Question

part a
order the elements sr, ca, and se in order of decreasing atomic size.
ca > se > sr
se > ca > sr
se > sr > ca
sr > ca > se

Explanation:

Step1: Recall atomic size trend

Atomic size increases down a group and decreases across a period.

Step2: Locate elements

Ca (Calcium) and Sr (Strontium) are in Group 2 (alkaline earth metals), with Sr below Ca. Se (Selenium) is in Group 16.

Step3: Compare sizes

Since Sr is below Ca, Sr > Ca. Across the period (same period as Ca and Se), Ca > Se (as atomic size decreases across a period). So Sr > Ca > Se.

Answer:

Sr > Ca > Se