Calculate the average mass for iron. 15.2% iron-55, 85.8% iron-56

Iron 55 (Fe-55) is the iron isotope whose nucleus consists of 26 protons and 29 neutrons. It is a radioisotope that disintegrates by electron capture in manganese 55.

Explanation:

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since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

the answer is b. dilution