So what is a mole? An Avogadro's constant? 6.023E23? # of particles in 12 grams of 12C?
Nah son. Those answers do nothing but create *%$ loads of other questions.
It is a number of atoms (or molecules) when the sum of the neutrons and protons in that particular atom (molecule) is numerically equal to the mass of that number of atoms.
Why no give real definition chem text books? That answer solves almost every single mystery surrounding moles, AMU, gm-mole, lb-mole and molar mass. AND it's not dependent on system of units.
More facts and explanation
There s a relationship between the mass and moles (number of basic building blocks of matter atoms, molecules ions etc..., identified as atoms for the sack of brevity ). Mass of the matter is related to the number of atoms by it's atomic weight. They never told you explicitly but the atomic weight is arbitrary assignment of only logical when considered relative to different types of molecules. Hydrogen has i proton (P) in it's nucleus and oxygen has total of 16 [8 N(Neutrons) + 8 P] making it 16 times as heavy as hydrogen atom. Now it doesn't matter how many number of oxygen atom you'll take, the total mass of oxygen atoms is always going to be 16 times heavier than the mass of the that many hydrogen atoms. Now they went ahead and mass of an single atom a new unit (Atomic mass unit) which is mass of a proton or neutron. So 1 AMU = mass of a proton/neutron. Now mass of the different are directly related to number of the Ns and Ps they have and are relative to each other.
Now it is possible to find the number of atoms where the total mass (in a different unit of mass, say gram) of the atoms will be numerically equal to the total number of Ns and Ps in that particular atom. Furthermore that number of atoms for a different element will be numerically equal to the total number of Ns and PS in that element. This sounds like a magic. Let's explore the above sentences numerically:
Hydrogen: Atomic mass = 1 AMU,
Oxygen: Atomic mass = 16 AMU
Hence mass of an oxygen atom/mass of a hydrogen atom = 16/1 =16
or mass of x atoms of oxygen/mass of x hydrogen atoms = 16*x/1*x = 16.
In other words ratio of the masses of different elements is constant as long as the number of atoms are equal. or if we flip it back, the mass of a number of the atoms of different elements will be always related by the ratio of their atomic masses. So when mole was defined, it was nothing but the attempt o find x where the total mass of x atoms equals atomic mass of that element in grams. The reason behind the mole has the same number of atoms regardless of the element is because the elements mass are defined relative to each other where 1 atom of hydrogen weighs 1 amu arbitrarily and all other elements are integer multiples of hydrogen atom mass.
Now on Volume and moles:
The equal number of atoms of different gaseous atoms at same pressure and temperature will occupy the same volume. 1 mole of any gas at stp will always occupy 22.4 L of space.
More on this coming up:
