Which one of the following elements has an atomic number greater than that of Calcium (atomic number 20)?

Introduction / Context:
The periodic table arranges elements in order of increasing atomic number, which is the number of protons in the nucleus of an atom. Questions that compare atomic numbers help students become familiar with this ordering. Calcium has atomic number 20, and the question asks which listed element has an atomic number greater than that of calcium. Recognising approximate positions of common elements in the periodic table is an important skill for chemistry exams.

Given Data / Assumptions:
- Calcium has atomic number 20, which is taken as a reference point.
- Four elements are provided: scandium, argon, chlorine, and sulphur.
- The task is to identify which of these has an atomic number higher than 20.
- Standard periodic table atomic numbers are assumed: sulphur 16, chlorine 17, argon 18, scandium 21.

Concept / Approach:
Elements are ordered in the periodic table by atomic number from left to right and top to bottom. Sulphur (S) has atomic number 16, chlorine (Cl) has 17, argon (Ar) has 18, calcium (Ca) has 20, and scandium (Sc) has 21. Among the options, only scandium lies after calcium in the periodic table and therefore has a greater atomic number. The others have smaller atomic numbers and are located in periods before or at the same level as calcium, but to the left.

Step-by-Step Solution:
Step 1: Recall the atomic number of sulphur, which is 16. Step 2: Recall the atomic number of chlorine, which is 17. Step 3: Recall the atomic number of argon, which is 18. Step 4: Recall the atomic number of calcium, given as 20. Step 5: Recall that scandium, which follows calcium in the periodic table, has atomic number 21. Step 6: Compare each atomic number with 20 and conclude that only scandium has an atomic number greater than calcium.

Verification / Alternative check:
Looking at a periodic table confirms the sequence of elements by atomic number: argon (18), potassium (19), calcium (20), and scandium (21). This shows that scandium directly follows calcium as the next element. Sulphur and chlorine appear earlier in the same period as argon, with atomic numbers 16 and 17. Therefore, they are clearly less than 20. This visual check on the periodic table aligns with the numerical comparison and verifies that scandium is the correct answer.

Why Other Options Are Wrong:
Argon: Atomic number 18, which is less than 20 and hence not greater than calcium.
Chlorine: Atomic number 17, again less than 20 and located earlier in the same period.
Sulphur: Atomic number 16, also less than 20 and clearly before chlorine and argon in the periodic table.

Common Pitfalls:
A frequent mistake is to confuse atomic mass with atomic number, especially because some heavier elements with lower atomic numbers may have greater relative atomic masses. Another error is to misremember the order of elements around calcium, leading to wrong conclusion that argon lies after calcium. To avoid such mistakes, students should memorise at least the first twenty or thirty elements in sequence and repeatedly practice reading the periodic table so that patterns become familiar.

Final Answer:
Among the given options, the element whose atomic number is greater than that of calcium is Scandium, which has atomic number 21.