Atomic structure

• Subatomic particles


  Particles within the atom
• Atom


  • Consists of protons and neutrons in the nucleus and electrons in the shells around nucleus
  • As number of protons increases, number of neutrons increases relatively faster
  • Neutrons act as a diluted- reducing repulsive forces between positive protons
• Protons


  Mass of 1, charge of +1, located in nucleus, symbol p+
• Neutrons


  Mass of 1, charge of 0, located in nucleus, symbol n0
• Electrons


  Mass of 1/1840, charge of -1, located in shells, symbol e-
• Atomic number


  Number of protons in the nucleus of an atom
• The atom is electrically neutral- number of electrons equals the number of protons
• Mass number


  Total number of protons and neutrons in the nucleus of an atom- always a whole number
• Isotopes


  Atoms which have the same atomic number but different mass number- due to extra neutrons in the nucleus
• Relative isotopic mass


  The mass of an atom of an isotope of an element relative to one twelfth of the mass of an atom of carbon 12
• Relative atomic mass


  Average weighted mean mass of an atom of an element relative to one twelfth of the mass of an atom of carbon 12
• Relative formula mass


  Average weighted mean mass of a formula unit relative to one twelfth of the mass of an atom of carbon 12- ionic compounds
• Relative molecular mass


  Average weighted mean mass of a molecule relative to twelfth of the mass of an atom carbon 12
• Mass spectrometer


  • Atoms too small to be weighed individually so the masses of atoms may be compared
  • Machine allows us to work out the relative atomic mass and isotopic abundances for elements
  • It makes it possible to determine the structure and relative molecular mass of compounds by studying masses of ions produced by a molecule and ways in which molecule fragments in machine
  • Sorts out charged particles of different masses as they pass through a magnetic field
  • Particles of smaller mass are deflected more than those of a greater mass
• Orbital electrons


  Electrons moving rapidly around the nucleus and traces out a 3D volume of space
• Orbital


  A region in an atom that can hold up to 2 electrons with opposite spin
• Sub-shell


  Groups of orbitals within a shell
• Shells


  • Distinguished by numbers and letters
  • Given a number known as principle quantum shells
  • Each shell contains n2 orbitals
• Orbitals


  • S orbital is spherical
  • P orbital is figure 8 or dumbbell
  • There exists a number of different types of orbitals that differ in shape
  • Maximum number of electrons which can be fitted into any one orbital is 2
  • The 2e- in a filled orbital make it a spin pair with each spinning in opposite direction
  • Orbitals occur in series of clusters
  • Distance from nucleus gradually increases
  • Number of orbitals that can be fitted into any one shell follows a fixed pattern
• Ordering of filling sub-shells

  1. Always put electrons into lowest available energy level

  2. 4s fills first and empties first

  3. 4s orbital is lower in energy than 3d orbitals

  4. When writing structure 3D is written before 4s
• Electronic configurations


  • Electrons fitted successfully into orbitals of lowest available energy
  • Atom is said to be in its ground state when all of its electrons occupy orbitals of lowest available energy
  • To deduce electronic configuration of an ion, do configuration of atom first then add/remove electrons to achieve configuration of ion
• Electronic configuration of chromium and copper are anomalous due to increase in stability that atom from half filled d sub-shell
• Blocks in periodic table

  • Group 1&2- s block- an element which has an atom with highest energy/outer electron in an s-sub-shell- can only have 2 electrons and is 2 units wide
  • Group 3-0- p block- an element which has an atom with the highest energy/outer electron in a p sub-shell- can hold 6 electrons and is 6 units wide
  • Middle bit- d block- an element which has an atom with highest energy/outermost electron in a d sub-shell- can hold 10 electrons- 10 units wide- transition metals
  • Outermost electron in f-block is in an f orbital- can hold 14 electrons- 14 units wide- inner transition elements- mostly radioactive elements- 1 row is lanthanides and other row is actinides
• Ionisation energy


  • Only forming positive ions
  • First ionisation energy- energy required to convert one mole of gaseous atoms into gaseous ions with a single positive charge- KJmol-1
  • Second ionisation energy- energy required to convert one mole of gaseous ions with a single positive charge into gaseous ions with a double positive charge
• Across the period


  • Electrons removed in same shell at same distance from nucleus
  • More protons in nucleus
  • Electrons attracted and pulled towards nucleus with an increased nuclear charge & radius of atoms decreased
  • Attraction of outermost electron increases
  • Ionisation energy increases
  • Electrons held more tightly
  • Shielding doesn't change
• Down the group


  • Atoms become larger and outermost electron is further from nucleus and less attracted to it
  • Number of shells of electrons between outer shell and nucleus increases their screening effect is greater
  • Electrons held less tightly
  • Distance and screening factors contribute to a decreased attraction of outer electrons to nucleus down a group
  • Decreased ionisation energy
• Evidence for existence of electrons in principle quantum shells comes from study of ionisation energy of atoms, compare first, second and successive ionisation energies of a particular element with ionisation number, log of ionisation energies plotted against ionisation number- see a sudden jump each time an electron is removed from a full shell- values increase as we move from electron in lowest energy shell- more energy required to remove an electro from a low energy level than from a higher energy level
• When removing successive electrons same shall the protons to electrons ratio increases- effective nuclear charge increases- remaining electrons more strongly attracted to nucleus and more difficult to remove
• Evidence for existence of sub-shells from first ionisation energies- graph consists of peaks (inert gas) and troughs (alkali metal)-drop in first ionisation energy immediately after each inert gas is explained as arising from alkali metal atom having its valence electron in a new shell- more easily removed
• General increase in first ionisation energy as additional electrons going into same main shell- nuclear charge is increased by 1 after each successive element
• Net effect of this increase nuclear charge is to draw electrons closer to nucleus making it slightly more difficult to remove