what is the mass number of an atom that consists of 20 protons, 20 neutrons, and 18 electrons?

what is the mass number of an atom that consists of 20 protons, 20 neutrons, and 18 electrons...

a) 18

b) 20

c) 38

d) 40

plz explain ur reasoning...thanks :)

4 Answers

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  • 8 years ago
    Best Answer

    20+20=40

    Mass no is the actual mass of the nucleus of the atom that consists of neutrons and protons.

  • Dr W
    Lv 7
    8 years ago

    wow.. 3 out of 3 got it wrong so far...

    ***********

    ATOMIC MASS is exactly that. It is the mass of an atom. It includes the mass of protons, electrons, neutrons, mass defect, etc. It is NOT found on a periodic table

    RELATIVE ATOMIC MASS. that is the number found on a periodic table. It is the weighted average atomic mass over all naturally occuring isotopes of that element. Weighted by relative abundance. It is the not the mass of any one atom EXCEPT if the number is in ( ). In which case, there are no known naturally occuring isotopes of that element and the number shown is is the mass of the isotope with the longest half life.

    MASS NUMBER. is not a "mass" at all. It is NOT the mass of the nucleus of an atom. It is simply the # protons + # neutrons. It is merely a way of defining which isotope you're dealing with. Mass # is NOT found on a periodic table.

    **************

    for this problem...

    mass # = # protons + # neutrons = 20 + 20 = 40.

    *************

    Don't believe anyone who tells you electrons don't count in atomic mass! That isn't true at all.

    Don't believe anyone who tells you mass # is the mass of anything. It isn't mass.

    Don't believe anyone who quotes relative atomic mass as mass number. They are not the same thing.

  • DaM
    Lv 6
    8 years ago

    The actual atomic mass of that isotope of calcium is 39.9626

    EDIT: That's true, we blew it. Ouch. The mass number (A) is "A characteristic property of a specific isotope of an element, equal to the sum of the number of protons and neutrons in the nucleus." so, 40.

    EDIT2: I didn't blow it really. My number was listed under the heading "atomic mass or weight", I didn't claim it was the mass number. The mass number is an approximation to the atomic mass.

    Source(s): CRC Handbook of Chemistry and Physics
  • maussy
    Lv 7
    8 years ago

    Electrons are not important for mass

    Add only sum of protons (called Z) and some of neutrons (called N)

    so answer 20+20=40

    d is correct

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