How are the elements arranged horizontally on the periodic table?

Answer 1

The period number of an element in the Periodic Table is the number of the highest electron shell that is occupied in the ground state of the element, and the column number, if the number has only one digit, or the last digit of the column number, if the number has two digits, is the number of valence electrons in the element, except for columns 8, 9, and 10. (For the elements in columns 3 through 11, the nominal number of valence electrons may or may not correspond to the most usual number of electrons donated, accepted, or shared in bond formation by an atom of the element.)

Each element is distinguished by having one more proton than the element to its immediate left in a row, or one more proton than the element at the far right of the preceding row if the element is in column 1 and therefore has no element to its immediate left. Hydrogen is in column 1 and the only other element in the first period, helium, is in column 18. In the next two periods, columns 3 through 12 are skipped in filling the table, and in the sixth and seventh periods, column 3 is expanded into a separate row of fourteen elements at the bottom of the main table before another atom is put into column 4.

Answer 2

The elements are arranged horizontally in rows called periods on the Periodic Table. Each row represents a different energy level of electrons in the atoms of the elements.