The video tutorial explains the concept of mass number, focusing on helium. It defines mass number as the sum of protons and neutrons in an atom's nucleus. Helium has an atomic number of two, indicating two protons. The video discusses helium's isotopes, helium-3 and helium-4, highlighting that helium-4 is more common. The mass number of helium-4 is four, with two protons and two neutrons, while helium-3 has a mass number of three, with one neutron. The average atomic mass is a weighted average of these isotopes, leaning towards four due to the prevalence of helium-4. The video concludes with a summary of these points.

What is the mass number of an atom defined as?

• The number of electrons

• The number of neutrons only

• The number of protons plus neutrons

• The number of protons plus electrons

What is the atomic number of helium?

• 4

• 1

• 3

• 2

How many protons are found in all helium atoms?

• Four

• Three

• Two

• One

What is the mass number of helium-4?

• 5

• 3

• 4

• 2

How many neutrons does helium-3 have?

• One

• Four

• Two

• Three

What is the main difference between helium-3 and helium-4?

• Number of protons

• Number of neutrons

• Number of electrons

• Number of atoms

What does the average atomic mass of an element represent?

• The mass of the heaviest isotope

• A weighted average of all isotopes

• The sum of all isotopes' masses

• The mass of the most common isotope

Why is the average atomic mass of helium closer to 4.00?

• Because helium-3 is more abundant

• Because helium-4 is more abundant

• Because helium has no isotopes

• Because helium-4 is less abundant