Atoms of elements at the top of a group on the periodic table are smaller than Atoms of elements at the bottom of the group how does this help explain the difference in reactivity of metals within a group

What is the mass of 0.46 mol of mgcl2

Word equation for k+ h2o yield koh + h2

I'm learning about the periodic tables and what each subject's configuration is. for example, hydrogen is 1s^1, but i don't understand how you get that. can someone me understand how to figure out how to figure this out? sorry if the question makes no sense, but it would really a lot if you could me understand! you so much if you can!