UNIT 1: SECTION 1 - ATOMIC STRUCTURE
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Define Relative Isotopic Mass
Mass of an atom of an isotope compared to 1/12th of the mass of C-12.
Define Relative Atomic Mass (Ar)
Mass of an average atom compared to 1/12th of the mass of C-12.
Define Relative Molecular Mass (Mr)
Average mass of a molecule on a scale where an atom of carbon-12 is 12.
3 uses of Mass Spectrometry?
• Provides structural information. • Identification of unknown compounds. • Can determine relative abundance of each isotope in an element.
Why is a TOF Spectrometer under vacuum?
It prevents the ions from colliding with air.
Two types of ionisation and how they occur?
1) Electrospray Ionisation: Sample dissolved in volatile solvent (H2O). Injected through hypodermic needle and high + voltage applied. Sample vaporises and left with +ly charged ions. 2) Electron Impact: Electron gun shoots beam of high charged electrons at sample; electron knocked off sample atoms; left with positive ions and fractionation may occur.
How does TOF MS work?
1) Ionisation 2) Acceleration: +ions -> -ly charged plate. All ions same Ek (mass dependent now) 3) Ion drift: pass through hole in -ly charged plate; forms ion beam and travels through flight tube. 4) Detection: Arrive at detector, +ions pick up electron and cause current to flow. Flight times recorded.
Formula to calculate Relative Atomic Mass?
(relative abundance x atomic number)n / sum of relative abundance .
Why do sub-shells exist in shells?
Because not all electrons in a shell have the same energy.
What does the principle quantum number refer to?
Energy level of an electron; Further the shell is, larger the PQN.
How do Chromium and Copper differentiate from the general electronic structure?
Chromium and copper both donate an electron from the 4s shell to the 3d shell.
Define 1st ionisation energy.
Energy required to remove an electron from each atom in 1 mole of gaseous atoms to form 1 mole of +1 gaseous ions.
True or false: Ionisation energy is always an endothermic process.
TRUE
What factors affect ionisation energy? 3
• nuclear charge: more protons, stronger attraction to outer electrons. • distance from nucleus: closer, stronger attraction • shielding: shells between outer electron and nucleus lessen attraction.
What do successive ionisation energies show?
They show electronic structure; 1) Ei increases successively - electron closer to nucleus, less shielding, increasingly positive ion. 2) Bigger jumps in Ei - new shell, big jump closer to nucleus.
How to calculate which group unknown ion is in from the graph?
Count the number of electrons removed before the first big jump in Ei.
Two common trends in Ei?
Down a group, Ei decreases. Across a period, Ei increases.
How does Ei decrease down G2?
Each element down group 2 has an extra shell - more shielding, distance from nucleus increases, EASIER TO REMOVE!
How does Ei increase across P3?
More protons - Stronger nuclear attraction, Extra shielding (slight)
What are the exceptions to the trend in P3?
Aluminium - Ei drops; • Outer electron in 3p not 3s. • 3p higher up than 3s, • extra shielding and further away from nucleus. • requires less energy to remove. Sulfur - Ei drops; NOT BECAUSE OF SHIELDING! - Electron removed from occupied orbital, - repulsion between 2 electrons, - easier to remove. ^Sulfur’s ionisation abnormality is evidence for sub-shells.