Chapter 2 - Atoms Ions and Compounds

Chapter 2 - Atoms Ions and Compounds

1. 2.1 Atomic structure and isotopes

   • Protons, Neutrons and electrons
     • Nuclear atom
       • Atom consists of a nucleus made of protons and neutrons
       • There are electrons that are arranged around the nucleus in shells
     • Properties
       • Mass
         • We have to work in relative mass as the mass is so small
         • Proton mass = neutron mass
         • Electron mass = negligible
       • Charge
         • Proton = positive
         • Electron = negative
           • -1.6x10^-19
             • But just use -1
         • Neutron = neutral
       • Building the atom
         • Nearly all of an atom's mass is in the nucleus
         • Atoms contain the same number of protons and electrons
           • Positive charge is cancelled out by negative charge
             • Overall charge of an atom = 0
     • Atomic Number
       • Every atom of the same element contains the same number of protons
       • Different elements contain atoms that have different numbers of protons
       • Number of protons = atomic number
         • No. protons = no. electrons
     • Isotopes
       • Isotopes are atoms of the same element with different numbers of neutrons and different masses
       • Representing isotopes
         • Mass number (A) = no. protons + no. neutrons
         • Atomic number (Z) = no. protons
       • Different isotopes may have small physical properties of the product of a reaction
     • Atomic structure of ions
       • An ion is a charged atom
         • No. electrons different from no. protons
       • Positive ions = cations they have fewer electrons tha protons
       • Negative ions = anions they have more electrons than protons

2. 2.2 Relative mass

   • Carbon-12
     • The mass of a carbon-12 isotope is defined as exactly 12 atomic mass units
       • The standard mass for atomic mass is 1u the mass of 1/12th of an atom of carbon-12
   • Relative isotopic mass
     • Relative isotopic mass is the mass of an isotope relative to 1/12th of the mass of an atom of carbon-12
     • The mass spectrometer can also record the accurate m/z ratio for each isotope so that accurate values of relative isotopic mass can be measured
   • Relative atomic mass
     • Most elements contain a mixture of isotopes
     • Relative atomic mass is the weighted mean mass of an atom of an element relative to 1/12th of the mass of an atom of carbon-12
     • The mass spectrometer:
       • 1. a sample is placed in the mass spectrometer
         • 2. The sample is vaporised and then ionised to form positive ions
           • 3. Ions are accelerated, heavier ions move more slowly and are more difficult to deflect than lighter ions so the ions of each isotope are separated
             • 4. The ions are detected on a mass spectrum as a mass-to-charge ratio (m/z) each ion reaching the detector adds to the signal, so the greater the abundance the larger the signal
       • Relative atomic mass = (contribution of one isotope*atomic mass) + (contribution of another isotope*atomic mass) all divided by 100

3. Work with masses to one decimal place

4. 2.3 Formulae and equations

   • Atoms of metals on the left of periodic table lose electrons to form cations
     • Atoms of non-metals on the right of the periodic table gain electrons to form anions
   • Binary compounds
     • Contains only two elements
     • To name: use the name of the first elemet but change the ending of the second elements name to -ide
   • Polyatomic ions an ion may contain atoms of more than one element bonded together
   • In a correct formula the overall charge is zero
   • Writing compounds
     • Diatomic molecules
       • Group 7
       • H2, N2, O2
       • P4, S8
         • But in equations leave S8 as just S