S1.2 The nuclear atomIB Chemistry HL: Flashcards

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How many electrons are in Se²⁻ (Z = 34)?

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How many electrons are in Se²⁻ (Z = 34)?
How do you calculate neutrons from a nuclear symbol?: Neutrons = mass number − atomic number (A − Z).
Define isotopes.: Atoms of the same element with the same number of protons but different numbers of neutrons.
What formula gives Ar from isotope data?: Ar = Σ(isotopic mass × % abundance) ÷ 100.
What does a mass spectrometer separate ions by?: Their mass-to-charge ratio, m/z.
What does the number of peaks in an element's mass spectrum tell you?: The number of isotopes (for 1+ ions).
What does the height of each peak show?: The relative abundance of that isotope.
Where would ²⁴Mg²⁺ appear in a mass spectrum?: At m/z = 24 ÷ 2 = 12.
Neon: m/z 20 (90.5 %), 21 (0.3 %), 22 (9.2 %). What is Ar?: 20.19.
Why does Br₂ give three molecular ion peaks at 158, 160 and 162?: The molecules can be ⁷⁹Br⁷⁹Br, ⁷⁹Br⁸¹Br or ⁸¹Br⁸¹Br.
Why is the 160 peak of Br₂ twice the height of the others?: The isotopes are about equally abundant and the mixed molecule can form in two ways.
Why can the full mass spectrum distinguish sources of an element when Ar cannot?: Different isotope compositions can give the same weighted mean, but the individual abundances differ.
Why is Ar calculated from whole-number m/z values slightly different from the data booklet value?: Actual relative isotopic masses are not exactly whole numbers.