Atomic Structure: Chemistry 9701 (Cambridge International AS & A Level)

The nuclide symbol for a bromide ion is $^{79}_{35}\text{Br}^-$.

How many protons, neutrons and electrons does this ion contain?

A newly isolated element is given the temporary label Q while it is investigated. A sample of Q is analysed in a mass spectrometer. The mass spectrum contains three peaks, at $m/z = 24$, $m/z = 25$ and $m/z = 26$, with relative peak heights of $20.0$, $2.0$ and $3.0$ units respectively. No other isotopes of Q are present in a measurable amount.

(a) Calculate the percentage abundance of each of the three isotopes of Q. [2]

(b) Calculate the relative atomic mass, $A_r$, of this sample of Q, giving your answer to three significant figures. [2]

(c) Explain why all three isotopes of Q react identically with a given reagent, yet a sample enriched in the $m/z = 26$ isotope would have a measurably higher density than a sample enriched in the $m/z = 24$ isotope. [2]

Chromium has proton number 24.

Which row shows the correct full electronic configuration of a ground-state chromium atom?

Phosphorus has proton number 15.

(a) Write the full electronic configuration of a phosphorus atom, using subshell notation. [1]

(b) The three electrons in phosphorus's outermost occupied sub-shell each occupy a separate orbital, with parallel spins, before any orbital contains a pair of electrons. State the name of the rule that describes this behaviour, and explain, in terms of electron-electron repulsion, why electrons arrange themselves this way. [2]

(c) Deduce the full electronic configuration of the phosphide ion, $\text{P}^{3-}$, and name the noble gas that has the same electronic configuration as this ion. [2]

An unknown element, given the temporary label T, has the following successive ionisation energies for the first five electrons removed (values in $\text{kJ mol}^{-1}$):

$IE_1 = 580 \quad IE_2 = 1060 \quad IE_3 = 4900 \quad IE_4 = 6300 \quad IE_5 = 8000$

(a) Define the term first ionisation energy of an element. [2]

(b) Use the data to deduce the number of electrons in the outermost occupied shell of element T, explaining the reasoning behind your answer. [2]

(c) State the group of the Periodic Table to which element T belongs. [1]

(d) Suggest why $IE_2$ is greater than $IE_1$ for element T. [1]

(e) The first ionisation energy of aluminium is lower than that of magnesium, even though an aluminium atom has one more proton than a magnesium atom. Explain this anomaly in terms of electronic configuration and sub-shells. [3]

Which row correctly gives the relative charge and relative mass of a proton, a neutron and an electron?

In a time-of-flight (TOF) mass spectrometer, a sample is first ionised, then the resulting ions are accelerated through the same accelerating potential difference, allowed to drift along a flight tube of fixed length, and finally detected. The kinetic energy gained by an ion from this accelerating field depends on both the potential difference and the ion's own charge.

(a) Explain why the sample particles must be ionised before they can be accelerated by the electric field and detected. [2]

(b) An ion $\text{X}^+$ has mass number 40 and charge $1+$. An ion $\text{Y}^{2+}$ has mass number 80 and charge $2+$. Show that $\text{X}^+$ and $\text{Y}^{2+}$ have the same mass-to-charge ratio, and use this, together with the fact that $\text{Y}^{2+}$ gains twice the kinetic energy of $\text{X}^+$ from the accelerating field, to explain why the two ions take the same time to travel the length of the flight tube, even though a $\text{Y}^{2+}$ ion has twice the mass of an $\text{X}^+$ ion. [3]

(c) State and explain the effect on the time of flight of all the ions in the spectrometer if the accelerating voltage is increased. [1]

Iron has proton number 26. A ground-state iron atom has the electronic configuration $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^6\,4s^2$.

Which row shows the correct full electronic configuration of the $\text{Fe}^{3+}$ ion?

(a) State the maximum number of electrons that can occupy a single atomic orbital, and explain this maximum in terms of electron spin. [2]

(b) State the number of orbitals contained in each of the $s$, $p$ and $d$ sub-shells, and hence state the maximum number of electrons each of these sub-shells can hold. [3]

(c) Using your answer to part (b), show that the maximum number of electrons the third shell ($n=3$) can hold is 18. [2]

(d) Oxygen has proton number 8. Describe the arrangement of the four electrons in the $2p$ sub-shell of a ground-state oxygen atom across its three orbitals, and state how many of oxygen's electrons are unpaired. [2]

(a) State and explain the general trend in first ionisation energy across Period 2, from lithium to neon, in terms of nuclear charge and shielding. [3]

(b) This general increasing trend is not perfectly smooth: first ionisation energy decreases slightly from beryllium to boron, and again from nitrogen to oxygen. Explain each of these two exceptions in terms of sub-shells and electron-electron repulsion. [4]

(c) Sodium is directly below lithium in Group 1. Explain why the first ionisation energy of sodium is lower than that of lithium, even though a sodium atom has a much greater nuclear charge. [2]