The three naturally occurring isotopes of magnesium are: Mg-24 , Mg-25 and Mg-26.
The atomic mass and natural abundance of Mg-24 and Mg-25 are 23.9850 amu, 79% and 24.9858 amu, 10% respectively.
Let the abundance of Mg-26 be X thus, sum of natural abundance of all three isotopes will be 100 or,
%Mg-24+%Mg-25+%Mg-26=100
Or,
79%+10%+X=100
or,
X=11%
Therefore, natural abundance of Mg-26 is 11%.
The answer is: two atoms have same number of protons and neutrons, but different number of electrons.
Mass number (A) is the total number of protons and neutrons in a nucleus.
Atomic mass (ma) is approximately equivalent to the number of protons and neutrons in the atom.
For example, chlorine atom (Cl) and chlorine anion (Cl⁻).
Similarities:
1) atomic number is the same. Both, chlorine atom and anion, have atomic number 17. They have 17 protons in nucleus of an atom.
2) both are the same element.
Difference:
1) chlorine atom (Cl) has neutral change and chlorine anion (chloride ion Cl⁻) has negative charge, because chlorine anion has one electron more.