atomic structure

• atom

  -basic unit of matter

  -mostly made up of empty space around small, dense nucleus that contains protons and neutrons

  -overall a +ve charge

  - -ve charged electrons are found in orbitals in empty space around nucleus
• subatomic particles

  -Particles found within the atom, mainly protons, neutrons, and electrons

  -proton - +1 1

  -neutron - 0 1

  -electron - -1 1/1836
• atomic number

  the number of protons in the nucleus of an atom

  -symbol Z
• mass number

  the sum of the number of neutrons and protons in an atomic nucleus

  -symbol A

  -number of neutrons = mass no - atomic no
• isotopes

  Atoms with the same number of protons but different numbers of neutrons
• isotopes - chemical properties

  -isotopes of same element display same chemical characteristics

  -due to same number of electrons in their outer shell
• isotopes - physical properties

  -only difference is number of neutrons - this will only add mass to the atom

  -only different property will be their mass and density
• Relative atomic mass

  The mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12.
• time of flight mass spec

  -It is the most useful instrument for accurate determination of the relative atomic mass of an element
• (TOF) Stage 1: electron impact ionisation

  -used for lower molecular mass

  -sample is vaporised and then bombarded with high energy electrons

  -electrons are fired from and electron gun

  -an electron is knocked off forming 1+ ion

  -then attracted towards -ve charged plate which accelerates them through mass spec

  -ion can be broken down or fragmented
• (TOF) Stage 1: electrospray ionisation

  -used for higher molecular mass

  -fragmentation is unlikely to happen

  -sample is dissolved in a volatile solvent

  -solvent injected into mass spec by hypodermic needle - this produces a fine mist

  -needle attached to power supply so the particles are ionised by gaining a proton from the solvent

  -solvent evaporates and ions are attracted towards -ve charged plate

  -accelerated through mass spec
• (TOF) Stage 2: Acceleration

  -ions produced are accelerated using an electric field

  -all accelerated to have the same kinetic energy

  -their velocity will depend on their mass
• (TOF) Stage 3: Ion drift

  -ions will pass through a hole in the charged plate and move into the flight tube
• (TOF) Stage 4: Detection

  -once ions pass through mass spec they will hit a -ve charged detector plate

  -gain an electron when they hit the plate

  -gaining of electron discharges the ion and causes current to be produced

  -SIZE OF CURRENT IS PROPORTIONAL TO THE ABUNDANCE

  -detector plate is connected to a computer which produces mass spec
• electron shells

  -e- arranged in principal energy levels or principal quantum shells

  -quantum shells are split into subshells - s,p,d
• S subshell

  -has one orbital

  -total of 2 electrons

  -spherical shape
• P subshell

  -has 3 orbitals

  -total of 6 electrons

  -dumbbell shape

  -oriented perpendicular to eachother
• D subshell

  • 5 orbitals
  • 10 electrons
• F subshell

  • 7 orbitals
  • 14 electrons
• ground state

  -lowest energy state of an atom

  -most stable electronic configuration
• s-block elements

  -groups 1 and 2

  -have their valence electrons in an s orbital
• p-block elements

  -groups 13-18

  -have their valence electrons in a p orbital
• d-block elements

  -groups 3-12, transition metals

  -have their valence electrons in a d orbital
• f-block elements

  -lanthanides and actinides

  -have valence electrons in the f orbital
• Argon electron configuration

  1s2 2s2 2p6 3s2 3p6

  -accountable for 18 electrons
• electron configuration exceptions

  -chromium is [Ar] 3d5 4s1

  -copper is [Ar] 3d10 4s1

  -because they want to be energetically stable by completely or half filling the d orbital
• Ionisation energy

  the amount of energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous ions under standard conditions 298K and 101kPa

  -eg. Ca(g) ---> Ca+(g) + e-
• trends in first ionization energy

  -increases across a period, decreases down a group

  -size of first IE is affected by: size of nuclear charge, distance of outer electrons from the nucleus, shielding effect of inner electrons, spin-pair repulsion
• Ionisation energy across a period

  -IE increases

  -nuclear charge increases

  -causes atomic radius to decrease - distance between the nucleus and outer electrons decrease

  -shielding by inner shell electrons remain constant

  -it becomes harder to remove and electron as you move across a period
• (1stIE) from one period to the next

  -large decrease in IE between last element in one period and 1st element in the next

  -because increased distance between nucleus and outer electrons as a new shell is added

  -because increased shielding by inner electrons due to added shell

  -these factors outweigh increased nuclear charge
• (1stIE) down a group

  -decreases down a group

  -because number of protons increased so nuclear charge increases

  -But, the atomic radius of the atoms increases as you are adding more shells of electrons, making the atoms bigger

  -So, the distance between the nucleus and outer electron increases as you descend the group

  -The shielding by inner shell electrons increases as there are more shells of electrons

  -These factors outweigh the increased nuclear charge, meaning it becomes easier to remove the outer electron as you descend a group

  -ionisation energy decreases
• Second ionisation energy

  -the energy required to remove the second mole of electrons from each +1 ion in a mole of gaseous +1 ions, to form one mole of +2 ions

  -eg. Be+(g) ---> Be2+(g) + e-
• successive ionisation energies of an element

  -increases because once outer electron of atom is removed it forms a +ve ion

  -+ve ion more difficult to remove than neutral atom

  -as more e- are removed, the attractive forces increase due to decreasing shielding
• John Dalton (1803)

  Atoms are spheres and each element is made from a different sphere
• J.J. Thomson (1897)

  discovered the electron

  -atom wasn't solid, made from other particles

  -plum pudding model
• Ernest Rutherford 1909

  Discovered the nucleus

  Nucleus was very small and +ve charged

  Atom was empty space - gold leaf experiment

  Shot positive alpha particles at gold leaf, most went through and some deflected

  Atom was empty space made up negative cloud
• Neils Bohr (1913)

  Discovered problem with Rutherford model

  Cloud could collapse so said they were in fixed shells