Complete the sentences. Choose the answer A, B, or C which you think fits best

One of the most important modern chemical theories is the Periodic (… 1 …). It can simply be stated as follows: “The properties of the elements are a periodic function of the nuclear (…2...) of their atoms”.

In 1869 the great Russian scientist D.I. Mendeleyev came to the conclusion that by the (…3…) of the elements in order of (…4…) atomic weight the (…5…) and periodicity of properties of various valence groups of the elements were clearly seen.

There were several (…6…) spaces in Mendeleyev’s Periodic table. This allowed him to (…7…) the existence of six unknown elements (scandium, gallium, germanium, polonium, etc). His confidence in the new classification was clearly (…8…) in the predictions which he made of the chemical properties of these missing elements. And within fifteen years, gallium, scandium and germanium were discovered.

The periodic table has been modified hundreds of times. Many new discoveries, isotopes, rare gases, atomic numbers, and electron configurations have (…9…) the idea of the periodicity of the properties of the elements.