Why aren't all of the atomic mass numbers in the periodic table whole numbers?

Answer 1

Each atom of an element isn't always exactly the same as another; some can have slightly more mass, and some can have slightly less than others. Atoms with different masses are called isotopes, and in any naturally-occurring sample you get a certain proportion of each isotope. Then the Atomic Mass is actually the mass of each isotope, multiplied by the proportion of that isotope you'd get in a sample, all added together.

For example, chlorine has two isotopes, chlorine-35 and chlorine-37, but in a natural sample you'd get 75% chlorine-35 and 25% chlorine-37. So chlorine's atomic mass is 35*75/100 + 37*25/100. If you work that out, you'll get 35.5, which is the figure on the Periodic Table.

Answer 2

The weights on the table are averages because elements may have isotopes (atoms with different numbers of neutrons). Explained Further Elements don't have whole numbers for their atomic masses because the atomic mass of an element must reflect the amounts of different isotopes of that element that occur in nature. A given element always has a fixed number of protons in its nucleus. That how we identify that element; it's what gives it its elemental identity. But the number of neutrons in a given element's nucleus can vary. Atoms of a given element that have different numbers of neutrons represent different isotopes of that element.

If we look at hydrogen, we see it's the simplest of all the elements. It's basically one proton with an electron hanging around. Problem is, if we look at a whole bunch of hydrogen atoms, we find one every once in a great while that has a neutron hanging on to the proton in the nucleus. That atom of hydrogen has about twice the mass of a "regular" hydrogen atom. It's a distinct isotope of hydrogen. There are even some atoms of hydrogen that have two neutrons hanging on to that proton in the nucleus. The atoms of hydrogen with the neutrons are rare, but the difference in their masses and the fraction of the naturally occurring element that they make up must be accounted for.

As we go on up the periodic table to heaver elements, we find that pretty much all the elements have several isotopes that occur in nature, and some have significant portions distributed over two or more isotopes. Take Boron. It's atomic number is 5. How many neutrons are in a boron atom? Well, roughly 80% have 6 neutrons while about 20% have 5. When we weigh a sample of boron in the lab, we need to account for that.

If we look at tin, we find it has 10 naturally occurring isotopes! And we have to account for them all when we have a sample of tin because they are each in there in some amount. That's why the atomic masses of the elements aren't whole numbers. It would be simple if they did, but science demands we account for the variables in nuclear structure.

Another explanation is that protons do not have exactly the same mass as neutrons, though they are close, and electrons have less than an 18 hundredth the mass of a proton. All atomic mass units are normalized to the mass of carbon-12, which has a molar mass of 12 grams per Avogadro's number of atoms. Since the ratio of protons, neutrons, and electrons for other elements is not the same as carbon-12, you can expect slight variations in mass.

Additional detail Another reason is, except for carbon-12 (which is the standard and by definition has a mass of 12 amu exactly), the actual masses of atoms aren't whole numbers. The binding energy holding the nucleus together throws the total off slightly from what you would expect, due to energy-mass equivalence (E=mc2).

Answer 3

1. First cause: the atomic weight is the sum of the weights of protons, neutrons ans electrons.; they don't have masses as integers.

2. Second cause: also occurs the so-called mass defect.

Answer 4

The atomic number is always whole, atomic weight is not