Consider the stable elements through lead (Z = 82). In how many instances are the atomic weights of the elements out of order relative to the atomic numbers of the elements?

Video transcript

hey everyone in this example we're told that among the elements in the fourth period of our periodic table determine which elements have atomic weights that are not in order compared to their atomic numbers. So when we find period four on our periodic tables, we want to find the rows going across our periodic table at the number four where we have that row going across. And so to start off period four we would begin with the atom potassium with the atomic number 19. And we see that it has the corresponding atomic weight 39.10. Am us moving um Next across our period four we have the atom calcium which has atomic number 20 and has a higher atomic weight of point oh eight a.m. U. S. For our third element across period four we have scanned iem located in the D block with atomic number 21 which is a higher atomic number and a higher atomic weight of 44.96 A. M. U. S. So as you can see what they're explaining in the question is that as our atomic numbers are increasing the atomic weights across period four are also increasing in order Specifically in ascending order. Where we're getting towards greater values for both atomic weight and atomic number as we go more across to the right of period four. Now, what we should recognize is that within the D block we have the atoms cobalt and we have the atom nickel. And this is located within section eight B. Of the D. Block of our periodic table. So that would be the group number for these two atoms within the D. Block across period four. And we would recognize that for cobalt we have the atomic number 27. For nickel we have the atomic number 28. So those two values increase accordingly. However, when we look at the atomic mass of cobalt given we have a mass of 58.93 and when we look at the atomic mass and this is an AM. Us. When we look at the atomic mass for nickel we have a value of 58.70 A. M. Us. And as you can see, our trend of increasing atomic mass with increasing atomic number has ended here. And that is due to the fact that we have a lower atomic mass. Or sorry, a higher atomic mass for cobalt compared to nickel which has the higher atomic number. And so the atomic mass and the atomic weight for cobalt and nickel do not follow the order of the rest of the atoms in period four of our periodic table. And so this would be our answer to complete this example here the elements cobalt and nickel. So if you have any questions, please leave them down below. Otherwise, I will see everyone in the next practice video