Atomic structure and isotopes
Details of the three Sub-atomic (fundamental) Particles Particle Position Relative Mass Relative Charge Proton Nucleus 1 +1 Neutron Nucleus 1 0 Electron Orbitals 1/1800 -1 An atom of Lithium (Li) can be represented as follows: 7 3 Li Atomic Number Atomic Symbol Mass Number The atomic number, Z, is the number of protons in the nucleus. The mass number ,A, is the total number of protons and neutrons in the atom. Number of neutrons = A – Z Isotopes Isotopes are atoms of same element with the same number of protons, but different numbers of neutrons. Isotopes have similar chemical properties because they have the same electronic structure. They may have slightly varying physical properties because they have different masses. There are various
models for atomic
structure
models for atomic
structure
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2.1.1 Atomic structure and isotopes Learning outcomes Additional guidance Learners should be able to demonstrate and apply their knowledge and understanding of: Atomic structure and isotopes (a) isotopes as atoms of the same element with different numbers of neutrons and different masses (b) atomic structure in terms of the numbers of protons, neutrons and electrons for atoms and ions, given the atomic number, mass number and any ionic charge HSW1 Different models for atomic structure can be used to explain different phenomena, e.g. the Bohr model explains periodic properties. HSW7 The changing accepted models of atomic structure over time. The use of evidence to accept or reject particular models.
Relative mass
DEFINITION: Relative Isotopic mass is the mass of one isotope compared to one twelfth of the mass of one atom of carbon-12 The relative atomic mass quoted on the periodic table is a weighted average of all the isotopes R.A.M = (isotopic mass x % abundance) 100 For above example of Mg R.A.M = [(78.7 x 24) + (10.13 x 25) + (11.17 x 26)] /100 = 24.3 R.A.M = (isotopic mass x relative abundance) total relative abundance If relative abundance is used instead of percentage abundance use this equation Calculating the Relative Atomic Mass of an Element Percentage Abundance 78.7 10.13 11.17 Relative Isotopic mass 24.00 25.00 26.00 Isotope Mg24 Mg25 Mg26 DEFINITION: Relative molecular mass is the average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12 N Goalby chemrevise.org 2 Example: Calculate the relative atomic mass of Tellurium from the following abundance data: 124-Te relative abundance 2; 126-Te relative abundance 4; 128-Te relative abundance 7; 130-Te relative abundance 6 R.A.M = [(124×2) + (126×4) + (128×7) + (130×6)] 19 = 127.8 Example: Copper has two isotopes 63-Cu and 65-Cu. The relative atomic mass of copper is 63.5. Calculate the percentage abundances of these two isotopes. 63.55 = yx63 + (1-y)x65 63.55 = 63y +65 -65y 63.55 = 65 -2y 2y = 1.45 y = 0.725 %abundance 63-Cu =72.5% %abundance 65-Cu = 27.5% Cl has two isotopes Cl35 (75%) and Cl37(25%) Br has two isotopes Br79 (50%) and Br81(50%) These lead to the following spectra caused by the diatomic molecules 70 72 74 m/z relative abundance Cl35Cl35 +Cl35Cl37 +Cl37Cl37 + 158 160 162 m/z relative abundance Br79Br79 + Br79Br81 + Br81Br79 + Br81Br81 + Mass spectra for Cl2 and Br2 The 160 peak has double the abundance of the other two peaks because there is double the probability of 160 Br79 -Br81 + as can be Br79-Br81 and Br81-79. DEFINITION: Relative atomic mass is the average mass of one atom compared to one twelfth of the mass of one atom of carbon-12
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Relative mass (c) explanation of the terms relative isotopic mass (mass compared with 1/12th mass of carbon-12) and relative atomic mass (weighted mean mass compared with 1/12th mass of carbon-12), based on the mass of a 12C atom, the standard for atomic masses Definitions required. (d) use of mass spectrometry in: (i) the determination of relative isotopic masses and relative abundances of the isotope, (ii) calculation of the relative atomic mass of an element from the relative abundances of its isotopes M0.2, M1.2, M3.1 Knowledge of the mass spectrometer not required. Limited to ions with single charges. (e) use of the terms relative molecular mass, Mr , and relative formula mass and their calculation from relative atomic masses. For simple molecules, the term relative molecular mass will be used. For compounds with giant structures, the term relative formula mass will be used. Definitions of relative molecular mass and relative formula mass will not be required.
Credits: Neil Goalby