100%(1)1 out of 1 people found this document helpful
This preview shows page 1 - 5 out of 14 pages.
Edexcel Chemistry A-Level Topic 1: Atomic Structure and the Periodic Table Detailed Notes
Fundamental Particles The model for atomic structure has evolved over time as knowledge and scientific understanding changes.The current, accepted model of the atom consists of a small, dense central nucleussurrounded by orbiting electronsin electron shells. This was discovered in the Rutherford scattering experiment in 1911. The nucleus consists of protons and neutronsgiving it an overall positivecharge. It contains almost the entire massof the atom. In a neutral atom, the number of electrons is equal to the number of protons due to the relative charges. Particle Proton Neutron Electron Relative Charge +1 0 -1 Relative Mass 1 1 1/1840 The maximum number of orbiting electrons that can be held by any single shell, depends on the number of the shell. This can be calculated using 2n2wherenis the number of the shell. Example: Electrons in shell 2 = 2(22) = 8 electronsEach electron shell must fillbefore the next one can hold any electrons. Atomic Number and Mass Number Mass number is represented using Aand can be calculated as the sum of protons and neutronsin an atom. Atomic number is represented using Zand is equal to the number of protonsin an atom, hence it can be referred to as proton number.
Using these numbers, the quantity of each fundamental particle in an atom can be calculated.Example: Atomic number = 7 Mass number = 14 Proton number = 7 Neutron number = 14 - 7 = 7 IsotopesIsotopes are atoms of the same element with the same atomic number, but with a different number of neutrons, resulting in a different mass number. Neutral atoms of isotopes will react chemically in the same wayas their proton number and electron configuration is the same. The sharing and transfer of electrons is unaffected. However, the different mass number means they have different physical properties. Example:Hydrogen = 1 proton and 0 neutrons Deuterium = 1 proton and 1 neutron Tritium = 1 proton and 2 neutrons Relative MassesRelative atomic mass (Ar)is defined as: The mean mass of an atom of an element, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope. This takes the relative abundancesof the different isotopesof an element into account.
Relative isotopic massis defined as: The isotopic mass of an isotope relative to one twelfth of the mean mass of an atom of the carbon-12 isotope. Relative molecular mass (Mr)is defined as: The mean mass of a molecule of a compound, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope. It can be calculated for a molecule by adding together the separate Ar values of the component elements.Example: Relative formula massis similar to Mr but is used for compounds with giant structures.