The atomic number of chlorine is 17. Which of the following elements has an atomic number greater than that of chlorine?

Introduction / Context:
Atomic number is a fundamental concept in chemistry, defined as the number of protons in the nucleus of an atom. It determines the position of an element in the periodic table and its basic chemical identity. This question checks whether learners can recall relative atomic numbers of some common elements and compare them with that of chlorine, whose atomic number is given as 17.

Given Data / Assumptions:
- Chlorine has atomic number 17, meaning it has 17 protons in its nucleus.
- The elements listed are potassium, sulphur, aluminium, phosphorus and magnesium.
- Standard periodic table ordering is assumed, with atomic numbers increasing from left to right and top to bottom.
- Only one element in the list has an atomic number greater than 17 in basic school level sequences.

Concept / Approach:
To solve this, we recall or infer the approximate positions of the elements in the periodic table. Sulphur and phosphorus lie close to chlorine in the same period, but they come before chlorine, so their atomic numbers are slightly lower than 17. Aluminium and magnesium are in an earlier period and therefore have even lower atomic numbers. Potassium lies in the next period after argon and has an atomic number of 19. Therefore, among the given options, only potassium has an atomic number greater than that of chlorine.

Step-by-Step Solution:
Step 1: Recall that the atomic numbers around chlorine in the third period are: phosphorus 15, sulphur 16, chlorine 17, argon 18 and potassium 19. Step 2: Identify the atomic number of sulphur as 16, which is less than 17. Step 3: Identify the atomic number of phosphorus as 15, also less than 17. Step 4: Recall that aluminium and magnesium are in the third period but earlier, with atomic numbers 13 and 12 respectively. Step 5: Recognise that potassium is in the next period with atomic number 19, which is greater than 17, and conclude that potassium is the correct choice.

Verification / Alternative check:
An alternative approach is to write a short section of the periodic table from sodium to argon. In this row, sodium is 11, magnesium 12, aluminium 13, silicon 14, phosphorus 15, sulphur 16, chlorine 17 and argon 18. Going down to the next period, potassium comes immediately after argon and is assigned atomic number 19. This pattern confirms that aluminium, sulphur, phosphorus and magnesium all have atomic numbers lower than 17, while potassium has a higher atomic number than chlorine.

Why Other Options Are Wrong:
Sulphur has atomic number 16, which is one less than chlorine. Aluminium has atomic number 13, significantly less than 17. Phosphorus has atomic number 15, also lower than chlorine. Magnesium, with atomic number 12, lies even earlier in the table. None of these options meet the condition of having an atomic number greater than that of chlorine. Only potassium, with atomic number 19, satisfies the requirement.

Common Pitfalls:
Students sometimes confuse atomic number with atomic mass, leading them to guess based on relative mass rather than position in the periodic table. Another pitfall is memorising isolated facts about elements without seeing the sequential pattern in the table. Some may remember that sulphur and phosphorus are close to chlorine and assume they must have higher numbers. Using the periodic table sequence and remembering that atomic number increases steadily avoids these errors.

Final Answer:
The element among the options whose atomic number is greater than that of chlorine is Potassium.