2.7 The Periodic Table of the Chemical Elements

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2.7: Mendeleev's Periodic Table of the Chemical Elements -update 18 Nov. 2021

Here below, the first 4 rows of the Periodic Table of the Chemical Elements first formulated by the Russian chemist Dmitri Mendeleev in 1869 will be used to show its usefulness and meanings.

Group	1	2		3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Period
1	1 H 1.008																		2 He 4.0026
2	3 Li 6.94	4 Be 9.0122												5 B 10.81	6 C 12.011	7 N 14.007	8 O 15.999	9 F 18.998	10 Ne 20.180
3	11 Na 22.990	12 Mg 24.305												13 Al 26.982	14 Si 28.085	15 P 30.974	16 S 32.06	17 Cl 35.45	18 Ar 39.948
4	19 K 39.098	20 Ca 40.078		21 Sc 44.956	22 Ti 47.867	23 V 50.942	24 Cr 51.996	25 Mn 54.938	26 Fe 55.845	27 Co 58.933	28 Ni 58.693	29 Cu 63.546	30 Zn 65.38	31 Ga 69.723	32 Ge 72.63	33 As 74.922	34 Se 78.96	35 Br 79.904	36 Kr 83.798

Note the top 18 column numbers (If needed, use bottom right pointer to expose right side of screen). The vertical columns tell the Group number of a chemical element. The horizontal row numbers to the left of the table tell the period number into which the element falls.  

The first element of Group-1, Period 1 is Hydrogen identified by its Atomic Number 1, by the symbol, H, and by the Atomic Mass (Previously, Atomic Weight), 1.008. The Period-1 elements are only two, the Group-1, H, Hydrogen, and the Group-18, He, Helium. In Period-2, are the 8 elements Li (Lithium), Be (Beryllium), B (Boron), C (Carbon), N (Nitrogen), O (Oxygen), F (Fluorine) and Ne (Neon). From Periods-2 to -4 each period has 8 elements, but from Period-5 on (Not shown above but you may view the full table by Googling it on Internet), each period has up to 18 elements.

The Periodic Table gives good information. Consider the Atomic Number (AN) above each element's symbol. It is in the sequence (1, 2, 3, …) from your left to right and then down to the next period row. The Atomic Number gives you the number of protons in the element’s atomic nucleus and from it you know the number of electrons around the element’s neutral atom, which is the same number.

   Note the letter abbreviation of each element. In the full Table, each element’s data is in a box and the element is spelled out (e.g., “Hydrogen  H” ……. “Helium  He”). In some cases, the symbol is from the Latin (e.g., Natrium for Sodium) originally used for the element.

   The lowest part of the element data box has the mixed decimal number for the element’s averaged Atomic Mass (AM). By knowing the AN and AM integers of an element, you can find the number of neutrons in the element’s nucleus by subtracting AN from the integer of AM. 

Each period is composed of elements that contain the same number of  electron shells. For example, all elements in Period-1 have only one electron shell in the 1s orbital, a simple sphere shape; so, the 1s orbital’s limit of filling with only 2 electrons will set a limit of two elements for the Period-1. The Period-2 elements have 2 electron shells; the inner shell contains the one subshell that can hold up to two electrons; and then the outer shell fills with a 2s subshell that may hold up to 2 electrons and a 2p subshell that may hold up to 6 electrons. In the Period-2 elements, starting with the 2p subshell, one electron is added each time the Atomic Number increases from AN 3 to AN 10, until with AN 10 Neon (Ne), the three 2p orbitals are all filled with electrons. And so on in the higher Periods.

   The period number tells the number of shells in the element atom and its AN allows you to figure out the subshell and the sum number of electrons in all orbitals of the neutral atom, from which can be figured the reactive electron outer orbital, which is crucial for predicting an element’s chemistry. For example, the Li, Lithium’s being in Period-2 with an AN 3, tells that its atom has the inner 1s orbital filled with 2 electrons and the outer 2s orbital has only 1 reactive electron and no electrons at all in the empty, potential p subshell orbitals. Thus it can be predicted that Li will be an electron donor, or electron loser, in chemical reactions and become the ion Li⁺ in body fluid solutions. Also, as mentioned in Section 2.6b, either the paramagnetism or diamagnetism of an element  can be predicted from these numbers. Lithium is paramagnetic (attracted to an iron magnet) from its odd electron number.

Let us take the Period-2 elements as example of outer shell orbital electron reactivity. In Period 2, above, in the Periodic Table, you see on left side three elements that have 1, 2, or 3 electrons in the outer orbital respectively (Li 1, Be 2, B 3). In these cases the balance of electrical forces work so that such element’s atoms more easily lose their outer electrons – they are “electron donors or losers” in reactions and form the electro-positive ions (Li⁺ Be²⁺, B³⁺ because when an atom loses one or more electrons it loses a 1 or-more-minus charge, and the atom gets a 1+ or-more-electric charge and becomes a positive ion, a so-called anion. On the other hand, elements toward your right side in Period-2, especially the Atomic Number-8 Oxygen (O) or the AN 9 Fluorine (F) have a 2p outer orbital that lacks 2 electrons (Oxygen) or 1 electron (Fluorine) and, there, the balance of electrical forces favors a stronger attraction of electrons and a tendency to fill the empty parts of their outer orbital with electrons, so filling a one-electron-lacking orbital gives a 1– (one minus electric charge) ion and filling a 2-electron-lacking orbital gives a 2– (minus) ion, etc. Such elements, which lack one, two or three electrons to fill the outermost orbital shells are electron receivers – they pick up electrons from elements on the left side and form electro-negative ions. (Oxygen picks up 2 electrons to fill its outer orbital and becomes the electro-negative O^2–, Fluorine picks up one electron and becomes F^–)

   Note that elements in the extreme right vertical column (Group 18; i.e., He, Ne, Ar) have all orbitals completely filled by paired electrons. (Each orbital has its full complement of two spin-paired electrons) In that sense they are satisfied with a full set of electrons and tend neither to lose nor gain; they are chemically inert elements a.k.a. the Noble Elements or Gases because they are all gases that have no chemical reactions with other elements. 

As you may note from the last case, the elements of a same Group number have the same number of electrons in the outermost orbital subshells: Group 1 (H, LI, Na, …) has one electron in outermost orbital; Group 2, two electrons; Group 13, three electrons; ... Group 14, four electrons etc.  Elements within their own Group undergo similar reactions, e.g., Li and Na both easily lose their single outer subshell orbital electron and become Li⁺ and Na⁺ in salts; and, as already mentioned, same-Group elements O and S tend to pick up 2 electrons to fill outer subshell orbital and become double negative ions) and so on.

   I hint here of the value gotten from The Periodic Table, which will relate to good health. For example, the recent startling discovery that Arsenic (As) can replace Phosphorus (P) in the DNA molecule is really not so startling since the As atom is right below P in Group 15.

   This chapter is a starter for your independent study of the elements from the full periodic table. 

 END CHAPTER.  To read on now, click 2.8 Some Important Chemical Concepts in Medicine