Question

what is the mass of an atom that has 20 protons, 21 neutrons, and 20 electrons?
20 amu
41 amu
61 amu
40 amu

Explanation:

Step1: Recall mass - number formula

The mass number (A) of an atom is the sum of the number of protons (p) and neutrons (n). The mass of an electron is negligible compared to protons and neutrons, so it is not considered in the mass - number calculation. The formula is $A=p + n$.

Step2: Substitute values

Given that $p = 20$ and $n=21$, we substitute these values into the formula: $A=20 + 21$.

Step3: Calculate the mass number

$A = 41$. The mass of the atom is expressed in atomic mass units (amu), so the mass of the atom is 41 amu.

Answer:

B. 41 amu