AT_Periodic Table Lesson

Periodic Table

By this point, you have probably figured out that the periodic table is an integral tool of any chemist. Let's learn a little bit about its history before we explore the details.

History of the Periodic Table

Dmitri Mendeleev (1834-1907) was a Russian chemist. Although he was not the only scientist working on the creation of the modern periodic table, he is generally given the credit. German physicist Julius Meyer (1830-1895) was also creating a periodic table at the exact same time. Mendeleev published his results just a few months before Meyer, and the rest, as they say, is history. By arranging the known elements in order of increasing atomic mass, patterns developed. Certain properties began to repeat themselves in regular intervals. The periodic table that Mendeleev created is arranged into periods and groups. Groups of elements have similar properties because of their similar electron arrangements.

Mendeleev left gaps in his table that would later be filled in with elements that were not yet discovered during his creation of the table. Mendeleev arranged his periodic table by increasing atomic mass, and this gave him some problems. For example, iodine has a lower atomic mass than tellurium, so it should come before tellurium in Mendeleev's table. But in order to get iodine in the same group as other elements with similar properties such as fluorine, chlorine and bromine, he had to put it after tellurium. In 1913, the British chemist, Henry Moseley rearranged the periodic table in order of increasing atomic number instead of increasing atomic mass. This actually solved these anomalies that Mendeleev himself had predicted!

Organization of the Periodic Table

The first thing you probably noticed about the periodic table is that at the bottom of the periodic table there are 2 additional rows. These are actually part of periods 6 and 7. Notice the atomic numbers fit in with the elements in periods 6 and 7.

image of the periodic table

If we put all of the elements in periods 6 and 7 in one horizontal line, this is what the periodic table would look like:

image of wide periodic table

On the periodic table, metals, nonmetals, and metalloids are each grouped together.  

Metals are elements that are generally solids at room temperature (the only exception is mercury). They are ductile, malleable, conduct electricity, and they conduct heat very well. Nonmetals are most often found as parts of compounds. Some of them are solids at room temperature, while others are gases. One nonmetal element is a liquid at room temperature (bromine). Metalloids are found on the periodic table along a stair step line that divides the metals from the nonmetals. In many ways, metalloids behave as nonmetals, but they are more like metals concerning their electrical conductivity. Metalloids are known as semiconductors. They do conduct electricity, but not as well as metals.

image of corrected periodic table

Groups

In addition to being divided by metals, nonmetals, and metalloids, the periodic table is also arranged by family groups. Just like our families, element families have many characteristics in common.

image of the periodic table organized into families

Test your knowledge with this activity. Read the description at the bottom left and click on the matching location in the image.

Forming Ions

Recall that groups of elements that have the same number of valence electrons. Also recall which group of elements is the most stable. There is actually a relationship between these. There is an inherent stability in having an electron configuration that is like those of the noble gases, s2p6 or 8 valence electrons. This is known as the Octet Rule. (H, Li, Be, & B are exceptions to this rule as they will form ions that have the same electron configuration as He. He only has two valence electrons.)  

So, when atoms lose or gain electrons, they do so in an attempt to have the same electron configuration ending as the noble gases, or 8 valence electrons. When an atom loses or gains electrons, it becomes an ion. Let's look at sodium, for example.

What is the electron configuration of sodium? 

Answer: 1s², 2s², 2p⁶, 3s¹

How many valence electrons does it have? 

Answer: 1

So, when sodium forms an ion, the attempt is to have 8 valence electrons and the configuration of a noble gas. Let's look at the ways sodium can achieve this.  

Na has 1 valence electron. (Diagram 1) If it were to gain 7 electrons, it would have the full octet and have a charge of -7. (Diagram 2) But, there is another way to achieve this. What if Na loses just one electron? Look at what is left behind. (Diagram 3) Which of these two scenarios is more likely? Remember that Na has 11 protons in its nucleus and, in a neutral atom, that nucleus is pulling on 11 electrons. If Na were to gain 7 electrons, you would have 11 positive protons trying to hold on to 18 negative electrons - that would not be possible. Instead, it makes more sense for Na to lose one electron. That way the 11 protons are only holding on to 10 electrons AND it has an octet. Na Diagram 1 Na-7 Diagram 2 Na+1 Diagram 3

 

If you follow this method for each element, you will see a trend emerge. Below is what each group of representative elements does to become a stable ion.

image of the periodic table with gaining or losing an electron

Look closely at the chart above and complete the following statements:

  1. Metals_____  electrons to become stable. (Answer: lose)
  2. Nonmetals _____  electrons to become stable (Answer: gain)

Charges of the Ions Formed

Now we need to determine the charge of each ion formed. All you need to do is start with the charge of an atom (zero) and add or subtract the number of electrons either gained or lost.

Charge of ion = Charge of atom - (# of electrons lost)(charge of electron)

Charge of ion = Charge of atom + (# of electrons gained)(charge of electron)

So for the alkali metals,

Charge of ion = 0 - (1)(-1) = +1

When these atoms lose 1 electron, they form a +1 ion.

See if you can figure out the charges of the ions formed by the other representative elements (s and p block), then look below to see if you were correct.

image of periodic table including charges

Positive ions are called cations. Negative ions are called anions . (This is easy to remember because the + sign is present in the word ca t ion!)

Ions Formed By Transition Metals

Did you notice that we left out the transition metals? Why did we leave them out? The answer is that they don't follow the same pattern. Let's take a look.

When an ion forms, the first electrons to be lost come from the outer energy level. As electrons are removed from an energy level, they come from the highest energy subshell first. This means that the f subshell is emptied before the d subshell, etc.

Transition Metal Ionization - Iron The electron configuration for iron (Fe) is [Ar] 3d6, 4s2 When an ion is formed, the first electrons lost are the 4s2 electrons. For the metals of group IIA (the Alkaline Earth metals), losing these s-sublevel electrons gives them the same electron configuration as a noble gas. But, that doesn't happen for Fe, instead it still has 6 electrons in its outer shell. [Ar] 3d6 - this creates the Fe+2 ion If Fe were to lose another electron it would have this configuration: [Ar] 3d5= this creates the Fe+3 ion Why would Fe lose another electron? Doing that leaves the d orbital half full which is also a stable configuration. In the end, neither of these ions has the same configuration as a noble gas, so iron may form the +2 ion or the +3 ion.

Transition metals and post-transition metals (those metals that are in the p-block) often form ions of multiple charges, just like is shown here with Iron.

Electron Configuration of Ions

As should become increasingly obvious, the distribution of electrons is very important to determining the actual behavior of an atom or ion. Let's look at some ions more closely. We can predict not just how many electrons are lost or gained when an ion is formed, but also specifically the energy level and sublevel of those electrons.

Let's look at sodium.

Na 1s2, 2s2, 2p6, 3s1

When Na forms an ion, it loses 1 electron. So, the electron distribution is now:

Na 1s2, 2s2, 2p6

Let's look at chlorine.

Cl 1s2, 2s2, 2p6, 3s2, 3p5

When Cl forms an ion, it gains 1 electron. So the electron distribution is now:

Cl 1s2, 2s2, 2p6, 3s2, 3p6

Remember to work on the module practice problems as you complete each section of content.  

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