text: The periodic table is divided into sections by chemists in order to make it easier to talk about elements that behave in similar fashions.  The periodic table makes this possible by organizing elements by the number of electrons in each orbital.  The transition metals, the family that platinum and Iridium both belong in, is made up of elements that in their elemental (as opposed to compound) form have a d orbitals as their last filled orbital.  This gives them some similar properties and ways in which they will react.  Which is why scientists call them families.