Electronic Configuration
Before discussing any more about the placement of elements on the periodic table, it is important to understand how the electrons circling the nucleus of the atoms are configured. Through experiments, scientists have demonstrated that the electrons are arranged into levels consisting of different shells. The electrons in the innermost shell are held most tightly by the atom. The further from the nucleus, the weaker the attraction the electron feels. It is said that an electron further from the nucleus is at a higher energy level, and subsequently, has more energy.
Bne important fact about energy levels is that they cannot hold an infinite number of electrons. Each level and shell has a specific maximum capacity. Also, both are most stable when full. The first level can hold up to two electrons. The second level has a capacity of eight. The number increases with each level but even so, atoms strive to have eight electrons in their outermost shell.
Something else to remember is that only the electrons in the outermost level or shell interact in chemical reactions. If the outer shell is full, there is very little chance that the atom will react with another atom. On the other hand, if an atom has only seven electrons it will try to find an additional electron or if the outer shell has only one electron, the atom will try to release that extra electron to move to the full shell one level lower. Knowing these facts will aid in the understanding of the periodic table.
The Modern Periodic Table
In a modern periodic table, the periods of the table increase unevenly as the atomic number of the elements increase. This is a result of the way in which electrons are added to the atoms. The first energy shell in each level is known as the S. It is capable of handling only two electrons. The second shell is the P and can handle six more for a total of eight in the second period. The third level has an additional set of atomic orbitals called the D shells which can hold ten more electrons. This pattern continues toward infinity. Each new level adds twice the next odd number of electrons. If each level filled uniformly, this system would be easy to follow. Unfortunately, by looking at the chart it is easy to see that this is not happening. Looking at the table shows that the progression is 2, 8, 8, 18, 18, 32, 32, and so on. This problem might seem hard to comprehend at first, but in the end, it has a very simple explanation.
Filling The Electron Shells
Atoms try to find an electronic configuration which allows them to reach the lowest energy state. This helps explain the odd progression in the periodic table as well as the reasons for the formation of molecules. As it turns out, certain electron configurations allow a lower energy state than others. The following chart shows how the different electron levels are filled to allow for this lowest energy state.
By starting at the top of each column and reading diagonally to the left, it is possible to see how different levels fill in different orders. The number in parenthesis corresponds to the maximum number of electrons which can be placed in any one level.
As can be seen from the pattern of filling, it is more advantageous for an atom to fill the 4S shell before it fills the 3P shell. This deviation from the pattern allows for the odd, ever expanding, shape of the periodic table.
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