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In the modern periodic table:

• elements are arranged in rows, called periods, in order of increasing atomic number
• elements with similar properties are placed in vertical columns, called groups

The table is called the periodic table because elements with similar properties occur at regular intervals.

Resolving pair reversals

Mendeleev did not know about isotopes, but their existence explains pair reversals. The positions of iodine and tellurium were reversed in Mendeleev's table because:

• iodine has one naturally occurring isotope, iodine-127
• the most abundant tellurium isotopes are tellurium-128 and tellurium-130

The high relative abundance of these tellurium isotopes gives tellurium the greater relative atomic mass. The atomic number of tellurium is 52 and the atomic number of iodine is 53, so these elements are in the correct order in the modern periodic table.