The Periodic Table and Group Trends: Question 6

Worked solution

Step 1: Recall the organising principle of the Periodic Table

Every element has its own unique proton number (also called the atomic number), the number of protons in the nucleus of its atoms. The Periodic Table lists elements in order of increasing proton number, starting with hydrogen (proton number 1), then helium (proton number 2), and so on, with no gaps or repeats.

Step 2: Rule out the other quantities

• Relative atomic mass (B): this generally rises across the table too, but it is an average value affected by the mix of isotopes an element has. It is not the defining quantity used to fix each element’s exact place.
• Number of neutrons (C): this can be different for different atoms of the same element (isotopes of one element have the same proton number but different neutron numbers), so it cannot be used to fix a single, unique position for that element.
• Number of electron shells (D): this tells you which period (row) an element is in, but several different elements share the same number of shells, so shell number alone cannot fix one exact position. Only proton number can.

Final answer

• The Periodic Table is arranged in order of $\boxed{\text{proton number}}$, option A.