Electron Configuration and the Table

Electron Configuration and the Table

The "periodic" nature of chemical properties that Mendeleev had discovered is related to the electron configuration of the atoms of the elements. In other words, the way in which an atom's electrons are arranged around its nucleus affects the properties of the atom.

Bohr's theory of the atom tells us that electrons are not located randomly around an atom's nucleus, but they occur in specific electron shells (see our Atomic Theory II module for more information). Each shell has a limited capacity for electrons. As lower shells are filled, additional electrons reside in more-distant shells.

The capacity of the first electron shell is two electrons and for the second shell the capacity is eight. Thus, in our example discussed above, oxygen, with eight protons and eight electrons, carries two electrons in its first shell and six in its second shell. Fluorine, with nine electrons, carries two in its first shell and seven in the second. Neon, with ten electrons, carries two in the first and eight in the second. Because the number of electrons in the second shell increases, we can begin to imagine why the chemical properties gradually change as we move from oxygen to fluorine to neon.

Sodium has eleven electrons. Two fit in its first shell, but remember that the second shell can only carry eight electrons. Sodium's eleventh electron cannot fit into either its first or its second shell. This electron takes up residence in yet another orbit, a third electron shell in sodium. The reason that there is a dramatic shift in chemical properties when moving from neon to sodium is because there is a dramatic shift in electron configuration between the two elements. But why is sodium similar to lithium? Let's look at the electron configurations of these elements.

As you can see in the illustration, while sodium has three electron shells and lithium two, the characteristic they share in common is that they both have only one electron in their outermost electron shell. These outer-shell electrons (called valence electrons) are important in determining the chemical properties of the elements.

An element's chemical properties are determined by the way in which its atoms interact with other atoms. If we picture the outer (valence) electron shell of an atom as a sphere encompassing everything inside, then it is only the valence shell that can interact with other atoms - much the same way as it is only the paint on the exterior of your house that "interacts" with, and gets wet by, rain water.

The valence shell electrons in an atom determine the way it will interact with neighboring atoms, and therefore determine its chemical properties. Since both sodium and lithium have one valence electron, they share similar chemical