Elements are organized in the periodic table based on which property?

Answer 1

The simplest criterion for arranging the atoms on the periodic table is that each atom in a row or period, except the first or last atom on the row, has an atomic number one more than the atomic number of the atom to its immediate left and one less than the atom to its immediate right in the same period, if there is such an element. If an element is the rightmost one in its period, the element with an atomic number one greater than that goes into column 1 of the next higher period.

The most common wide form arrangement of columns includes 18 columns numbered from 1 through 18. The lightest element, hydrogen, is placed in the first period in column 1 and the next lightest element, helium, is placed in the first period column 18, leaving columns 2 through 17 unoccupied in the first period.

Period 2 begins with the next lightest atom, lithium, in column 1, and the next lightest element after that, beryllium, in column 2. In the second period, columns 3 through 12 are left unoccupied, so that the atom with one higher than atomic number than beryllium, boron, goes into column 13. The same columns 3 through 12 are also left unoccupied in the third period, but are occupied in the remaining periods.

The number of each period corresponds to the first principal quantum number of the electrons in the atom with the highest such number, and the reason for leaving unoccupied places in the first three periods is connected with the second principal quantum characteristic of the electrons in the atom, the one sometimes called "shape" and designated by a letter s, p, d, or f. In the first period, only s electrons are allowed, and either one or two only may be present. In all the remaining periods, p electrons are also allowed, up to a maximum number of 6 such electrons. In the third and higher numbered periods, up to 10 d electrons are allowed along with the s and p electrons, and in the fifth and higher numbered periods, up to 14 additional f electrons are also allowed. Thus, the "space" of ten columns left unoccupied in the second and third periods corresponds to the ten d electrons that are allowed in the fouth and higher periods, but are not allowed in periods 1 - 2 and are not actually occupied until period 4.

The possibility of f electrons beginning with the fifth period but not actually observed until the sixth period is accommodated by two independent rows usually located at the bottom of the periodically arranged table. The element lanthanum, with atomic number 58, occupies period 6 column 3 and is the heaviest element that does not contain any f electrons. The next fourteen elements, with atomic numbers 58 through 71, occupy the upper one of these independent rows and contain f electrons with a principal quantum number of 5. After element 71, no more f electrons with principal quantum number 5 are allowed, and element 72 is entered in column 4 of period 6, which continues to be occupied in the normal way through the element radon with atomic number 86. The second independent horizontal row begins after actinium, element 89, in column 3 period 7 and continues through lawrencium, element 103. Heavier elements discovered so far return to the regular row of period 7.

Answer 2

Elements are organized based on the number of valance electrons that they have.

Answer 3

Atomic number