Know these relationships across the table.Mendeleev’s table had the elements arranged by atomic weights with
recurring properties in a periodic manner. Where atomic weight placement
disagreed with the properties that should occur in a particular spot in the table,
Mendeleev gave preference to the element with the correct properties. He even
predicted elements for places that were not yet occupied in the table. These
predictions proved to be amazingly accurate and led to wide acceptance of his
table.
Periodic Law
Henry Moseley stated, after his work with X-ray spectra in the early 1900s, that
the properties of elements are a periodic function of their atomic numbers, thus
changing the basis of the periodic law from atomic weight to atomic number. This
is the present statement of the periodic law.
The Table
The horizontal rows of the periodic table are called periods or rows. There are
seven periods, each of which begins with an atom having only one valence
electron and ends with a complete outer shell structure of an inert gas. The first
three periods are short, consisting of 2, 8, and 8 elements, respectively. Periods 4
and 5 are longer, with 18 each, while period 6 has 32 elements, and period 7 is
incomplete with 22 elements, most of which are radioactive and do not occur in
nature.
In Table 4, you should note the relationship of the length of the periods to the
orbital structure of the elements. In the first period, the 1s^2 orbital is filled with
the noble gas helium, He. The second period begins with the 2s^1 orbital and ends
with the filling of the 2p^6 orbital, again with a noble gas, neon, Ne. The same
pattern is repeated in period three, going from 3s^1 to 3p^6. The eight elements from
sodium, Na, to argon, Ar, complete the filling of the n = 3 energy level with 3s^2
and 3p^6. In the fourth period, the first two elements fill the 4s^2 orbital. Beyond
calcium, Ca, the pattern becomes more complicated. As discussed in the section
“Order of Filling and Notation,” the next orbitals to be filled are the five 3d
orbitals whose elements represent transition elements. Then the three 4p orbitals
are filled, ending with the noble gas krypton, Kr. The fifth period is similar to the
fourth period. The 5s^2 orbital filling is represented by rubidium, Rb, and
strontium, Sr, both of which resemble the elements directly above them on the
table. Next come the transition elements that fill the five 4d orbitals before the
next group of elements, from indium, In, to xenon, Xe, complete the three 5p
orbitals. (Table 3 should be consulted for the irregularities that occur as the d