By understanding how elements are arranged in the periodic table, we can begin to understand why they do this—why they always combine in fixed ratios. We can also begin to predict in what ratios they will combine.
The history of the Periodic Table is, perhaps, fascinating. Students of chemistry would serve themselves well by researching the topic and seeing how Mendeleev first constructed the table. Even with, in the 1860s, the incomplete knowledge of the elements, he was able to arrange them into something useful.
The first thing he did was put them in order by increasing mass from left to right and top to bottom. Thus, beginning at top left and moving across the rows, then down, the elements increase in atomic mass sequentially.
Secondly, he put into columns those elements that behaved similarly in reactions and that had other similar properties. Where there were missing elements in his time, he left spaces, and in time, those spaces were filled in with newly discovered elements.
In time, new discoveries were made and the Periodic Table was adapted to incorporate them. Much information was gained by the discovery of protons and eventually electrons.
The Modern Periodic Table
The modern Periodic Table arranges the elements in order by increasing numbers of protons.
The number of protons is called the Atomic Number. This is the order by which the modern periodic table is arranged.
The columns became organized to match the arrangement of electrons in the various orbitals. This is very important because an element's electron configuration determines how it will interact with other elements.
The number of protons is called the Atomic Number. This is the order by which the modern periodic table is arranged.
The columns became organized to match the arrangement of electrons in the various orbitals. This is very important because an element's electron configuration determines how it will interact with other elements.
Each column of similar elements is called a group and each element is in the group because it has a similar electron configuration.
Each row of the table is called a period. The periods correlate to the orbitals that are sequentially filled by electrons.
The Periods (Rows) and Groups (Columns)
For any element, all electrons must exist in a set, specific orbital—though exactly where in that orbital is a matter of probability, not certainty. Various orbitals have different energy levels and can hold only a certain number of electrons:
For any element, all electrons must exist in a set, specific orbital—though exactly where in that orbital is a matter of probability, not certainty. Various orbitals have different energy levels and can hold only a certain number of electrons:
Energy Level Number of Orbitals Maximum Electrons 1 1 2 2 4 8 3 9 18 4 16 32
The elements of the Periodic Table are arranged into periods (rows) based on which orbitals exist in an atom of that element.The period number is equal to the number of orbitals that are present.
The elements of the Periodic Table are organized into groups (columns) based on how the electrons are filling the available orbitals.
The first period represents the filling of the first, lowest-energy-level orbital. Recalling that each orbital holds only two electrons, it follows that only two elements (H and He) can be in the first period (and located in the first row of the Periodic Table).
Periods 2 and 3 (rows 2 and 3) each add a set of orbitals in the second energy level. Thus, row two is made up of the elements has a filled the single orbital of the first period and is filling 4 more orbitals. The elements on row three have all of the electrons on the first orbital, all of the electrons on the first set of four orbitals from period 2, and is filling four more orbitals.
Period 4 begins with 18 electrons and adds a set of orbitals from the third energy level (18 more spaces.) It would be simply far to easy for the additional electrons to fill the 9 new orbitals! Instead, chemistry hocus pocus happens, and there are only openings for 8 electrons.
In the link above, you see the electron configuration for each element. The 18 electrons scooted down leaving the set of 8 electrons to interact with other elements. So, for example, iron (Fe, atomic number 26) has 2 electrons on the first orbital (Level 1), 8 electrons on the second set of orbitals (Level 2), then 14 electrons on the third set of orbitals (level 3), and only 2 electrons on the fourth set of orbitals. By the time you get to the end of the 4th row, krypton (Kr, atomic number 36) has electrons located in 4 sets of orbitals as follows: 2, 8, 18, 8.
If you are confused, that's not surprising.
A few summary thoughts:
- Columns 3 through 12 are just weird.
- Rows 6 and 7 are also weird.
- The electrons in the "outer shell" establish how it is going to react with other elements.
- Column 1 always has 1 electron in its "outer shell."
- Column 2 always has 2 electron in its "outer shell."
- Column 13 always has 3 electron in its "outer shell."
- Column 14 always has 4 electron in its "outer shell."
- Column 15 always has 5 electron in its "outer shell."
- Column 16 always has 6 electron in its "outer shell."
- Column 17 always has 7 electron in its "outer shell."
- Column 18 always has 8 electron in its "outer shell."
A more appropriate name for the "outer shell" is to recognize that it is the group of electrons in the element's highest energy level. The electrons in an element's highest energy level are called valence electrons. Elements in any group (column on the periodic table) react similarly and have similar properties because they have the same number of valence electrons.
Because of the valence electrons, atoms can be assigned relative valence charges. Thinking about elements based on these charges is useful for predicting how they will react.
This periodic table shows charge information (number in upper right corner):
The valence charge represents the tendency of the atom to either give up its electron in a reaction or take on the electrons from another atom. Atoms want to combine so that in a sense, between all the atoms involved, eight valence electrons are present.
It is sort of like this…
Once upon a time, there was an unhappy sodium (Na) atom who had just one lonely valence electron (+1). It would be happier if someone would take it from it. It met up with chlorine, who was missing 1 electron (valence = -1). The chlorine needed that one, lonely valence electron to fill its own orbital to eight. So, it combined with sodium, holding on to sodium's electron, which gave the chlorine eight valence electrons. Sodium, having "given up" its lonely electron, was then "full" on the second set of orbitals. Both elements were happy, so long as they stuck together!
Sort of…
Another example with chlorine is when it combines with aluminum.
Because aluminum has a valence charge (or just valence) of +3 and chlorine has a valence of -1, they combine in a fixed, set ratio of 3:1. It takes 3 chlorines to combine with one aluminum to form aluminum chloride (AlCl3).
The vital things to understand about the periods and groups are:
- The elements in each column (group) have something similar in their electron configuration.
- The elements in each row (period) are filling a particular set of orbitals.
- Based on how the electrons arrange, each element has a valence charge which is generally common for all elements in a group.
- Columns 3 - 12 are weird.
- Each element in a group will react with elements in other groups in a way to adjust the electron configuration so that a full set of valence electrons exists.
Atomic Mass
One more piece of information on the Periodic Table that is very important is the Atomic Mass. The Atomic Mass is the sum of the masses of all the protons and neutrons in the atom. (To find out how many neutrons are in an atom, subtract the Atomic Number (number of protons) from the Atomic Mass.)
The Atomic Mass is also the mass in grams of one mole of the element's atoms.
Summary Remarks
- The Atomic Number is the number of protons. Elements are arranged on the Periodic Table according to Atomic Number. It is an integer, the count of the number of protons, and is usually located at the top of the element listing.
- The elements of the Periodic Table are arranged into periods (rows) based on the the orbitals that exist in the atom.
- The elements of the Periodic Table are organized into groups (columns) based on how the electrons are filling the available orbitals.
- Each element in a group will react with elements in other groups in a way to adjust the electron configuration so that a full set of valence electrons exists.
- The valence charge represents the tendency of the atom to either give up its electron in a reaction or take on the electrons from another atom. Atoms want to combine so that in a sense, between all the atoms involved, eight valence electrons are present. Some Periodic Tables include this, and is an integer with a + or - in front of it.
- The Atomic Mass is the sum of the masses of all the protons and neutrons in an atom, and it is also mass in grams of one mole of the element's atoms. It is a decimal number, and usually located at the bottom of the element listing.
