C2 - Atomic structure and the periodic table

QUESTION 1 6 marks Criterion A

Medium

Describing an ion

An ion contains 17 protons, 18 electrons and 20 neutrons.

a. Determine its atomic number and mass number.

[2]

b. Write the ion symbol, including mass number and charge.

[2]

c. Explain which subatomic particle determines the element's identity.

[2]

Show complete worked solution

(a)

\[Z=17\]\[A=17+20=37\]

(b)

The particle has one more electron than protons, so its charge is \(-1\).\[{}^{37}_{17}\mathrm{Cl^-}\]

(c)

The number of protons determines the atomic number. Changing the proton number changes the element.

QUESTION 2 9 marks Criterion B

Hard

Testing a model of the atom

Students model Rutherford scattering by rolling small steel balls toward a hidden circular obstacle beneath a board.

a. State a focused research question for the model.

[2]

b. Describe how the students should collect reliable quantitative data.

[5]

c. State one way the model represents an atom and one limitation.

[2]

Show complete worked solution

(a)

How does the distance of closest approach to the hidden obstacle affect the deflection angle of a steel ball rolled at constant initial speed?

(b)

Mark several controlled approach lines at measured offsets from the obstacle center. Release the same ball from the same ramp height for constant initial speed. Record the incoming and outgoing paths on paper and measure the deflection angle. Repeat each offset at least three times and calculate mean angles.

(c)

The small obstacle represents a concentrated nucleus that is tiny compared with the whole atom. The rolling ball is macroscopic and interacts by contact rather than electrostatic force, so the mechanism is not realistic.

QUESTION 3 6 marks Criterion C

Medium

Charge and mass evidence

Four particles have the following compositions: W: 8 p, 8 n, 8 e; X: 8 p, 10 n, 10 e; Y: 9 p, 10 n, 10 e; Z: 10 p, 10 n, 8 e.

a. Identify the pair that are different elements but have the same mass number.

[2]

b. Determine the charges of X, Y and Z.

[3]

c. Identify which particle is neutral and justify your answer.

[1]

Show complete worked solution

(a)

X and Y both have mass number 18, but their proton numbers are 8 and 9, so they are different elements.

(b)

\[X:8-10=-2\]\[Y:9-10=-1\]\[Z:10-8=+2\]

(c)

W is neutral because it has equal numbers of protons and electrons.

QUESTION 4 6 marks Criterion D

Hard

Using an atomic model in medicine

A hospital proposes using a beam of positively charged particles to treat a tumor. A public leaflet says the particles are 'tiny atoms carrying spare protons.'

a. Evaluate the scientific accuracy of the phrase 'atoms carrying spare protons.'

[3]

b. Suggest a clearer explanation for a non-specialist audience.

[3]

Show complete worked solution

(a)

A positive ion normally forms by losing electrons, not by gaining spare protons. Adding a proton would change the atomic number and therefore the element. The leaflet should describe positively charged ions or nuclei.

(b)

The beam contains particles with positive electric charge. Electric and magnetic fields can guide them, and they transfer energy to tumor tissue. The dose and position must be controlled to limit damage to healthy cells.

QUESTION 1 6 marks Criterion A

Medium

Relative atomic mass from isotopes

Copper has two naturally occurring isotopes. The spectrum shows their mass numbers and relative abundances.

a. State what is meant by isotopes.

[2]

b. Calculate the relative atomic mass of copper from the graph.

[3]

c. Explain why the value is closer to 63 than to 65.

[1]

Show complete worked solution

(a)

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

(b)

\[A_r=\frac{(63)(69)+(65)(31)}{100}=63.62\]

(c)

The isotope with mass number 63 has the greater abundance, so it contributes more to the weighted mean.

QUESTION 2 9 marks Criterion B

Hard

Estimating isotope abundance

A student wants to model the relative atomic mass of chlorine using bags of identical counters labelled 35 or 37.

a. Describe a sampling method that can test how sample size affects the calculated value.

[5]

b. State the independent and dependent variables.

[2]

c. Predict the effect of increasing sample size.

[2]

Show complete worked solution

(a)

Prepare a large population with the intended isotope ratio and mix it thoroughly. Draw random samples of several sizes, replacing and remixing counters after each trial. For every sample, record the numbers labelled 35 and 37 and calculate the weighted mean. Repeat each sample size many times and compare the spread of results.

(b)

The independent variable is sample size. The dependent variable is the calculated relative atomic mass or its deviation from the population value.

(c)

Larger samples should give a smaller spread and values closer to the population mean because random sampling variation has less influence.

QUESTION 3 5 marks Criterion C

Medium

Interpreting repeated isotope data

Five measurements of the abundance of isotope M-24 are 78.4%, 78.9%, 79.1%, 78.7% and 84.6%.

a. Identify the anomalous value.

[1]

b. Calculate the mean of the remaining four measurements.

[2]

c. Explain why the anomalous result should be investigated rather than automatically deleted.

[2]

Show complete worked solution

(a)

84.6%.

(b)

\[\text{mean}=\frac{78.4+78.9+79.1+78.7}{4}=78.775\%\approx78.8\%\]

(c)

It may result from contamination, calibration error or a genuine sample difference. The cause should be checked and the measurement repeated before deciding whether exclusion is justified.

QUESTION 4 5 marks Criterion D

Medium

Choosing an isotope tracer

Tracer A has a half-life of 6 hours and emits detectable radiation. Tracer B has a half-life of 18 years and produces the same signal strength per decay. A process inside a water-treatment plant lasts one day.

a. Recommend a tracer and justify the recommendation scientifically.

[3]

b. State two other factors required before the tracer can be approved.

[2]

Show complete worked solution

(a)

Tracer A is preferable because its half-life is long enough for measurements during the one-day process but short enough for its activity to fall rapidly afterward. Tracer B would remain radioactive for many years.

(b)

Its chemical behavior must follow the water pathway, and the radiation type, dose, containment, detection limit and disposal procedure must be assessed.

QUESTION 1 6 marks Criterion A

Medium

Comparing electronic structures

The diagram shows particles P, Q and R with electronic structures 2,7; 2,8; and 2.

a. Identify the particles most likely to be chemically unreactive.

[2]

b. Predict the ion formed by P.

[2]

c. Explain why gaining one electron is more favorable for P than losing seven.

[2]

Show complete worked solution

(a)

Q and R have full outer shells and are therefore likely to be unreactive.

(b)

P gains one electron to obtain a full outer shell, forming a \(1-\) ion.

(c)

Gaining one electron requires a smaller rearrangement and gives the stable electronic structure 2,8. Removing seven electrons requires much more energy.

QUESTION 2 7 marks Criterion B

Hard

Flame-test controls

Students use flame tests to distinguish sodium, potassium and calcium ions. The first trials give mixed colors.

a. Suggest a cause of the mixed colors.

[1]

b. Describe an improved method including a check that the loop is clean.

[5]

c. State one safety precaution.

[1]

Show complete worked solution

(a)

The wire loop or solutions may be contaminated, especially with traces of sodium.

(b)

Use separate labelled solutions. Clean a nichrome or platinum loop in hydrochloric acid, heat it in a non-luminous flame and repeat until no color is seen. Dip the clean loop into one sample and place it at the same position in the flame. Record the color, clean again and repeat. Test known reference ions and repeat observations.

(c)

Wear eye protection and keep flammable materials away from the burner; hydrochloric acid should be handled carefully.

QUESTION 3 5 marks Criterion C

Hard

Ionization-energy sequence

An element has successive ionization energies of 738, 1451, 7733 and 10540 kJ mol\(^{-1}\).

a. Identify the significant feature in the data.

[1]

b. Infer the number of outer-shell electrons.

[2]

c. Predict the charge of the common ion and explain.

[2]

Show complete worked solution

(a)

There is a very large increase between the second and third ionization energies.

(b)

Two. After two electrons are removed, the third electron comes from an inner shell and requires much more energy.

(c)

The element forms a \(2+\) ion by losing its two outer electrons and reaching a stable filled inner shell.

QUESTION 4 6 marks Criterion D

Medium

Evaluating shell diagrams

A learning app always draws electrons as stationary dots on circular paths and states that this is a photograph of an atom.

a. State one useful feature of the model.

[1]

b. Explain two ways the statement is misleading.

[3]

c. Recommend how the app should label the representation.

[2]

Show complete worked solution

(a)

It communicates the number of electrons in each energy level and can help predict ion formation and chemical behavior.

(b)

The diagram is a simplified model rather than a photograph. Electrons are not stationary particles on fixed planetary paths, and the sizes and distances are not to scale.

(c)

Call it an electron-shell model and state explicitly what it represents and what it does not represent.

QUESTION 1 7 marks Criterion A

Hard

Explaining a periodic trend

The graph shows first ionization energy across Period 3.

a. Describe the overall trend.

[1]

b. Explain the overall trend using nuclear charge and shielding.

[3]

c. Identify the two decreases shown and state why a perfectly smooth rise is not expected.

[3]

Show complete worked solution

(a)

First ionization energy generally increases from sodium to argon.

(b)

Proton number increases across the period while added electrons enter the same main shell. Shielding changes little, so effective nuclear attraction increases and atomic radius decreases. More energy is needed to remove an outer electron.

(c)

The decreases occur from Mg to Al and from P to S. They arise from changes in subshell energy and electron pairing, so electron configuration modifies the overall nuclear-charge trend.

QUESTION 2 7 marks Criterion B

Medium

Comparing Group 1 reactivity

Plan an investigation comparing the reactions of lithium, sodium and potassium with water. Direct measurement of gas volume is permitted, but only very small metal pieces may be used.

a. State a suitable dependent variable.

[1]

b. Describe how to make the comparison valid and reasonably safe.

[5]

c. Predict the order of reactivity.

[1]

Show complete worked solution

(a)

The initial rate of hydrogen production, such as volume produced per second.

(b)

Use equal measured moles or equal surface-area pieces of freshly cut metal, equal volumes of water at the same temperature, and identical apparatus. Collect hydrogen with a gas syringe and record volume at short fixed intervals. Use a safety screen, eye protection and forceps, keep quantities very small and repeat each metal.

(c)

Lithium < sodium < potassium.

QUESTION 3 6 marks Criterion C

Medium

Analyzing halogen displacement

Chlorine water turns potassium bromide solution orange and potassium iodide solution brown. Bromine water does not change potassium chloride but turns potassium iodide brown.

a. Use the observations to rank chlorine, bromine and iodine by reactivity.

[2]

b. Write the ionic equation for chlorine reacting with bromide ions.

[2]

c. Predict the result of iodine solution added to potassium bromide and justify it.

[2]

Show complete worked solution

(a)

Chlorine is more reactive than bromine and iodine; bromine is more reactive than iodine. Therefore chlorine > bromine > iodine.

(b)

\[\mathrm{Cl_2+2Br^-\rightarrow2Cl^-+Br_2}\]

(c)

No displacement occurs because iodine is less reactive than bromine and cannot oxidize bromide ions.

QUESTION 4 6 marks Criterion D

Hard

Predicting an unknown element

A newly reported element is claimed to belong to Group 1 because its oxide dissolves in water to give an alkaline solution. No reaction-rate or electronic-structure data are provided.

a. Evaluate whether the evidence is sufficient.

[3]

b. Identify two additional types of evidence and explain how each would help.

[3]

Show complete worked solution

(a)

An alkaline oxide is consistent with Group 1 behavior, but other metals can also form basic oxides. One property is not sufficient to establish group placement.

(b)

Electronic structure should show one outer electron. Reactions with water or halogens and comparison of melting point or ionization energy with group trends would provide independent evidence of similar chemical and physical behavior.

QUESTION 1 6 marks Criterion A

Medium

Classifying elements from properties

Element J conducts electricity, is malleable and forms a colored ion. Element K is a gas that exists as single atoms and rarely reacts. Element L is a brittle solid and forms covalent molecules.

a. Classify J, K and L.

[3]

b. Explain which observation most strongly supports J being a transition metal.

[2]

c. Relate K's low reactivity to electronic structure.

[1]

Show complete worked solution

(a)

J is a metal, likely a transition metal. K is a noble gas. L is a non-metal.

(b)

Formation of a colored ion is characteristic of many transition metals; conductivity and malleability are general metallic properties.

(c)

K has a complete outer electron shell.

QUESTION 2 9 marks Criterion B

Hard

Corrosion comparison

A manufacturer must compare corrosion of an iron alloy and an aluminum alloy in salty water.

a. Write a testable hypothesis based on protective oxide layers.

[2]

b. Describe a quantitative method.

[5]

c. Explain why mass loss per unit area should be used.

[2]

Show complete worked solution

(a)

Under identical salty-water conditions, the aluminum alloy will show a smaller mass loss than the iron alloy because its adherent oxide layer limits continued corrosion.

(b)

Prepare equal exposed surface areas, clean, dry and weigh each specimen. Immerse replicate specimens in equal volumes of salt solution at the same concentration, temperature and oxygen exposure for a fixed time. Remove corrosion products by a standardized method, rinse, dry and reweigh. Calculate mass loss per unit area and compare means and ranges.

(c)

It controls for any remaining difference in specimen size and allows corrosion rates to be compared fairly.

QUESTION 3 6 marks Criterion C

Medium

Transition-metal catalyst data

The same reaction takes 240 s without a catalyst, 82 s with catalyst X and 79 s with catalyst Y. Repeated values for Y are 79, 121 and 76 s.

a. Compare the evidence for catalysts X and Y.

[2]

b. Calculate the percentage reduction in time using catalyst X.

[2]

c. State what should be done before claiming Y is better than X.

[2]

Show complete worked solution

(a)

Both appear to increase rate compared with 240 s. X gives 82 s. Y gives two similar values near 78 s but one anomalous value at 121 s, so its evidence is less consistent.

(b)

\[\%\,\text{reduction}=\frac{240-82}{240}\times100=65.8\%\]

(c)

Repeat both catalysts under identical conditions, investigate the anomalous Y result and compare mean rates with uncertainty. The current difference of only 3 s may not be significant.

QUESTION 4 6 marks Criterion D

Hard

Selecting a catalyst metal

Catalyst P gives a 92% yield and lasts 2 years but contains a scarce toxic metal. Catalyst Q gives an 88% yield, lasts 5 years and is made from abundant iron. Both operate at the same temperature.

a. Compare the scientific and resource evidence.

[3]

b. Recommend a catalyst for large-scale use and state one further datum needed.

[3]

Show complete worked solution

(a)

P gives 4 percentage points higher yield, but it has a shorter lifetime and uses a scarce toxic metal. Q gives slightly lower yield but lasts 2.5 times as long and uses an abundant material.

(b)

Q is the stronger provisional choice because the modest yield loss may be outweighed by longer life, lower resource pressure and reduced toxicity. A full decision needs production rate, cost, energy demand, recyclability and life-cycle emissions.

C2 - Atomic structure and the periodic table

C2.1 - Protons, neutrons, electrons and ions 4 questions

C2.2 - Isotopes and relative atomic mass 4 questions

C2.3 - Electronic structure and ion formation 4 questions

C2.4 - Groups, periods and periodic trends 4 questions

C2.5 - Metals, non-metals, transition metals and noble gases 4 questions