A star that explodes is called a supernova.
Valence shell
The outer shell of a plane can be called the skin. The smoother the skin the faster the plane passes through the air.
An egg with an outer hard shell is called the cuticle or bloom. It helps keep out bacteria and dust.
valence shell
It is a shell, as in a cockle shell or oyster shell.
The outer shell is called the valence shell
the shell
Long, long before the sun blows off its outer shell ("explodes", if you like) earth will have become uninhabitable for humans. By the time the sun does blow of its outer shell, we will be long gone already.
Valence shell and Ultimate Shell
The electrons located in the outer shell are called "valence electrons".
The key to "happiness" for an atom is a full outer electron shell. (The outer electron shell is called the valence shell.) There are two conditions that cause a shell not to be full. Either it has only an electron or two (or three) in the outer electron shell or it's short an electron or two in that outer shell. The direct answer to the question is that if an element is chemically active, its outer electron shell is incomplete or is not full.
alkali metals