Worked examples: The Periodic Table of Elements

Work through each example by covering the answer, attempting it yourself, then checking your method against the solution. Every example flags the slip that most often costs marks. These are original questions in SPM style, not past-year papers, and each one trains the same skill: reading a position and turning it into a prediction.

Example 1, Reading position from proton number

Question. An element X has proton number 12. State its group, period, and number of valence electrons.

Solution. The electron arrangement is 2.8.2, so X has 3 occupied shells and 2 valence electrons. Number of shells gives the period: 3. Number of valence electrons gives the group: 2.

Common slip: reading the group from the total number of electrons rather than the valence electrons. The group number matches the valence electrons only.

Example 2, Predicting metal or non-metal

Question. Element Y has the electron arrangement 2.8.7. Predict whether it is a metal or a non-metal and name the group it belongs to.

Solution. With 7 valence electrons Y is in Group 17, the halogens, so it is a non-metal. It tends to gain one electron to reach a stable octet.

Common slip: calling any element that “reacts strongly” a metal. Reactivity is not the test; the number of valence electrons and whether the element loses or gains electrons is.

Example 3, Comparing Group 1 reactivity

Question. Sodium and potassium are both in Group 1. Predict which is more reactive with water, and explain your answer.

Solution. Potassium is more reactive. It lies lower in the group, so it has more shells and a larger atom; its single valence electron is further from the nucleus and held less tightly, so it is lost more easily. Losing that electron is what the reaction with water requires.

Common slip: saying potassium is more reactive “because it is bigger” without linking size to how easily the valence electron is lost. The reason carries the marks.

Example 4, A Group 1 reaction with water

Question. Sodium is added to water. State two observations and write the word equation.

Solution. Observations: the sodium moves rapidly across the surface and melts into a ball, and a gas is released that produces a squeaky pop with a lit splint. Word equation: sodium + water → sodium hydroxide + hydrogen. The resulting solution is alkaline and turns red litmus blue.

Common slip: naming the gas as oxygen. A Group 1 metal reacting with water releases hydrogen, not oxygen.

Example 5, Comparing Group 17 reactivity

Question. Chlorine and bromine are both in Group 17. Predict which is more reactive, and explain.

Solution. Chlorine is more reactive. It lies higher in the group, so it has fewer shells and a smaller atom; the nucleus attracts an incoming electron more strongly, so chlorine gains an electron more easily. Gaining an electron is what a halogen reaction requires.

Common slip: applying the Group 1 rule in reverse by mistake. In Group 1 reactivity increases down; in Group 17 it increases up, because one group loses an electron and the other gains one.

Example 6, A displacement reaction

Question. Chlorine water is added to colourless potassium bromide solution. State what you would observe and explain why it happens.

Solution. The solution turns brown (orange) because bromine is displaced. Chlorine is more reactive than bromine, so it takes the place of bromine in the salt: chlorine + potassium bromide → potassium chloride + bromine. The brown colour is the free bromine formed.

Common slip: predicting no change. A more reactive halogen always displaces a less reactive one from its salt solution.

Example 7, Explaining noble-gas inertness

Question. Explain why argon does not react with other elements.

Solution. Argon is in Group 18 and has a stable, full valence shell of 8 electrons. Because its electron arrangement is already stable, it has no tendency to lose, gain or share electrons, so it is chemically inert. It also exists as single atoms.

Common slip: writing “it is a gas” as the reason. Being a gas is a physical fact; the chemical reason is the full, stable valence shell.

Example 8, Identifying a transition element

Question. A grey metal forms a green solution in one compound and a yellow-brown solution in another, and speeds up a reaction without being used up. Identify the type of element and give the property shown.

Solution. It is a transition element. Two different coloured solutions from the same metal show variable oxidation numbers and coloured compounds; speeding up a reaction without being used up shows it acts as a catalyst. Iron fits this description (Fe²⁺ green, Fe³⁺ yellow-brown).

Common slip: naming the colour without naming the property. The mark is for identifying the property, coloured ions, variable oxidation number, catalysis or complex-ion formation.

Example 9, A trend across Period 3

Question. Going across Period 3 from sodium to chlorine, describe how the atomic size and the metallic character change, and explain the atomic-size trend.

Solution. The atomic size decreases and the elements become less metallic (more non-metallic). The size decreases because, across the period, the proton number increases while the electrons are added to the same shell, so the greater nuclear charge pulls the shells inward.

Common slip: claiming the number of shells changes across a period. Across a period the number of shells stays the same; only the nuclear charge and the number of valence electrons change.

Example 10, Why Mendeleev left gaps

Question. Mendeleev arranged the known elements by increasing relative atomic mass but left some empty spaces in his table. Explain the purpose of these gaps and why his arrangement was accepted.

Solution. The gaps were left for elements not yet discovered. Because Mendeleev grouped elements by their chemical properties, an element that did not fit was placed by property, leaving a space where a missing element should be. He then predicted the properties of those missing elements, and when they were later discovered with the predicted properties, his table was accepted.

Common slip: saying the gaps were “mistakes” or “empty for decoration”. The gaps were deliberate predictions, the strongest evidence that the arrangement was sound.

Using these examples

Notice that every answer states a position, converts it to structure (shells and valence electrons), and only then predicts a property, the exact order the examiner rewards. Once the method feels automatic, move on to the practice questions and mark yourself the same way. A one-to-one teacher can check that your trend answers always carry an electron-arrangement reason, which is precisely where marks are gained or lost in the SPM Chemistry written papers.