The short-period elements refer to the 18 elements of the first to third periods, including hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminum, silicon, sulfur, phosphorus, chlorine, and argon. The position of these elements in the periodic table has a certain regularity, and the characteristics of the position of the short-period elements are described in detail below. The periodicity of short-period elements is very obvious, and with the increase of atomic number, the properties of the elements show periodic changes. In the first to third cycles, the physical and chemical properties of the elements change regularly with the increase of atomic number. For example, in the first cycle, hydrogen is special with helium; In the second cycle, from left to right, the atomic ratio gradually decreases, the metallicity decreases, and the non-metallicity increases. The change law of the third period is similar to that of the second cycle, so the change law of element properties is particularly prominent in the short-period elements. In short-period elements, the regularity and particularity of the elements coexist. On the one hand, with the increase of atomic number, the properties of elements show obvious regular changes. On the other hand, due to the limited number of short-period elements, there are peculiarities in the properties of some elements. For example, in the second cycle, oxygen and fluorine are the most electronegative non-metallic elements, while neon is one of the most difficult noble gaseous elements to undergo chemical reactions. These peculiarities allow short-period elements to exhibit colorful properties in chemical reactions.
The elements whose ordinal number is equal to the number of periods are h, be, al.
Elements whose ordinal number is twice the number of periods are c and s.
An element whose ordinal number is 3 times the number of periods is O.
An element whose number of periods is twice as large as the ordinal number of the group is li.
An element whose period number is 3 times the number of the family ordinal is na.