Atomic number and atomic mass

[webplodder]

What is the difference between atomic number and atomic mass?

[swansont]

What is the difference between atomic number and atomic mass?

 

Atomic number is the number of protons in the nucleus. The atomic mass number is the number of protons + neutrons in the nucleus. The mass of a particular isotope is not just the mass of the protons plus mass of the neutrons, because the nucleus is a bound system and must release energy were it to be formed from its constituent particles, and E=mc^2. (The mass difference is known as the mass defect). But an element can have more than one stable isotope, so the atomic mass is the weighted average of the masses of the stable isotopes. (or, IIRC, the mass of the longest-lived isotope if there are no stable ones)

[SMF]

Apparently the way that elements will be displayed in the future in, for example, the periodic table will be changing to reflect different isotopes more accurately. The International Union of Pure and Applied Chemistry commission in charge of atomic weights are suggesting that atomic weights should be displayed as ranges, or more accurately as pie charts that represent the abundance of the different isotopes. There is a good short review of this proposal, its advantages, and some history regarding chemistry by committee in Science News, 2011, Jan 29, pp 5. SM

[webplodder]

Atomic number is the number of protons in the nucleus. The atomic mass number is the number of protons + neutrons in the nucleus. The mass of a particular isotope is not just the mass of the protons plus mass of the neutrons, because the nucleus is a bound system and must release energy were it to be formed from its constituent particles, and E=mc^2. (The mass difference is known as the mass defect). But an element can have more than one stable isotope, so the atomic mass is the weighted average of the masses of the stable isotopes. (or, IIRC, the mass of the longest-lived isotope if there are no stable ones)

 

 

Thanks for that swansont, but you've covered a lot of ground there. What do you mean by isotope?

[Pwyll Pendefig Dyfed]

Thanks for that swansont, but you've covered a lot of ground there. What do you mean by isotope?

 

An isotope means it has the same number of protons (i.e. atomic number - which defines it as that element) but the number of neutrons (and hence the atomic mass) varies. For example there are three isotopes of Hydrogen. Each has an atomic number of 1. One isotope has an atomic mass of 1 (1 proton, 0 neutrons), another isotope (deuterium) has and atomic mass of 2 (1 proton, 1 neutron); the third isotope (tritium) has an atomic mass of 3 (1 proton, 2 neutrons).

[swansont]

An isotope means it has the same number of protons (i.e. atomic number - which defines it as that element) but the number of neutrons (and hence the atomic mass) varies. For example there are three isotopes of Hydrogen. Each has an atomic number of 1. One isotope has an atomic mass of 1 (1 proton, 0 neutrons), another isotope (deuterium) has and atomic mass of 2 (1 proton, 1 neutron); the third isotope (tritium) has an atomic mass of 3 (1 proton, 2 neutrons).

 

That should be atomic mass number of 1, 2 or 3.

[Pwyll Pendefig Dyfed]

That should be atomic mass number of 1, 2 or 3.

 

An isotope means it has the same number of protons (i.e. atomic number - which defines it as that element) but the number of neutrons (and hence the atomic mass) varies. For example there are three isotopes of Hydrogen. Each has an atomic number of 1. One isotope has an atomic mass of 1 (1 proton, 0 neutrons), another isotope (deuterium) has an atomic mass of 2 (1 proton, 1 neutron); the third isotope (tritium) has an atomic mass of 3 (1 proton, 2 neutrons).

 

[swansont]

Mass has units. The mass of H-1 is not 1, the mass of H-2 is not 2. Those are the mass numbers. The mass of H-1 is 1.0078250 atomic mass units (using C-12 as exactly 12), and the mass of H-2 is 2.0141018 amu.

[Pwyll Pendefig Dyfed]

Mass has units. The mass of H-1 is not 1, the mass of H-2 is not 2. Those are the mass numbers. The mass of H-1 is 1.0078250 atomic mass units (using C-12 as exactly 12), and the mass of H-2 is 2.0141018 amu.

 

Ah, get you, sorry, I misunderstood you

I meant atomic mass numbers, but wrote it as atomic mass (as oppose to mass number)

Edited February 23, 2011 by ceribethlem

[lemur]

Mass has units. The mass of H-1 is not 1, the mass of H-2 is not 2. Those are the mass numbers. The mass of H-1 is 1.0078250 atomic mass units (using C-12 as exactly 12), and the mass of H-2 is 2.0141018 amu.

Is the amu mass of an element reflected in molarity? I think 1 mole of a substance refers to a large but specific number of atoms, and thus the mass of this amount of a substance would reflect the exact mass of each atom. Now I'm wondering how you would count the required number of atoms to test the mass of a single mole of atoms.

[Pwyll Pendefig Dyfed]

Is the amu mass of an element reflected in molarity? I think 1 mole of a substance refers to a large but specific number of atoms, and thus the mass of this amount of a substance would reflect the exact mass of each atom. Now I'm wondering how you would count the required number of atoms to test the mass of a single mole of atoms.

 

 

Not molarity exactly, that's to calculate the number of moles in solution.

 

Number of moles = mass / Mr

 

1 mole contains 6 x 10^23 particles - Avogadros number. This is the equivalent to the number of atoms in 12 grams of the Carbon - 12 isotope.

[mississippichem]

Is the amu mass of an element reflected in molarity? I think 1 mole of a substance refers to a large but specific number of atoms, and thus the mass of this amount of a substance would reflect the exact mass of each atom. Now I'm wondering how you would count the required number of atoms to test the mass of a single mole of atoms.

 

Careful, molarity isn't the same things as the number of moles.

 

Molarity is the number of moles of substance in a solution divided by the volume of the solution. Molarity is a concentration unit while moles are a "count unit".

 

[math]molarity = \frac{ moles}{sln. volume}[/math]

 

[math]moles= \frac{mass}{MW}[/math]

 

where "MW" is the molecular or atomic weight per 1 mole.

[lemur]

Thanks for the clarification on the term, "molarity." Still, my question remains how it is possible to weigh a mole of a substance? I suppose if you can isolate two substances, such as carbon and oxygen, and you combine them to make CO and nothing is left over, then you know there was the same number of atoms of each. I guess I should google avogadro to find out how he discovered the number of atoms in mole.

 

edit: It was hard to find a satisfactory, but it seems that Faraday was able to measure the charge of a single electron and when this number was divided into the charge of a molar quantity of some substance, the result came out as avogadro's number. I still don't see how Avogadro came up with it by measuring ideal gas weights and volumes, though. It seems like it would be an arbitrarily small number if you had no basis for establishing the exact size/mass of a single atom/molecule itself.

Edited February 23, 2011 by lemur