Arrange the following ions in order of increasing size:

Br- Ca2+ K+ Se2-

Taking electrons away makes the ions smaller than the atom. Adding electrons makes the ions larger than the atom.

why is the atomic radius of a noble gas larger than a halogen?

same response.

To arrange the given ions in order of increasing size, we need to compare their atomic radii. Atomic radius generally increases as you move down a group (from top to bottom) in the periodic table, and it decreases as you move across a period (from left to right).

Let's start comparing the ions:

1. K+ (potassium ion): Potassium is in Group 1 of the periodic table. It has lost one electron, resulting in a positive charge. Since potassium is located at the bottom of Group 1, it has the largest atomic radius among the given ions.
2. Ca2+ (calcium ion): Calcium is in Group 2, and it has lost two electrons, resulting in a 2+ charge. Even though calcium has a greater positive charge than potassium, it is located higher up in the periodic table. Therefore, it has a smaller atomic radius compared to potassium but larger than the remaining ions.
3. Se2- (selenium ion): Selenium is in Group 16 of the periodic table and has gained two electrons, resulting in a 2- charge. Since selenium is located at the top right of Group 16, it has a smaller atomic radius than potassium and calcium but larger than bromine.
4. Br- (bromide ion): Bromine is in Group 17 of the periodic table and has gained one electron, resulting in a 1- charge. Being at the top right of Group 17, bromine has the smallest atomic radius among the given ions.

So, the ions can be arranged in order of increasing size as follows:

Br- < Se2- < Ca2+ < K+