# Video: Recalling the Organizing Principle of the Periodic Table

The elements on the modern periodic table are organized left to right according to which property?

02:52

### Video Transcript

The elements on the modern periodic table are organized left to right according to which property?

Let’s have a look at our periodic table. The structure of the periodic table is quite complicated. The periodic table has vertical columns, called groups, and horizontal rows, called periods. However, the more complex features of the periodic table do not apply to this question. We’re only interested in the organizing principle, left to right. This means we’re looking for a feature that puts the elements in sequence, left to right. So when we get to the end of one row, we can start at the extreme left to the next and go all the way to the right and so on. This gives us the sequence hydrogen, helium, lithium, beryllium, and so on.

Now, the box for each element contains two numbers, the atomic number and the element’s atomic mass. If we look at these two candidates going left to right, we can see that they both steadily increase, one to two to three to four, for the atomic numbers, and about one, four, seven, and nine for the atomic masses. So, on the modern periodic table, which property is it that the elements are organized by?

Well, as I’m going to show, the atomic number, left to right, consistently increases, while increases in the atomic mass are not necessarily consistent. The first inconsistency arises with argon and potassium. Argon has an atomic number of 18. It has 18 protons in its nucleus, while potassium has an atomic number of 19. So we’re seeing an increase from left to right. However, the atomic mass of potassium is actually less than that of argon. A similar inconsistency occurs with cobalt and nickel and tellurium and iodine.

So, by examining our modern periodic table, we can see that the organizing principle, left to right, is ordering by atomic number. The atomic number is the number of protons in each nucleus of atoms of a given element. It is the atomic number that dictates the identity of an atom. The number of protons in an atom dictates the number of electrons it has to be neutral which dictates a great deal of its chemistry. Therefore, it makes sense that we organize the periodic table first by atomic number.