MAI- Atomic Structure PPQS
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What does M(g)->M2+(g)+2e- represent (1)
the first and second ionisation energies
explain why sodium’s second ionisation energy is significantly higher than neon’s first ionisation energy (3)
• Na+ and Ne are isoelectronic • Sodium has 1 more proton than Neon (higher nuclear charge) • therefore stronger attraction between outer electrons and nucleus than Ne (so requires more energy to overcome)
Why does lithium have a higher first ionisation energy than sodium? (3)
• sodium’s outer electron is in a higher energy level than lithium’s so its further from the nucleus • sodium has more electron shells so has more shielding than lithium • shielding and atomic radii outweigh sodium’s greater nuclear charge (due to having more protons) (so lithium has a higher first ionisation energy)
Explain why the second ionisation energy of lithium is significantly greater than the first ionisation energy of helium (4)
• Li+ and He are isoelectronic • Li has a greater nuclear charge due to more protons • both removed from 1s2 so outer electrons in same energy level with no shielding • greater attraction means more energy needed
DOWN A GROUP rank which factor influencing ionisation energy has the largest influence
• shielding • atomic radii • nuclear charge
Across a period, why does atomic radius decrease (2)
nuclear charge increases, number of electrons in outer shell increases -therefore there is greater attraction between nucleus and outer electrons so they are attracted closer to the nucleus
(Graph of successive ionisation energies) How would you prove existence of X quantum shells (3)
• large increase in ionisation energy means electron gets closer to the nucleus • these jumps show the boundary between shells • arrangement (eg 2,8,2) confirms X number of shells
the highest peak has an m/z value of 74 for X+. State the relative molecular mass of X and explain how this value is obtained from the mass spectrum data (2)
• 74 • the charge is 1+ so m/z = relative molecular mass
State the change that occurs to a molecule during mass spec to form the molecular ion, M+, that is detected in a mass spectrometer (1)
ionised so loses 1 electron to form a positive ion
which peak on a mass spectra is used to determine relative molecular mass
molecular ion peak (peak with highest m/z value)
Explain in terms of electronic interactions, the three essential features required for a hydrogen bond to form between 2 methanol molecules (2)
• O-H covalent bond is polar due to oxygens higher electronegativity (than H) • H has a slight delta+ charge • electrostatic attraction occurs between delta + hydrogen and lone pair of electrons on a neighbouring methanol’s oxygen
Explain in terms of electrostatic interactions, the features of hydrogen fluoride molecules that give rise to hydrogen bonding in the liquid state (3)
• Fluorine is a very electronegative element, resulting in a large difference in electronegativity in the H-F bond • creates a highly polarised bond, creates delta+ charge on hydrogen and delta- charge on fluorine • electrostatic attraction occurs between delta+ Hydrogen and a neighbouring molecule’s lone pair on its fluorine atom
Explain why hydrogen bonding is not a significant intermolecular force in liquid hydrogen chloride, leading so its much lower boiling point compared to hydrogen fluoride (2)
• The electronegativity different between Cl and H is much smaller than between H and F (smaller positive charge on H) • Chlorine is a larger atom than fluorine so its lone pairs have a lower charge density and are less effective at attracting the delta+ hydrogen of a neighbouring molecule
Explain in terms of intermolecular forces why hydrogen bonds form in liquid ammonia but not in liquid phosphine (PH3) (3)
• N is very electronegative, P is not very electronegative • N-H bonds are very polar, P-H bonds arent polar • H bonds are electrostatic attraction between delta+ H and a neighbouring molecule’s lone pair on the N atom
Explain why water can form 4 hydrogen bonds but HF can only form 2 by considering the number of hydrogen bond donors and acceptors in each molecule (3)
• water has 2 donors and 2 acceptors • HF has 1 donor and 3 acceptors • bonds are limited by the donor in HF as it’s in shorter supply
Explain in terms of electronic structure, the underlying reason for the existence of periodicity (2)
• repeating pattern in the outer shell configuration • starting a new shell at the beginning of each period
A student stated ‘lithium and sodium both have one outer electron, their chemical properties are identical. Periodicity is the term for this observation’ Critique this statement. Your answer should explain why the properties are not identical and give a more complete description of periodicity. (3)
• properties are similar not identical • sodium’s outer electron is further from nucleus and has more shielding • periodicity is a gradual trend across a period