The Periodic Table

• Elements


  Substances that cannot be broken down into simpler substances by chemical means
• Periodic table


  Arrangement of elements in order of increasing atomic number, with elements with similar properties placed in vertical columns
• Mendeleev's approach to organising elements

  1. Arranged elements in order of increasing relative atomic mass

  2. Left gaps for undiscovered elements

  3. Placed elements with similar chemical properties in vertical columns
• Mendeleev sometimes swapped the positions of elements if he thought it better suited their chemical properties and those of their compounds
• Mendeleev placed iodine after tellurium even though iodine has a lower relative atomic mass, to align it with other elements with similar chemical properties
• Mendeleev assumed that elements would continue to be discovered, so he left gaps for them in his table
• An entire group of inert or very unreactive elements, the noble gases, were discovered near the end of the 19th century and easily fitted into the periodic table as group 0
• Atomic number


  The number of protons in the nucleus of an atom, which is a physical property of the element
• Moseley showed that an element's atomic number, not its relative atomic mass, determines its position in the periodic table
• The elements in the modern periodic table are arranged in order of increasing atomic number
• The iodine-tellurium pair reversal is explained by the fact that tellurium has several different isotopes, with about two-thirds of its atoms being Te-128 and Te-130, which have higher relative atomic masses than iodine
• Electronic configuration


  The arrangement of electrons in an atom, with electrons occupying shells around the nucleus
• The electrons in a sodium atom do not all occupy the same shell, they occupy different shells according to the shell-filling rules
• The number of occupied shells in an element's electronic configuration is equal to the period number in the periodic table
• The number of electrons in the outer shell of an element is equal to the group number in the periodic table, except for group 0 elements which have full outer shells