Periodic tables have been around for over 150 years, and some of them look very different from the one we have today. What is it about periodic tables that makes them periodic? (A) They are tables of information that are periodically updated to reflect our current knowledge. (B) They group elements based on their behaviors, which repeat periodically from the lighter to the heavier elements. (C) They have rows as well as columns. (D) They organize elements based on their periods of reactivity. Or (E) they arrange elements in reference to their reactivity with iodine. They are per-iodic.
The word periodic relates to the word period, which is another word for interval. In the 1800s, chemists started to see patterns in the behavior of the elements when they were put in order by atomic mass. This was before the atomic model, which taught us about nuclei, protons, and atomic numbers. John Newlands, who lived between 1837 and 1898, was a British chemist and was the first to identify that these similarities occurred in fixed intervals. He saw similarities between every eighth element and put them in groups of seven, giving rise to the name the Law of octaves. Because, as in music, moving eight positions down the line would move you to a comparable element.
There were inaccuracies in Newlands’ table, but it was a good start. And a lot of the patterns he identified exist in the modern periodic table.
Which of the statements describes what makes periodic tables periodic? The best answer is the second statement. Periodic tables group elements based on their behaviors, which repeat periodically from the lighter to the heavier elements.
Of course, the modern periodic table is ordered by atomic number, the number of protons in the nucleus, and not by atomic mass, the average mass of an atom. But the periodicity from lighter to heavier elements is still broadly true. So this is our answer.
While it’s true that periodic tables are periodically updated, this is not the source of the name. As for having rows and columns, well, that’s just what makes it a table. What about they organize elements based on their periods of radioactivity? Well, periodic tables don’t actually contain information about the radioactivity of the elements. This varies depending on the isotope and is not a simple property of the element. What about the last statement? They arrange elements in reference to their reactivity with iodine. They are per-iodic. Well, iodine was discovered in 1811. And while it reacts with many other elements, it has never been used as a basis for the periodic table.
So of the five statements given, the one that describes what makes periodic tables periodic is that they group elements based on their behaviors, which repeat periodically from the lighter to the heavier elements.