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    2016年A-Level化学-基本定义Basic Definitions

    国际学校网    http://www.mjhb.net    2016年04月08日
    Revision: Basic Definitions
      The atomic number of an element is the number of protons in the nucleus of its atom and it defines the element and its place on the periodic table.
      The mass number is the number of protons and the nucleus of its nucleus of its atom plus the number of neutrons relative to 1/12th the mass of carbon-12.
      Isotopes are atoms with the same atomic number but different mass number. They have same number of protons but a different number of neutrons.
      The relative atomic massis the mass of a single isotope of an atom relative of carbon-12.
      The relative isotopic mass is the mass of a single isotope of an atom relative to carbon-12 taking into account the abundance of the isotopes of the atom.
      The relative molecular mass is the sum of all the relative atomic masses of the elements, which make up the compound's formula.
      The mass spectrometer
      Work at low pressures, avoid inaccuracy, removes other particles that may collide with the particles of the sample.
      Ionization: Achieved by removing electrons from the gaseous atoms to form positively charged ions. Electrons are removed by bombarding them an electron X- +2e-àgun. Theyknock off electrons at 90 degrees. X + e-
      Acceleration: Positive ions accelerate towards negatively charged plates. There is a small slit, which the ions travel through at constant velocity. The electrons that hit the plates gain electrons and become atoms.
      Deflection: The ions travel towards a bend in the tube with an electromagnet wrapped around the bend. The degree of deflection depends upon the m/e ratio.
      Detection: Only ions with a certain m/e are focused on the detector at any given time so only certain m/e ratios can reach the detector. The detector is connected to an ammeter, which records the flow of electrons.
      Electronic configuration
      Three sub-levels:'s' –contains 1 orbital 'p' –contains 3 orbital 'd' –contains 5 orbital
      1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 4d10
      1st ionization energy
      E+ (g) =E (g) – e-
      1mol of gaseous atoms
      1st electron affinity
      E-(g)=E(g) + e-
      1mol gaseous atoms
      First ionization energy definition: Energy change required to remove one electron from one mole of gaseous atoms to form positively charged ions.
      First electron affinity definition:
      Energy change required to remove one electron to one mole of gaseous atoms to form negatively charged ions.
      There is a general increase in the ionization energy going across a period. This is because of the atoms increased nuclear charge without an increase in the inner shielding electrons.
      There are slight decreases after group 5 because the repulsion of the two electrons in the p orbital makes it easier to remove one of them.  
      There is a decrease in ionization energy going down the group. This is caused by the outer electron being further from the nucleus. This is caused by the outer electron being further from the nucleus. The extra nuclear charge is balanced by the same extra number of inner shielding electrons.
      Ionic Bonding
      Definition: An ionic bond is the electrostatic attraction that occurs between an atom that has lost electrons and one that has gained.
      Covalent Bonding
      Definition: A covalent bond occurs when two atoms share a pair of electrons. It is formed by the overlap of atomic orbital of that, electron density increases between the bonded atoms. Dative covalent bonding is when both electrons in a bond are from the same atom.
      Polarity is the distortion of the electron cloud of one atom by another.
      Cations cause polarization- they cause the distortion.
      Anions are polarized- they are polarized.
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