Question

select the correct answer from each drop - down menu.
much of the mass of an atom is concentrated in its nucleus. the nucleus contains
reset
next

Explanation:

To solve this, we recall atomic structure: the nucleus holds protons and neutrons (massive particles), while electrons (much lighter) occupy the outer part (electron cloud). So the first dropdown (outer part of atom) should be "electrons", and the nucleus contains "protons and neutrons".

Answer:

First dropdown (outer part of atom): electrons
Nucleus contains: protons and neutrons