A sweet tasting element,
but cancer I can give.

Brittle am I, like an old bone

You can find me lingering in the Earth's crust,
every 2 to 6 parts a million.

Flex me and bend me, like a gymnast.

My appearance is metallic
A mysterious gray.

When mixed with other elements
I become stunning gems.

Because of me you know the neutron.

As I stay in Earth's crust,
Iḿ secretive and shy.

Earth is my turtle shell.

My atomic number is 4, 4 protons and 5 neutrons.
Don't forget 4 electrons too!