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Scientists continued to revise Mendeleev’s periodic table.

• The Modern periodic table is arranged in order of increasing atomic number , rather than atomic weight.
• there are now over 100 elements and no gaps in the periodic table.
• a group of unreactive elements – the noble gases – were discovered at the end of the 1800s.
• the modern periodic table includes actinides and lanthanides.

Groups and periods

The modern table is called a “periodic” table because the elements with similar properties appear at regular intervals.

• the vertical columns are called groups
• the horizontal rows are called periods