periodicity and classification

concerning Na - Ar

how do we classify elements? how is this determined?

• according to their block - s, p, d or f

• block is determined by proton no. (or last orbital in e^- configuration)

how does atomic radius change across a period? why?

• decreases

• as no. of protons increases, +ve charge of nucleus increases

• this means outer e^- are pulled closer, making radius smaller

• w/ same shielding (as even though we are adding extra e^-, they are added to the same shell, so they don’t increase the size or shield the +ve charge of the nucleus)

  

what is first ionisation energy?

minimum energy needed to remove 1 mole of e^- from 1 mole of atoms in the gaseous state

what is the general trend of ionisation energy across a period? why?

• generally increases - takes more energy to remove an e^-

• as no. of protons increases, so +ve charge of nucleus increases

• so pulling force on outer e^- increases

  

why may first IE sometimes decrease across a period?

an e^- may be being removed from a different subshell, so requires less energy to be removed

how does the mpt change across period 3 from Na → Al? why?

• there is a general increase from Na → Al as metallic bonding gets stronger, so more energy is required to break the metallic bonds

• this is because we go from Na⁺ to Mg²⁺ to Al³⁺ so there is a smaller radius and more delocalised e^-

• so there is a stronger attraction between the +ve ions and the delocalised e^-

how does the mpt change across period 3 when we get to Si? why?

• increase - has highest mpt

• as Si is a macromolecular compound w/ very strong covalent bonds

• which require a large amount of energy to break

how does the mpt change across period 3 from P → Ar? why?

• P₄, S₈ and Cl₂ are all molecular substances, so their mpt depends on the strength of the IMF - in this case VDW - between the molecules

• the more atoms, the stronger the IMF, so S₈ > P₄ > Cl₂

• Ar is monatomic so has a very low melting point