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Chemistry LibreTexts

Homework 33

  • Page ID
    28948
  • Question 1

    Determine which atom is larger in each pair of elements.

    A) Na or Mg

    B) Ga or Al

    C) As or Cs

    D) Br or Fe

    Things you need to complete this problem:

    To complete this type of problem you will need a periodic table of elements handy to easily access each element to be able to tell where it is located in comparison to other elements.

    Strategy:

    To complete this problem there are a few rules and guidelines you will need to know about before starting.

    Firstly, we need to be able to locate each element in comparison to the other on our periodic table of elements. We know when we look at the periodic table that there are rows (horizontal) and there are columns (vertical).

    For this problem, we are locating elements and determining their atomic radius to see which is larger of the elements chosen. The atomic radius for each individual element is defined as a measure of the size of each individual atom, normally the mean (average) or the distance from the middle of the nucleus to the boundary of the neighboring cloud of electrons.

    The rules for deciding which element has a larger atomic radius are the following:

    It decreases as you go left to right on the periodic table because of the greater amount of protons. The higher the atomic number, the greater number of protons. The protons will pull the electrons closer to its nucleus thus resulting in a smaller atomic radius. It increases as you go down the group because as you go down the group, you add another principal energy level. The more energy levels, the further the electrons will be from the nucleus resulting in a a smaller atomic radius. Also, the elements in the same column as they move down closer to the bottom of the table, the radius gets larger. For example, if you look at the two elements: Iodine and Bromine, out of the two elements Iodine would have the larger radius because it is located below Bromine on the periodic table.

    So to recap: On the periodic table the elements starting from the left side have higher atomic radius, it decreases as you go across to the right and as you go down the table the radius increases. So usually, the closer to the top of the periodic table in comparison to the ones below it is smaller in radius!

    Answers:

    A) Na or Mg:

    If we look at the periodic table to the far left you will find both Sodium and Magnesium. If you look in the first row you will find sodium and to the right of it you will find magnesium. When looking back at our rules, left to right the atomic radius decreases, thus sodium has a higher atomic radius!

    Na is larger than Mg.

    B) Ga or Al

    When locating the two elements on the periodic table you will notice these two are in the same column. From our rules we know that the further down the element is on the periodic table the larger the radius! Thus, out of the two elements Gallium is the larger of the two when it comes to the atomic radius!

    Ga is larger than Al.

    C) As or Cs

    With Cesium on the first row, and almost all the way down the table of elements, we assume from our rules listed above that Cesium carries a high atomic radius. We locate the As which is two rows below and all the way to the right side of the table. The rules state left to right atoms decrease, and down the gradient of the table they increase. So for both cases of these rules makes Cesium have a larger radius thus making it the larger atom.

    Cs is larger than As.

    D) Br or Fe

    Located on the same row, we find that Iron is a located closer to the left of the periodic table than our other element Bromine. The rules state once again that as elements move left to right the atomic radius decreases, making bromine the smaller atom because of its location on the periodic table compared to Iron. Iron in this case is the bigger atom.

    Fe is larger than Br.

    Question 2

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