<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> A period 4 element is one of the chemical elements in the fourth row (or period) of the periodic table of the elements . The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns . The fourth period contains 18 elements, beginning with potassium and ending with krypton . As a rule, period 4 elements fill their 4s shells first, then their 3d and 4p shells, in that order; however, there are exceptions, such as chromium . </P> <P> Every single one of these elements is stable, and many are extremely common in the Earth's crust and / or core; it is the last period with no unstable elements at all . Many of the transition metals in period 4 are incredibly strong, and therefore commonly used in industry, especially iron . Copper is one of three metals that are not silver or gray in color, along with caesium and gold . Three adjacent elements are known to be toxic, with arsenic one of the most well - known poisons, selenium being toxic to humans in large quantities, and bromine, a very toxic liquid . Many elements are essential to humans' survival, such as calcium being what forms bones . </P>

The number of elements in the fourth period is