An element is a substance whose atoms all have the same number of protons, or the same atomic number (Z). Chemical properties of elements depend on their atomic number which can be read from the periodic table of the elements (often referred to simply as the “periodic table”). The atomic number is a whole number often shown to the left or just above the symbol, and the name of each element is usually found below the atomic symbol. For example, oxygen (O) has atomic number 8; uranium (U) has atomic number 92.
The periodic table is organized into horizontal rows and vertical columns. The rows are referred to as periods. Elements in period 1 are hydrogen and helium; period 3 begins with sodium (Na) and ends with argon (Ar).
In the periodic table, elements have something in common if they are in the same row or period. All of the elements in a period have the same number of electron shells or levels at which electrons orbit the nucleus. For example, every element in the top row (the first period) has one shell for its electrons. All of the elements in the second row (the second period) have two shells for their electrons. Seven shells are enough to include all of the elements in the periodic table.
Moving from left to right in a given period, the chemical properties of the elements gradually change. At the end of each row, a drastic shift occurs in chemical properties. The next element in order of atomic number is more similar (chemically speaking) to the first element in the row above it; thus a new row begins on the table.
|4||This is period 4|
The columns are referred to as groups or families. Elements in groups have similar properties based on placement of electrons. The groups are numbered from 1 to 18 starting from the left.
It was the observation of such chemical trends and similarities among groups of elements that lead Russian chemist Dmitri Mendeleev to invent the Periodic Table in 1869.
Sources: Masterton & Hurley, Chemistry Principles and Reactions, Third Edition, (1997, 1993, 1989 Saunders College Publishing)