Question

the mass number of an atom is 15, and its atomic number is 7. the atom probably has... 8 neutrons in the nucleus. 7 units of negative charge in the nucleus. at least 15 electrons. about as much mass in electrons as in protons. 7 electrons in the nucleus. part b which statement is true of atoms? most of an atoms volume is filled with matter. protons repel electrons. electrons determine the atoms size. protons attract other protons. all of the above.

Explanation:

For Part A: The mass number of an atom is the sum of protons and neutrons. Given atomic number (number of protons) is 7 and mass - number is 15, so number of neutrons = mass number - atomic number = 15 - 7=8. Protons are in the nucleus with positive charge, electrons are outside the nucleus, and electrons have negligible mass compared to protons.
For Part B: Most of an atom's volume is empty space. Protons are positively charged and electrons are negatively charged, so they attract, not repel. Protons repel other protons due to like - charges. Electrons, with their electron - cloud, determine the atom's size.

Answer:

Part A: 8 neutrons in the nucleus.
Part B: Electrons determine the atom's size.