let's review a couple of terms
- atomic number = number of protons
- mass number is a sum of protons + neutrons.. it is a count of nucleons. not a true "mass"
- atomic mass is the mass of an atom. it includes the mass of protons + mass neutrons + mass electrons + mass defect.
- relative atomic mass is the weighted average isotopic mass of all stable isotopes of any given element. That is the number you see for atomic mass on a periodic table unless it is in ( ). In which case that element has no known stable isotopes and the isotopic mass of the most stable isotope is shown in ( ).
- molecular mass is the mass of 1 molecule. it is the sum of the relative atomic masses of the atoms in a molecule
- formula unit mass is the mass of 1 formula unit of an ionic compound. It is the sum of the relative atomic masses of the atoms in that formula unit.
- molar mass is the mass of 1 mole of whatever we're referring to. that is the mass of 6.022x10^23 individual units of whatever. 6.022x10^23 atoms, 6.022x10^23 molecules, 6.022x10^23 formula units... or whatever. it is mass of 1 mole (6.022x10^23) of ___.
in your example
atomic # = # protons = 2
mass # = # protons + # neutrons = 2 + 2 = 4
atomic mass = 4.00260325413(6)
the (6) means +/- 1 std dev = +/- 6 in the rightmost digits
it is NOT correct to say
mass electrons are negligible are aren't included in atomic mass
atomic mass He-4 = 4. That's the mass number not the atomic mass.
it is correct to say
atomic mass of He-4 is approximately equal to mass number = 4
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let's use an example. copper.
copper has these known isotopes
https://en.wikipedia.org/wiki/Isotopes_of_copper the known stable isotopes of copper are Cu-63 and Cu-65
the relative atomic mass of Cu reported on a periodic table = 63.546
that is calculated by 0.6915 * 62.9295975 + 0.3085*64.9277895 = 63.546
those atomic masses (aka isotopic masses) of Cu-63 and Cu-65 include masses of electrons
now this molecule
https://en.wikipedia.org/wiki/Butanone MEK (methyl ethyl ketone)
has molecular mass (it's a molecule right?) = 4*12.011 + 8*1.008 + 1*15.999 = 72.107
now this substance
https://en.wikipedia.org/wiki/Potassium_bromide note this substance is not molecular. it is a 3d lattice of K+ and Br- ions
it has no limit to how big it can get.. so we refer to the smallest repeating unit....
KBr as the "formula unit"
since it's not a molecular substance... it does not have a molecular mass
but it has formula unit mass = 1*K + 1*Br = 119.002
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next.. units
atomic, molecular and formula unit masses are commonly given with units of "amu"
or "daltons". Amu is atomic mass unit.
by definition, 1 amu = 1/6.022x10^23 g
by definition, 1 mole = 6.022x10^23 entities..
so ... 1 amu / atom = 1g / mole of atoms (note the #'s are the same, units are different!)
let's use examples.. and I'll abbreviate "relative atomic mass" as "atomic mass" and "relative molecular mass" as "molecular mass" etc. Both will refer to the weighted average mass from a periodic table. This is "conversational chemistry" as I like to call it.
copper.
atomic mass = 63.546 amu.... (i.e. 63.546 amu per 1 atom)
molecular mass = not applicable... not a molecule
formula unit mass = not applicable.. not an ionic substance
molar mass = 63.546 g / mole of atoms.... (i.e. 63.546g / 6.022x10^23 atoms)
notice the 63.546 amu/atom and 63.546 g/mole of atoms have the same numbers but different units? that is because of how amu's and moles are defined.
MEK
atomic mass = not applicable not an individual atom
molecular mass = 72.107 amu.... (72.107 amu per 1 molecule)
formula unit mass = not applicable.. not an ionic substance
molar mass = 72.107 g / mole of molecules... (72.107g per 6.022x10^23 molecules)
KBr
atomic mass = not applicable
molecular mass = not applicable
formula unit mass = 119.002 amu... (119.002 amu per 1 formula unit)
molar mass = 119.002 g / mole of formula units (119.002 g / 6.022x10^23 formula units)
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now to confuse things more we often drop the "of atoms" and "of molecules" and "of formula units" and let the reader figure out what "molar mass" refers to.
CH3OH.. methanol
molecular mass = 32.04 amu
molar mass = 32.04 g/mol (mol "of molecules" is understood)
MgO.. magnesium oxide (this exists as a repeating ionic compound in a crystal lattice)
formula unit mass = 40.304 amu
molar mass = 40.304 g/mol (mol of "formula units" is understood)
copper
atomic mass = 63.546 amu
molar mass = 63.546 g/mol (mole of atoms is understood)
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likewise, if I had a mole of 1000kg cars
car unit mass = 1000kg
molar mass = 6.022x10^26 kg/mol (the mass of 6.022x10^23 cars)
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I'm done.. ask questions if you need to.