Hello asked in Science & MathematicsChemistry · 7 years ago

# Chemistry help? 3 Questions?

3. An electrically neutral atom of oxygen has eight electrons. What other facts do you know for sure about this atom?

There are eight neutrons in the nucleus.

There are eight protons.

There is a charge of +8 on an atom of beryllium.

There is a charge of 8 on an atom of beryllium.

8. An atom of gallium has 31 protons and a mass number of 70. What is the atomic number for an atom of gallium?

31

35

39

70

9. Mercury has an atomic number of 80 and a mass number of 200. How many neutrons are in one atom of mercury?

40

80

120

200

Please explain how to do the last 2! Thank you!

Relevance
• 7 years ago

3. We know the atom is electrically neutral, so the number of electrons must equal the number of protons to make the atom neutral (recall that electrons and protons are equal but opposite in charge). Therefore, there must be eight protons in the atom to balance with the eight electrons.

8. The atomic number is simply equal to the number of protons. The number of protons (the atomic number) is the defining characteristic of an element... atoms of a certain element can vary in number of neutrons and electrons but they ALWAYS have the same number of protons, so this is their atomic number. Therefore, the atomic number is 31.

9. # of neutrons = Mass number - atomic number.

Therefore the number of neutrons in one atom of mercury is 200-80 = 120.

To further elaborate on this, it is mainly protons and neutrons that make up the mass of an atom, electrons have a negligible mass. Protons and neutrons have roughly the same mass, which is about 1 amu. So, the mass number (in amu) of the atom is simply the mass of protons + the mass of neutrons. Since all protons and neutrons are about 1 amu in mass, we can also say that the mass is equal to the number of protons + the number of neutrons. Since the number of protons is also defined as the atomic number, the mass number can finally be expressed as the atomic number + the number of neutrons. This gives us the method to solve this question.

Hope these little rants help to somewhat clarify why the answers are what they are :)

• mcgray
Lv 4
4 years ago

Mass Number Facts