He estimated that the outermost shell — the one we live on — was 500 miles thick. (discovermagazine.com)
Earth's outermost shell is a fragmented jigsaw puzzle of rocky plates sliding over the mantle below. (sciencenews.org)
The number of electrons in the outermost shell, and its distance from the nucleus and the other shells, are the main factors that determine an element's chemical behaviour. (newscientist.com)