IB Chemistry S1.2: The nuclear atom worked solutions

Which description identifies a neutron?

A A light particle of relative charge -1 outside the nucleus. B A particle of relative mass about 1 and charge 0 in the nucleus. C A particle of relative mass about 1 and charge +1 in the nucleus. D A particle of negligible relative mass and charge 0 outside the nucleus.

A species contains 12 protons, 13 neutrons and 10 electrons. Write its complete nuclear symbol; use section 7 of the data booklet.

A neon specimen initially contains only neutral {}^{20}_{10}\mathrm{Ne} atoms. Which added species introduces a second isotope of neon, rather than merely another charge state of the original isotope?

A   {}^{20}_{10}\mathrm{Ne}^{+}

B   {}^{20}_{9}\mathrm{F}

C   {}^{22}_{10}\mathrm{Ne}

D   {}^{20}_{10}\mathrm{Ne}^{2+}

For {}^{31}_{15}\mathrm{P}^{3-}, state the neutron count and the electron count.

Compare {}^{48}_{20}\mathrm{Ca}^{2+} with {}^{45}_{21}\mathrm{Sc}^{3+}. Which statement is correct?

A They have equal neutron counts and equal electron counts. B They have equal neutron counts but different electron counts. C They have different neutron counts and different electron counts. D They have different neutron counts but equal electron counts.

Neutral magnesium-24 and magnesium-26 have very similar characteristic chemistry. Explain this using their electron arrangements.

A titanium mixture contains 30.0\% titanium-46 atoms and 70.0\% titanium-48 atoms. Treat the relative isotope masses as 46 and 48. What is the sample mean?

A 46.6 B 47.0 C 47.4 D 94.0

An antimony spectrum has atomic peaks at m/z=121 and 123 with relative signals 55 and 100, respectively. Both ions have charge 1+ and equal response. What is the antimony-123 atom percentage?

A 35.5\% B 50.0\% C 64.5\% D 100.0\%

State what A counts in one nucleus. Give the mass reference used to define A_r.

A prepared two-isotope sample has mean relative mass 71.8. Which nominal isotope pair could give this mean?Options in order A–D: {68,70}{70,74}{72,76}{74,78}

A zinc isotope is used as a material label. A neutral atom has nuclear symbol {}^{64}_{30}\mathrm{Zn}; it subsequently loses two electrons.

(a) Find its neutron count.   [1]

(b) Find the electron count after the loss.   [1]

(c) Give the nuclear charge in units of the elementary charge, and state whether this charge changed during electron loss.   [2]

Two boron samples contain only neutral boron-10 atoms and only neutral boron-11 atoms, respectively. Both occupy the same volume and contain the same number of atoms. Approximate each atomic mass by its mass number.

(a) Define isotopes.   [2]

(b) Determine the ratio of the density of the boron-11 sample to that of the boron-10 sample.   [2]

(c) Explain why their characteristic chemical behaviour is very similar.   [2]

An enriched rubidium sample contains 40.0\% rubidium-85 atoms and the remainder rubidium-87 atoms. Use mass numbers as approximate relative isotope masses.

(a) Calculate the percentage of the heavier isotope.   [1]

(b) Calculate the relative atomic mass of this sample to two decimal places.   [2]

An enriched palladium sample gives singly charged atomic-ion peaks at m/z=106 and 108, with relative signals 100 and 25. Use these positions as relative isotope masses. A production target is a mean of 107.0. How does this sample compare with the target?

A   Its mean is 0.6 below the target.

B   Its mean equals the target.

C   Its mean is 0.6 above the target.

D   Its mean is 1.0 below the target.

An artificial isotope mixture of X contains nominal masses 40 and 42 with atom fractions 0.25 and 0.75. Assume independent random pairing and intact singly charged X_2 ions only.(a) [ 2 ] {Give the three peak positions and their fractions.}(b) [ 2 ] {Explain the factor of two in the middle peak; check that the fractions sum to one.}

Positive probes cross a thin layer of neutral atoms. Most paths change little; a small minority turn through large angles.(a) [ 2 ] {Explain what these observations imply about the volume occupied by the nucleus and the concentration of atomic mass.}(b) [ 2 ] {Explain the large turns using the sign and location of atomic charge.}

A laboratory must determine an enriched element' s A_r without assuming its natural isotope composition. Name a suitable technique, identify the two kinds of data needed, and explain how to combine them.

A supplied spectrum model has atomic fragment peaks at m/z=40,42 and an intact singly charged molecular cluster at 80,82,84. The molecular signals are 9,6,1, respectively.(a) [ 2 ] {Calculate the relative molecular mass from the appropriate cluster.}(b) [ 1 ] {Explain why the two lower-mass peaks are excluded.}

A monatomic ion has charge 2+ and 62 nucleons. Its neutron count exceeds its electron count by six.

(a) Use the two conditions to deduce the numbers of electrons, protons and neutrons.   [3]

(b) Identify the element and write the full nuclear symbol for the ion.   [1]

(c) Compare its electron and neutron counts with those of {}^{64}_{30}\mathrm{Zn}^{3+}.   [2]

Element Q has just two isotope masses, r and r+2. In sample L, the heavier-isotope atom fraction is 0.60 and A_r=73.20. Sample M has A_r=72.70. Work within this supplied model.(a) [ 2 ] {Deduce both isotope masses.}(b) [ 2 ] {Calculate the heavier-isotope atom fraction in M.}(c) [ 2 ] {Equal numbers of atoms from L and M are mixed. Find the new A_r.}(d) [ 1 ] {Explain why mixing equal sample masses need not give that result.}

An enriched zirconium batch contains only isotopes with relative masses approximated as 90 and 92. Its mean rises from 90.60 to 91.36 after adjustment. What change in the percentage of atoms of the heavier isotope produces this rise?

A   An increase of 38 percentage points

B   An increase of 68 percentage points

C   A decrease of 38 percentage points

D   An increase of 76 percentage points

A xenon spectrum contains only singly charged atomic-ion peaks at m/z=129,131,132. Their relative signals are 40,h,20. Signals are proportional to atom populations. The measured mean relative atomic mass is 130.50; approximate isotope masses by the peak positions.

(a) Determine the missing signal h.   [3]

(b) Find the atom percentage of xenon-131.   [1]

(c) Explain why the unweighted mean of the three peak positions is unsuitable.   [1]

(d) If each detected isotope instead formed a singly charged negative ion, would its relative isotope mass change significantly? Explain.   [1]

A detector reports intact singly charged X_2 ions at nominal masses 100,102,104 with signals 4,12,9. X has only two isotopes, which pair independently; responses are equal.(a) [ 2 ] {Infer the two atomic isotope masses and their atom fractions.}(b) [ 3 ] {Calculate A_r and M_r separately and check their relationship.}(c) [ 2 ] {Another operator adds atomic-fragment signals to the molecular average. Explain the error and why doubling every molecular signal leaves your M_r unchanged.}

S1.2 + S1.3 + S2.1   Two fluorine isotope labels, {}^{19}_{9}\mathrm F and {}^{18}_{9}\mathrm F, are followed into fluoride ions in an ionic lattice. Ignore nuclear decay.(a) [ 2 ] {Give the neutron count and electron count for each 1- ion.}(b) [ 2 ] {Write their shell electron arrangements and explain why the labels show similar characteristic chemistry.}(c) [ 2 ] {Explain the attraction of either ion to a sodium ion. Does unchanged ionic charge establish equal isotope masses?}

S1.2 + S1.4 + R2.1   An enriched magnesium sample contains only isotopes of nominal masses 24 and 26 in atom fractions 0.60 and 0.40. Treat these numbers as isotope molar masses in \,\mathrm{g\,mol^{-1}}. All 0.620\,\mathrm{g} reacts in excess acid: \mathrm{Mg}+2\mathrm{H}^{+}\rightarrow\mathrm{Mg}^{2+}+\mathrm{H}_2.(a) [ 2 ] {Calculate the sample' s mean molar mass and amount of magnesium.}(b) [ 2 ] {Determine the amount of hydrogen formed and of hydrogen ions consumed.}(c) [ 2 ] {A student uses 24.0\,\mathrm{g\,mol^{-1}} throughout. Calculate the percentage overestimate in the amount of hydrogen.}

S1.2 + S2.2 + S1.4 [HL]   A supplied model pairs chlorine isotopes independently into covalent Cl_2. Nominal isotope masses are 35 and 37; the chlorine-35 atom fraction is 0.60. Detect intact singly charged molecules with equal responses.(a) [ 2 ] {Predict the molecular peak positions and fractions.}(b) [ 2 ] {Calculate M_r and the amount in 1.44\,\mathrm{g}; take its numerical value as the molar mass in \,\mathrm{g\,mol^{-1}}.}(c) [ 2 ] {Explain whether isotope substitution changes the bond' s shared-electron-pair model or its molecular mass.}(d) [ 2 ] {A report gives middle-peak fraction 0.24. Test this against the model and identify a likely counting error.}