A period 5 element is one of the chemical elements in the fifth 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 fifth period contains 18 elements, beginning with rubidium and ending with xenon. As a rule, period 5 elements fill their 5s shells first, then their 4d, and 5p shells, in that order, however there are exceptions, such as rhodium.

This period contains technetium, one of the two elements until lead that has no stable isotopes (along with promethium), as well as molybdenum and iodine, two of the heaviest elements with a known biological role,^[1][2] and Niobium has the largest magnetic known penetration depth of all the elements. Zirconium is one of the main components of zircon crystals, currently the oldest known minerals in the earth's crust. Many later transition metals, such as rhodium, are very commonly used in jewelry due to the fact that they are incredibly shiny.^[3]

This period is known to have a large number of exceptions to the Madelung rule.

Chemical element			Chemical series	Electron configuration
37	Rb	Rubidium	Alkali metal	[Kr] 5s1
38	Sr	Strontium	Alkaline earth metal	[Kr] 5s2
39	Y	Yttrium	Transition metal	[Kr] 4d1 5s2
40	Zr	Zirconium	Transition metal	[Kr] 4d2 5s2
41	Nb	Niobium	Transition metal	[Kr] 4d4 5s1 (*)
42	Mo	Molybdenum	Transition metal	[Kr] 4d5 5s1 (*)
43	Tc	Technetium	Transition metal	[Kr] 4d5 5s2
44	Ru	Ruthenium	Transition metal	[Kr] 4d7 5s1 (*)
45	Rh	Rhodium	Transition metal	[Kr] 4d8 5s1 (*)
46	Pd	Palladium	Transition metal	[Kr] 4d10 (*)
47	Ag	Silver	Transition metal	[Kr] 4d10 5s1 (*)
48	Cd	Cadmium	Transition metal	[Kr] 4d10 5s2
49	In	Indium	Poor metal	[Kr] 4d10 5s2 5p1
50	Sn	Tin	Poor metal	[Kr] 4d10 5s2 5p2
51	Sb	Antimony	Metalloid	[Kr] 4d10 5s2 5p3
52	Te	Tellurium	Metalloid	[Kr] 4d10 5s2 5p4
53	I	Iodine	Halogen	[Kr] 4d10 5s2 5p5
54	Xe	Xenon	Noble gas	[Kr] 4d10 5s2 5p6

(*) Exception to the Madelung rule

Rubidium is the first element placed in period 5. It is an alkali metal, the most reactive group in the periodic table, having properties and similarities with both other alkali metals and other period 5 elements. For example, rubidium has 5 electron shells, a property found in all other period 5 elements, whereas its electron configuration's ending is similar to all other alkali metals: s¹.^[4] Rubidium also follows the trend of increasing reactivity as the atomic number increases in the alkali metals, for it is more reactive than potassium, but less so than caesium. In addition, both potassium and rubidium yield almost the same hue when ignited, so researchers must use different methods to differentiate between these two 1st group elements.^[5] Rubidium is very susceptible to oxidization in air, similar to most of the other alkali metals, so it readily transforms into rubidium oxide, a yellow solid with the chemical formula Rb₂O.^[6]

Strontium is the second element placed in the 5th period. It is an alkaline earth metal, a relatively reactive group, although not as nearly reactive as the alkali metals. Like rubidium, it has 5 electron shells or energy levels, and in accordance with the Madelung rule it has two electrons in its 5s subshell. Strontium is a soft metal and is extremely reactive upon contact with water. If it does come in contact with water, though, it will combine with the atoms of both oxygen and hydrogen to form strontium hydroxide and pure hydrogen gas which quickly diffuses in the air. In addition, strontium, like rubidium, oxidizes in air and turns a yellow color. When ignited, it will burn with a strong red flame.