Why in The Periodic Table each period contains 18 new elements EXACTLY?

[dimamuzhetsky]

What do we see in it?a period is over after 18 elements.Next one-after 36.Another period-& 54 elements are enumarated & so on :(...How is it that we start counting from 0 & procceed having 18 new elements after a period is counted?Reasons for this?Thanks

[Old_Geezer]

Didn't do chemistry but I suspect it is a result of how elements get their characteristics, which relates to the number of electrons in its outer shell (the inner shells being all full). Unsure why that ends up with 18 but I'm sure it can be worked out.

[Zeuhl]

You might be better reposting this in 'science' instead of the section devoted to 'advertising' which as an industry is very selective in its use of science - mostly shampoo commercials.

[jomifl]

wot not 17.5?

[factor-fiction]

Is it something to do with the numbers of electrons in outer shells- I recall the third shell had up to 18 electrons.

According to wikipedia:
"The main value of the periodic table is the ability to predict the chemical properties of an element based on its location on the table. It should be noted that the properties vary differently when moving vertically along the columns of the table than when moving horizontally along the rows.[1]

The layout of the periodic table demonstrates recurring ("periodic") chemical properties. Elements are listed in order of increasing atomic number (i.e., the number of protons in the atomic nucleus). Rows are arranged so that elements with similar properties fall into the same columns (groups or families). According to quantum mechanical theories of electron configuration within atoms, each row (period) in the table corresponded to the filling of a quantum shell of electrons. There are progressively longer periods further down the table, grouping the elements into s-, p-, d- and f-blocks to reflect their electron configuration.[1]"
"

[ABerrant]

The question contains a fundamental error; although the Periodic Table consists of 18 groups, each period is not made up of 18 elements, as indicated by this list:

Period 1 = 2
Period 2 = 8
Period 3 = 8
Period 4 = 18
Period 5 = 18
Period 6 = 32
Period 7 = 32