINTS | TIPS | HACKS The higher the energy level, the greater the amount of energy the electrons in them possess. Electrons fill orbitals from the lowest energy level to the highest. The orbital that houses the LAST electron tells us the “block” of the periodic table that the element is in. The total number of electrons in the highest energy level tells us which group the element is in. The 4s orbital is always filled before the 3d orbital. Exceptions: Chromium (Cr) is 1s2 2s2 2p6 3s2 3p6 4s1 3d5 and NOT 1s2 2s2 2p6 3s2 3p6 4s2 3d4 Copper (Cu) is 1s2 2s2 2p6 3s2 3p6 4s1 3d10 and NOT 1s2 2s2 2p6 3s2 3p6 4s2 3d9 You do not need to explain why, just remember them! How To Write Electronic Structures for Atoms & Ions AQA www.chemistrycoach.co.uk © scidekick ltd 2024