Mass number is the sum of protons and neutrons inside an atom.
Atomic mass = no. of protons + no. of neutrons
Answer: The mass of electrons is mostly ignored because electrons are extremely small compared to neutrons and protons.
Explanation: A proton is about 1,836 times the size of an electron.
On the periodic table, the atomic number for each element can be found. This number is found by measuring the weight of 6.02 x 10^23 atoms of the element in grams. Electrons aren't ignored when finding exact math, but for the sake of simplification high school teachers will generally have you only count the number of protons and neutrons when calculating the mass of atoms.
The average the natural abundances of the various isotopes of carbon to arrive at the fractional mass.
Accelerate, decelerate, and changing directions.
