[Valevalerio]

Hi there! I would like to apologize in advance for my English, I'm not a native speaker. I was trying to understand what mass is from a chemical point of view so there must be no forces involved. Considered that electrons have negligible mass, can I say that an atom that has 2 neutrons and 2 protons has a mass of 4? If so what's the unit of measure that must be used?

I don't understand the formula mass=molar mass*moles. To me, the mass is just how many protons and neutrons there are in an object so mass=atomic mass*number of atoms. Does anybody care to explain?

Am I overthinking? Maybe the mass of an object is just how many times it balances the cylinder of platinum-iridium located in a museum in France. Like they put the cylinder on one side of the scale and the object to be measured on the other side. If they perfectly balance each other then that object has a mass of 1 kg. Otherwise it would be a multiple or submultiple of 1 kg.

[MNIO]

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

*******
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

********
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)

******
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)

******
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)

******
I'm done.. ask questions if you need to.