15. The element lithium (Li) has an atomic mass of 3 and a mass number of 7. Explain how you know that the diagram above represents a lithium atom.

What you already know:

C on this diagram is a proton and B points to the electron. A note the nucleus that contains the protons and the center of orbit to the electrons. D notes the electron cloud, radius, or the bounds to which the electron obitals exist.

If you count the number of protons this atom has it adds up to 3, which, therefore comfirms that this molecule is Li. The number of protons is specific to the atom. The question states that this element has a mass number of 7, this is because the mass number is the sum of both protons and electrons.

the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.