How does elements atomic mass differ from its mass number?

•Atomic mass is also known as atomic weight. Atomic mass is the weighted average mass of an atom of an element based on the relative natural abundance of that element's isotopes.

•The mass number is a count of the total number of protons and neutrons in an atom's nucleus.

Explanation:

Atomic Mass Versus Mass Number

1.The mass number is the sum of the number of protons and neutrons in an atom. It is a whole number.

2.The atomic mass is the average number of protons and neutrons for all natural isotopes of an element. It is a decimal number.

3.Atomic mass value sometimes change over time in publications as scientists revise the natural isotope abundance of elements.

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explanation:

metals have excess of electrons whereas non-metals have deficiency of electrons. thus, in order to gain stability metals easily donate their electrons to the non-metals.

for example, electronic configuration of sodium is and electronic configuration of phosphorous is .

when a phosphorous atom and sodium atom combine together then sodium will donate is extra one electron to the phosphorous atom. as a result there will be formation of ionic bond between sodium and phosphorous.

the reaction will be as follows.

there, are three sodium sodium atoms and each of them will donate its one extra electron to phosphorous atom. thus, the sub-shell of phosphorous gets completely filled.  

thus, we can conclude that electrons are transferred from atoms of sodium to atoms of phosphorous. this transfer makes the sodium atoms donate one electron and the phosphorous atoms to gain one electron. as a result, the sodium and phosphorus atoms strongly attract each other.