Why does the atomic weight decrease when the atomic number increases from cobalt to nickel and from tellurium to iodine?

Answer 1

The atomic weight is found by taking the Atomic Mass of each isotope, weighted by that isotopes abundance in nature. And so the reason for the drop in atomic weight is the distribution of isotopes for those elements is different.

Cobalt just has one isotope with any significant abundance: cobalt-59. Nickel on the other hand has several isotopes with significant abundances: nickel-58 (68%), nickel-60 (26%), nickel-61 (1%), nickel-62 (4%), nickel-64 (1%). Because the largest contributor to the atomic weight of nickel is the nickel-58 isotope, which is lighter than cobalt-59, the overall atomic weight comes out light despite nickel having an extra proton.

It is the same thing with tellurium and iodine. Tellurium has numerous isotopes with significant abundances: 122Te (2.6%), 123Te (1%), 124Te (5%), 125Te (7%), 126Te (19%), 128Te (32%), 130Te (34%). Iodine only has one isotope with any significant abundance in nature: 127I. As before, the larger abundances of the heavier isotopes of Te tend to shift its atomic weight to be heavier, where-as the only isotope of iodine is relatively light.

See the Web Links to the left for a complete table of the isotopes and their abundances in nature. Using this sheet, you can actually prove to yourself why this strange exception to the trend of increasing atomic weight happens!

Answer 2

As Atomic Number increases, the number of electrons around the nucleus goes up also. As the size of the electron cloud increases, the outer electrons are more shielded from the protons, so it requires less energy to overcome the positive-negative attraction forces. The outer electrons are also in a higher energy state to begin with.

Answer 3

Because there are more protons in the nucleus so the nucleus has a stronger positive charge and exerts a stronger attractive force on the negatively charged electrons, pulling them in closer to the nucleus, therefore reducing the atomic radius (size).

Answer 4

It doesn't. Atomic weight goes up as the atomic number goes up. In fact, atomic weight goes up faster than the atomic number does.

Hydrogen atomic number 1, atomic weight 1.0079g/mole

Uranium atomic number 92, atomic weight 238.02891g/mole

Answer 5

Atomic mass depends on the number of protons and neutrons. As the number of protons and neutrons increases, so does atomic mass. The larger number of protons in the nucleus attracts the surrounding electrons more. This is a sort of 'pulling' phenomenon that decreases the radius of the atom.

Answer 6

Because the effective nuclear charge increases.

Across a period, the atomic number increases, the effective nuclear charge increases, So the force of attraction between the nucleus and the valence electrons increases, and the atomic size decreases.

Answer 7

Atomic size decreases even as atomic number and mass increases (within a period) because a greater number of electrons means a greater amount of attraction between the electron cloud and the positively charged nucleus. This increase in electronegativity draws the outer level of the atom a bit closer to the nucleus, making the overall size of the atom smaller.

Answer 8

Negatively charged electrons and positively charged protons are attracted to each other. As you go right on the Periodic Table, the number of electrons increases. Therefore, the level of attraction between the protons and electrons increases, pulling the atoms closer together and decreasing the size of the atom.

Think of two magnets. If you put them close enough, they'll stick together. Some magnets are stronger than others. They stick to each other better than weak magnets.

The more electrons there are, the stronger the "magnet" is.

Answer 9

This is because there is an increase in positive charge in the nucleus.