PERIODIC TABLE OF THE ELEMENTS - Atomic Structure and the Periodic Table of the Elements - REVIEW OF MAJOR TOPICS - SAT Subject Test Chemistry

SAT Subject Test Chemistry




Atomic Structure and the Periodic Table of the Elements



The history of the development of a systematic pattern for the elements includes the work of a number of scientists such as John Newlands, who, in 1863, proposed the idea of repeating octaves of properties.

Dimitri I. Mendeleev in 1869 proposed a table containing 17 columns and is usually given credit for the first periodic table since he arranged elements in groups according to their atomic weights and properties. It is interesting to note that Lothar Meyer proposed a similar arrangement about the same time. In 1871 Mendeleev rearranged some elements and proposed a table of eight columns, obtained by splitting each of the long periods across into a period of seven elements, an eighth group containing the three central elements (such as Fe, Co, Ni), and a second period of seven elements. The first and second periods of seven across were later distinguished by use of the letters A and B attached to the group symbols, which were Roman numerals. This nomenclature of periods (IA, IIA, etc.) has been revised in the present Periodic Table, even in the extended form of assigning Arabic numbers from 1–18 as shown in Table 4.


Mendeleev is given credit for the first Periodic Table. It was based on placement by properties.

Table 4. Periodic Table Properties


Periods are the horizontal rows 1–7.

Groups are the vertical columns 1–18.

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.


Know these relationships across the 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 1s2 orbital is filled with the noble gas helium, He. The second period begins with the 2s1 orbital and ends with the filling of the 2p6 orbital, again with a noble gas, neon, Ne. The same pattern is repeated in period three, going from 3s1 to 3p6. The eight elements from sodium, Na, to argon, Ar, complete the filling of the n = 3 energy level with 3s2 and 3p6. In the fourth period, the first two elements fill the 4s2 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 5s2 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 orbitals fill.) The sixth period follows much the same pattern and has the filling order 6s2, 4f14, 5d10, 6p6. Here, again, irregularities occur and can best be followed by using Table 3.

The vertical columns of the Periodic Table are called groups or families. The elements in a group exhibit similar or related properties. In 1984 the IUPAC agreed that the groups would be numbered 1 through 18.