Chemistry topic 2

Fundamental Particles

  • Atoms have a central nucleus (protons and neutrons) surrounded by orbiting electrons in shells.
  • Protons have a +1 charge, neutrons have 0 charge, and electrons have a -1 charge.
  • Relative mass: proton = 1, neutron = 1, electron = 1/1840.
  • Maximum electrons per shell: 2n^2 (n = shell number).
  • Atomic number (Z) = number of protons.
  • Mass number (A) = sum of protons and neutrons.

Isotopes

  • Isotopes: atoms of the same element with the same atomic number but different neutron numbers.
  • Same chemical behavior due to identical proton and electron configurations.
  • Different physical properties due to varying mass numbers.

Relative Masses

  • Relative atomic mass (Ar): Mean mass of an element's atom relative to 1/12 of carbon-12.
  • Relative isotopic mass: Isotopic mass relative to 1/12 of carbon-12.
  • Relative molecular mass (Mr): Mean mass of a compound's molecule relative to 1/12 of carbon-12, calculated by adding Ar values of component elements.
  • Relative formula mass: Similar to Mr, used for giant structures.

Ions and Mass Spectrometry

  • Ions are formed when atoms gain or lose electrons, resulting in an overall charge.
  • Mass spectrometry identifies isotopes and determines relative atomic mass.

Time of Flight (TOF) Mass Spectrometry

  1. Ionization: Vaporized sample ionized, forming +1 ions.
  2. Acceleration: Ions accelerated towards a negative plate.
  3. Ion drift: Magnetic field deflects ions.
  4. Detection: Ions hit the detector, producing a current proportional to abundance.
  5. Analysis: Spectra generated, displaying isotope abundance.
  • Ions with a 2+ charge have half the expected m/z ratio.
  • Ar = \frac{\sum(m/z \times abundance)}{\sum abundance}.

Predicting Mass Spectra

  • Spectra can be predicted from isotope abundances.
  • Example: Chlorine spectra show characteristic patterns due to the presence of ^{35}Cl and ^{37}Cl isotopes.

Ionisation Energy

  • Ionization energy: Minimum energy to remove one mole of electrons from one mole of gaseous atoms (kJmol^{-1}).
  • Na(g) \rightarrow Na^+(g) + e^-
  • Successive ionization energies increase due to increasing electrostatic attraction.
  • Factors influencing ionization energy: number of protons, electron shielding, and subshell.
  • Ionization energy increases across a period and decreases down a group.
  • Sudden large increases in successive ionization energies indicate a change in energy level.

Electron Orbitals

  • Electrons are held in orbitals (s, p, d, f), each holding up to 2 electrons with opposite spins.

Electron Configurations

  1. Fill the lowest energy orbital first.
  2. Electrons with the same spin fill orbitals before pairing.
  3. No orbital holds more than 2 electrons.
  • Exceptions: Half-full or completely full d sublevels are more stable.
  • Chromium: 1s^22s^22p^63s^23p^63d^54s^1
  • Copper: 1s^22s^22p^63s^23p^63d^{10}4s^1

Periodicity

  • Periodicity: Repeating patterns of physical/chemical properties in the Periodic Table.
  • Elements in the same period have the same number of electron shells.
  • Elements in the same group have the same number of outer electrons.
  • Blocks: s-block (groups 1, 2), p-block (groups 3-0), d-block (transition metals), f-block (radioactive elements).
  • Atomic radius decreases across a period and increases down a group.
  • Ionization energy increases across a period and decreases down a group.

Physical Properties of Period 2

  • Melting points peak towards the middle due to bond strength and structure.
  • Li and Be: Metallic bonding, MP increases (Be > Li).
  • B and C: Giant covalent lattices, high melting points.
  • N, O, F, Ne: Simple covalent molecules, low melting points due to weak van der Waals forces.
  • Ionization energies generally increase across the period (exceptions: B and O).

Physical Properties of Period 3

  • Melting points: Na, Mg, Al (metallic, MP increases), Si (macromolecular, very high MP), P, S, Cl (simple covalent, low MP), Ar (noble gas, very low MP).
  • Ionization energies generally increase across the period (exceptions: Al and S).