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$^2$2s$^2$2p$^6$3s$^2$3p$^6$3d$^5$4s$^1$
• Copper: 1s$^2$2s$^2$2p$^6$3s$^2$3p$^6$3d$^{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).