The Periodic Table of the Elements is used for reference values in chemistry courses (lists atomic mass, or covalent radii, etc.)

By folding the periodic table anyone can see why sodium forms a compound with chlorine in a ratio of one-to-one (sodium chloride, common table salt whose chemical formula is NaCl)...Water however requires that an oxygen atom bond with TWO hydrogen atoms (H2O). This is easy to visualize by the OCTET (8) RULE for valence (outermost only) electrons shown below like empty puzzle pieces for the electrons....

The first stop is the electron pair, that is TWO electrons bond two hydrogen atoms and there are two electrons for the stable helium atom and even in a lithium ion (Li+)......So pairs of electrons are the first stable stopping place in small molecules and atoms (and as you'll see almost all molecules are bound by a pair of electrons to hold the positive nuclei together...

Now a new shell of electrons is envisioned until there is an octet (8 e-s) around the atom (say a NEON atom ,so notice that there are 10 e-s total but only 8 in the outermost "shell",a nd two in the inner resembling he electron count of a helium atom)

so count only the valence=outermost e-s as a shell of eights for compounds...So picture a salt of say lithium fluoride LiF (solid)...now picture the surface of the salt underwater ...... where the lithium is pulled into water by Mickey MOLE's hands, the lithium leaves behind an electron such that it keeps only two of the three it has (for it's 3 protons to be an elemental form of lithium...so this creates the cation, positively charged Li+.)

Fluoride will dissolve with an extra electron as a negative one ion (F-)...

fluoride anion (-1) gets its 8th electron from water or lithium???????

so this is the way to balance equations in chemistry to find how much stuff to react (MIX)...

Notice that for the ionic compounds that this averages out the electrons to be predicted by the OCTET RULE but these salts are in crystals with many neighboring atoms ... However in the other type of bond for the elements on the right side of the table like NITROGEN, CARBON, or OXYGEN...

these atoms bond with the density of the electron pair in between only two atoms such that chains are formed for sugars, fats, DNA/RNA, etc this is where the OCTET RULE IS most useful!!!!!!!!!