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Why were there Changes in Periodic table? : In Brief

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One of the science blogs I read started off with news of the Periodic Table being changed by the IUPAC in December 2010. The chart is changed every time someone creates a new element in a laboratory setting, but this change is more fundamental than documenting an obscure, unstable element that very few people care about. The molecular weight of ten common elements was changed from a fixed number to a range. Why?

To understand why the Periodic Table was changed, we need to go back and review a little bit of atomic theory.

Protons and the atomic number define the element.

Each atom has a certain number of positively charged ions called protons. The number of protons in an atom determines the characteristics of that element, its name, and its place on the periodic table. The number of protons in an atom is that atom's atomic number. The chart is arranged in ascending order of the atomic numbers, from the first atom, Hydrogen with one proton, Helium with two protons, Lithium with three protons, and so on.

For the sake of this explanation, protons have a weight of 1. (The units are atomic mass units. )

Electrons are the next most important subatomic element.

Electrons are negatively charged ions, and they have very little weight. Their weight in scientific notation goes out to ten to the minus 19. So, for many purposes, such as calculating the weight of an element and for this explanation, you can ignore the weight of the electrons. A neutrally charged element has the same number of electrons as there are protons.

Ions are charged particles. If an atom has a different number of electrons than it has protons, then that atom is also an ion. Atoms with less electrons than protons are positively charged and are called cations. Atoms with more electrons than protons are negatively charged and are called ions.

While the number of protons in an atom identifies the element, the number of ions in the element contribute to how reactive the element is.

Neutrons are part of the atom's nucleus.

Another subatomic particle, besides the proton and electron, is called the neutron. A neutron weighs slightly more than a proton, but the difference between the weights is small (about the weight of an electron). The neutrons reside in the nucleus of the atom with the protons.

Remember, the name and characteristics of the element are related to the atomic number, the number of protons, in the element. The number of neutrons in the atom does not change the properties of the atom much. (Obviously, it changes the weight of the atom, because for every neutron, the weight goes up by 1.)

Carbon has an atomic number of 6. That means it has 6 protons. A neutral carbon atom has 6 electrons. How many neutrons does it have? Well, some carbon atoms have 6 neutrons. Some carbon has 8 neutrons per atom. Carbon with 6 neutrons is called Carbon-12. Carbon with 8 neutrons is called Carbon-14. Both of these forms of carbon are fairly stable and exist naturally in our world. (Over time Carbon-14 loses its two "extra" neutrons and turns into Carbon-12. This is a slow and steady process that has a constant known rate and carbon dating is based on this constant rate.) Carbon-12 and Carbon-14 are isotopes. Isotopes are atoms with the same number of protons but a different number of neutrons.

The atomic weight equals the weight of the protons, neutrons, and electrons.

Carbon with 6 neutrons has a weight of 12. Six protons (1) + six neutrons (1) + 6 electrons (0) equals twelve. Carbon with 8 neutrons has a molecular weight of 14. Both forms of this element exist in nature. Up until this recent change, Carbon was given a molecular weight of 12.0106 grams per mole, to average in the weights of both isotopes.

How does this relate to the IUPAC's changes?

The IUPAC recently changed the Periodic Table. Instead of Carbon being listed as 12.0106 grams per mole, it will have two numbers: 12.0096 and 12.0016, because those are the weights of carbon in its most common ratios (of Carbon-12 and Carbon-14).

Overall, I think this is a wonderful change to Mendeleev's famous chart. However, I am glad that I am out of school and don't have to memorize the ten additional numbers.
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  1. [FE].Zatak's Avatar
    good post .. ty
  2. PsP's Avatar
    Welcome ;D
  3. aaron's Avatar
    nice about the Periodic table