By Robert M. Hazen, Ph.D., George Mason University
The structure of the periodic table reflects the arrangements of electrons in atoms. In particular, atoms that have 2, or 10, or 18 electrons, and so forth, have filled electron shells and, thus, are particularly stable. Every atom or group of atoms adopts a strategy to achieve one of these magic numbers of electrons.
Why the Periodic Table Works
The development of the Bohr atom led to a deeper understanding of why the periodic table works. The periodic table systematizes two key properties of the elements: their weight and their chemical properties. The weight, or the mass, of elements increases from left-to-right, top-to-bottom of the periodic table.
Mass results primarily from particles in the atom’s nucleus. This is what Rutherford found, that it’s the nucleus that carries the mass of the atom. Therefore, heavier elements generally have more protons in the nucleus.
Rutherford showed that since most of the mass is in that positively charged nucleus, then mass and the number of protons could be characterized as sort of a 1:1 thing. Increasing weight corresponds to increasing the size of that nucleus.
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Positive Charge and Atomic Number
The lightest element is hydrogen. It has one positive charge, and thus, one proton in the nucleus. Successive elements have additional positive charges. So helium is element 2, it has two protons; lithium is element 3, it has three protons, and so forth. More and more massive nuclei in different atoms correspond to a different number of positive charges in the nucleus.
This is the element’s atomic number. It represents both its position in the periodic table and the number of positive charges. So the atomic number defines the element name as well as its chemical properties. Hydrogen is a shorthand name for the element with one proton. Gold is a shorthand name for the element with 79 protons.
For an atom to be electrically neutral, it has to have the same number of negatively charged electrons in orbit around the positive nucleus. Any difference in the number of protons, which is fixed, and the number of electrons, which can be added or stripped away, makes an ion. If there are more electrons than protons in the nucleus, this is a negative ion. If there are fewer electrons, if some electrons have been stripped away, this has more positive charges, and it’s a positive ion.
Learn more about semiconductors and modern microelectronics.
The Chemical Behavior of Elements
The chemical behavior of an element is a reflection of the interaction between two or more atoms. Since an atom is almost entirely empty space, this means that it is the arrangement of the electrons that are circling on the outside—sort of the outer guard—specifically the outermost electrons of that arrangement, that determines the chemical behavior.
The periodic properties of the chemical elements then suggest that similar chemical properties are the result of atoms that have similar arrangements of outer electrons. Remember that the elements are arranged in columns according to similar properties. That’s why from a chemical point of view, lithium, sodium, potassium, and rubidium have similar chemical properties because they have a similar electron arrangement.
The Implications of Each Row of the Periodic Table
It is now known that electrons combine in shells around the nucleus. Each row of the periodic table corresponds to one of these electron shells. Moving from left to right in the table, in the first row, there is hydrogen, with one electron and one proton, helium with two electrons and two protons, and that’s the first row, completely filled. So apparently, the innermost shell of electrons can hold up to two electrons.
Similarly, the second row has eight different elements, and so the second shell of electrons apparently can hold up to eight electrons. The third through the sixth rows have 8, then 18, then 18, then 32 more electrons. Because successively, electrons are added and added moving farther and farther down and from left to right in that periodic table.
Learn more about isotopes and radioactivity.
The Magic Number of Electrons
The key to understanding how atoms combine is that there are certain magic numbers of electrons. And these magic numbers of electrons appear to be more stable than any other numbers. By far, the most stable electron configurations are atoms that have two, or 10, or 18, or 36, or 54, or 86 electrons.
The first five rows of the table end in inert gases. Inert gases are elements that exist as isolated atoms. They don’t bond to anything. They are particularly stable. And those are elements two, 10, 18, 36, 54, and 86, the magic numbers. Each of these elements has a completely filled outer shell of electrons.
These inert gases don’t need to bond to anything. All the elements in the first column have a single electron in their outer shell. These atoms need to lose an electron in order to get a filled outer shell. So they bond in similar ways, that first column. All the elements in the next to the last column have electron shells that are one electron short of being full. That’s like the chlorine and the fluorine and so forth. So all of those elements need to get one more electron to fill their outer shell.
Common Questions about the Implications and Consequences of the Periodic Table
The periodic table systematizes two key properties of the elements: their weight and their chemical properties. The weight, or the mass, of elements increases from left-to-right, top-to-bottom of the periodic table.
The atomic number in the periodic table represents an element’s position as well as the number of positive charges. In other words, the atomic number identifies the name of the element and its chemical properties. The atomic number is one of the implications and consequences of the periodic table.
In the periodic table each row relates to one of the electron shells. Moving from left to right in the periodic table, the number of electron shells increases. The same thing happens when moving from the top to the bottom of the table.
