Paragraphs 10-14: 10:Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most helium atoms is 4 atomic mass units (2 amu for the protons + 2 amu for the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two. Therefore, they have a different mass number than the one given in the figure above..

11:Q: What is the mass number of a helium atom that has three neutrons?

12:A: The mass number is the number of protons plus the number of neutrons. For helium atoms with three neutrons, the mass number is 2 (protons) + 3 (neutrons) = 5.

13:Q: How would you represent this isotope of helium to show its atomic number and mass number?

14:A: You would represent it by the element’s symbol and both numbers, with the mass number on top and the atomic number on the bottom:
5 He
2

Question:
There are two atoms of nitrogen: atom A and atom B. Nitrogen has an atomic number of 7. Atom A has a mass number of 15 and atom B has a mass number of 16. Are the two atoms isotopes? Why or why not?

EXTRA HELP

What is an isotope? What is different about atoms A and B? Use information from paragraphs 10-14 for additional help.