Periodicity + trends Flashcards
(13 cards)
Q: Why does atomic radius decrease across Period 3?
-Nuclear charge increases.
-Electrons added to the same shell (no extra shielding).
-Stronger attraction pulls electrons closer to the nucleus.
Q: Why does first ionisation energy generally increase across Period 3?
- Nuclear charge increases.
- Same shielding.
- Attraction between nucleus and outer electrons increases.
- More energy needed to remove an electron.
Q: Why is the first ionisation energy of aluminium lower than that of magnesium?
1) Al’s outer electron is in a 3p orbital (higher energy)
2) Easier to remove than Mg’s 3s electron
3) so ionisation energy is lower.
Q: Why is the first ionisation energy of sulfur lower than that of phosphorus?
1) In sulfur, 3p electrons start to pair up.
2) Electron-electron repulsion makes it easier to remove one.
3) So lower ionisation energy than phosphorus.
Q: Why does silicon have a higher melting point than phosphorus?
-Silicon: giant covalent structure with strong covalent bonds.
-Phosphorus: simple molecular structure with weak van der Waals forces.
-More energy needed to break Si’s covalent bonds.
Q: Explain the trend in melting points from sodium to aluminium.
- All are metals with metallic bonding.
- From Na to Al, charge on ion increases and more delocalised electrons.
- Stronger metallic bonding = higher melting points.
Q: Why does argon have the lowest melting point in Period 3?
- Argon is monoatomic (exists as individual atoms).
- Only weak van der Waals forces.
- Very little energy
Q: Order these Period 3 elements by melting point: Si, Cl₂, Na, P₄. Explain the order.
- Si: giant covalent = very strong bonds.
- Na: metallic bonding (moderately strong).
- P₄ and Cl₂: simple molecules with van der Waals (P₄ larger → stronger forces).
- Cl₂ smallest → weakest forces → lowest MP.
Q: Why does sodium have a lower melting point than aluminium?
- Al has a higher charge (+3) and more delocalised electrons.
- Stronger metallic bonding than Na (+1).
- More energy needed to break bonds in Al.
Q: Explain why the atomic radius of sulfur is smaller than that of phosphorus.
- Sulfur has more protons = greater nuclear charge.
- Electrons added to the same shell → shielding stays the same.
- Stronger attraction pulls electrons in → smaller radius.
Q: Define first ionisation energy.
“The energy required to remove one mole of electrons from one mole of gaseous atoms.”
Q: State and explain the trend in atomic radius across Period 3.
- Atomic radius decreases.
- Nuclear charge increases, shielding stays the same.
- Electrons pulled closer to nucleus.
Q: What is meant by “periodicity”?
“The repeating pattern of physical and chemical properties of the elements when arranged by increasing atomic number.”
