An atom’s atomic number is determined by its number of protons.  An atom’s atomic mass is the sum of its protons and neutrons.  By counting the dots in the diagram, we can see that there are 9 protons and 11 neutrons.

Therefore, the atomic number of the atom is 9 (# of protons) and the atomic mass is 20 (protons + neutrons).  The correct choice is number 2.

If you chose # 1, you used the number of neutrons (instead of protons) as the atomic number and added incorrectly to find the mass.

If you chose #4, you used the number of neutrons (instead of protons) as the atomic number but correctly found the atomic mass.

If you chose #3, you correctly identified the atomic number as the number of protons but failed to add the neutrons and protons to find the atomic mass.

(3) is the correct answer.  In this sample, all compounds are identical – composed of 2Z’s and 1X.]

Answer 1 contains compounds that are composed of Z and X, but not both compounds.

Answer 2 is only made of atoms.  In answer 4, 3 of the compounds are composed both elements and 2 are only composed of unique elements.

(3) an electron is the correct answer

A proton is positive. An electron has an equal but opposite, and thus negative, charge. The assignment of positive and negative is a convention; there is nothing inherently "negative" about the charge on an electron, except that it is the opposite charge from a proton's, which we consider to be "positive."

1. An atom is mostly empty space with a dense positively charged nucleus

Rutherford fired alpha particles at a piece of gold foil.  When he looked at the beams on a fluorescent screen, he noticed that most of the particles went straight through the foil and hit the screen because gold atoms were mostly empty space. Rutherford hypothesized that the small dense positively charged nucleus of the atom deflected a small percentage of the atoms.

Incorrect answers:

2. An atom is composed of at least three types of subatomic particles

B is not correct because Rutherford and his colleagues didn’t discover the third subatomic particle until a later experiment with nitrogen and alpha particles.

3. An electron has a positive charge and is located inside the nucleus

C is not correct because an electron has a negative charge and is not located inside of the nucleus.

4. An electron has the properties of both waves and particles

D is not correct because it is a conclusion of Young’s Quantum Mechanics.