Question

a reference isotope is an isotope whose mass can be measured accurately and is used as the basis for a scale to compare the masses of other atoms.
understanding atomic mass
how is the mass of an atom determined?
carbon - 12 the isotope carbon - 12 is used as a standard to compare the relative masses of atoms. carbon - 12 has 6 protons, 6 neutrons, and 6 electrons. scientists have assigned carbon - 12 a mass of exactly 12 atomic mass units.
piano key to piano if you measure the mass of a piano with and without a piano key, you likely wouldnt see a difference between the two masses. the mass of a piano key is negligible compared to the mass of the whole piano.
even though the mass is insignificant, each key is essential to the function of the piano.
electron to atom similarly, the mass of an electron is negligible compared to the mass of a proton or neutron.
each electron is essential to the properties of an atom.
mass of proton 1.67×10^(-24) g
mass of neutron 1.67×10^(-24) g
mass of electron 9.11×10^(-28) g
atomic mass unit an atomic mass unit (amu) is one twelfth of the mass of a carbon - 12 atom.
1 amu = 1/12×12 amu 1 amu = about the mass of a proton or neutron
14 sep develop models each helium atom has two protons. sketch models of helium - 3 and helium - 4, which have approximate masses of 3 amu and 4 amu, respectively. label and differentiate protons, neutrons, and electrons in your models.

Explanation:

Helium - 3 has 2 protons, 1 neutron (since mass number = protons + neutrons, 3 - 2 = 1), and 2 electrons to balance the positive charge of protons. Helium - 4 has 2 protons, 2 neutrons (4 - 2 = 2), and 2 electrons. For the sketch, represent protons as positively - charged spheres, neutrons as neutral spheres, and electrons as small dots around the nucleus.

Answer:

For helium - 3: Sketch a nucleus with 2 positively - charged spheres (protons) and 1 neutral sphere (neutron). Draw 2 small dots around the nucleus to represent electrons. For helium - 4: Sketch a nucleus with 2 positively - charged spheres (protons) and 2 neutral spheres (neutrons). Draw 2 small dots around the nucleus to represent electrons.