Magnesium has three stable isotopes. The most commonly occurring isotope, 24 Mg, has an isotopic mass of 23.985 u and makes up 78.99 % of naturally occurring magnesium atoms. The isotope 25 Mg makes up 10.00 % of magnesium atoms and has an isotopic mass of 24.986u. The isotope 26 Mg makes up 11.01 % of magnesium atoms and has an isotopic mass of 25.983 u. Using the isotopic composition provided, calculate the average atomic mass of magnesium.

Average atomic mass = 24.3051 amu

Explanation:

Average Atomic Mass = (Mass of Isotope 1 x Fractional Abundance of Isotope 1) + (Mass of Isotope 2 x Fractional Abundance of Isotope 2) +

For 24 Mg

Mass = 23.985 amu

Fractional Abundance = 0.7899

Mass * Fractional Abundance = 18.9458

For 25 Mg

Mass = 24.986 amu

Fractional Abundance = 0.10

Mass * Fractional Abundance = 2.4986

For  26 Mg

Mass = 25.983

Fractional Abundance = 0.1101

Mass * Fractional Abundance = 2.8607

Average atomic mass = 24 Mg + 25 Mg + 26 Mg

Average atomic mass = 18.9458 + 2.4986 + 2.8607

Average atomic mass = 24.3051 amu

the answer is c.the valence of the atoms combine .