# Relative atomic mass number/mass number

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#1
Are these the same thing, or are they different? I have different books which use both to describe the top number of an element on the periodic table. Thanks
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6 years ago
#2
(Original post by daviem)
Are these the same thing, or are they different? I have different books which use both to describe the top number of an element on the periodic table. Thanks
They're subtly different – the relative atomic/molecular mass has no unit because it's the mass of 1 mole of atoms (units: mass) divided by 1/12 the mass of 1 mole of carbon-12 (units: mass). Because you're dividing a mass by a mass, the resultant is unit-less.

The atomic/molecular molar mass is just the mass of 1 mole of an atom/molecule. It has units of mass, so grams usually.

Of course they have the same numerical value, just the units that are different.

Edit: Just realised, you said something different again! The mass number of an atom is the number of protons + the number of neutrons. It is again not a mass, as it's just the number of particles in the nucleus. I'd think of that, and the relative atomic mass ass the same thing, especially if you're doing GCSEs! They're only different because a neutron has a ever so slightly greater mass, but you can definitely ignore this unless you were doing some very accurate experiment!

Scientists get very anal about terminology like this; the words have very precise meanings even though in many, many cases you'd be able to interchange the terms.
0
6 years ago
#3
(Original post by daviem)
Are these the same thing, or are they different? I have different books which use both to describe the top number of an element on the periodic table. Thanks
Relative atomic mass is the mass of an average atom of an element (taking into account all of the isotopes and their abundances), compared to the mass of a 12-C isotope that is assigned a mass of 12.00000 (i.e. exactly 12).

As mentioned above it has no units, being a relative measurement.

The mass number is just the sum of the protons and the neutrons in one specific isotope of an element.
0
6 years ago
#4
The term mass number is applied to isotopes e.g. chlorine has 2 isotopes - 35Cl and 37Cl but its relative atomic mass is 35.5 which takes into account the amounts of each isotope present in naturally occurring chlorine
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#5
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