Rank these elements according to the atomic radius from largest to smallest: K, Kr, Ca, Se, As, Ga, Ge, Br

To rank the elements by atomic radius, we consider the periodic trends that affect atomic size. Atomic radius generally decreases across a period from left to right due to increased nuclear charge, which pulls electrons closer to the nucleus. Conversely, atomic radius increases down a group due to the addition of electron shells.

Now, let’s analyze the elements:

• K (Potassium) – Located in group 1, K has the largest atomic radius because it has fewer protons than the others listed, which means less nuclear pull on the outermost electrons.
• Ca (Calcium) – Also in group 2, Ca comes next as it has a smaller radius than K due to the higher nuclear charge, but larger than nonmetals.
• As (Arsenic) – As is a metalloid in group 15. Its atomic radius is smaller than that of K and Ca as it’s further to the right in the periodic table.
• Ge (Germanium) – In group 14, Ge has a radius smaller than As but larger than nonmetals like Se. It follows due to its position in the periodic table.
• Br (Bromine) – As a halogen in group 17, Br’s atomic radius is smaller than that of Ge because it is further right.
• Se (Selenium) – Also in group 16, Se is smaller than Br, though they are close in size.
• Ga (Gallium) – In group 13, Ga’s radius is smaller than both Se and Br influenced by the d-block contraction.
• Kr (Krypton) – Finally, Kr is a noble gas and has the smallest atomic radius among the listed elements due to being farthest right on the periodic table.

Thus, the order from largest to smallest atomic radius is:

1. K
2. Ca
3. As
4. Ge
5. Br
6. Se
7. Ga
8. Kr