Answer:

a. Shell

Explanation:

Principal Energy Level (n)

-"shells"

Indicates the relative size

and energy of atomic

orbitals

n=integers: n= 1, 2, 3, etc.

• As n increases:

> orbital becomes larger

> electron spends more

time farther away

from nucleus

> atom's energy level

increases

Source:

Quantum Mechanical Model of the Atom