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All elements in a period have the same number of electron shells. Each next element in a period has one more proton and is less metallic than its predecessor. Arranged this way, groups of elements in the same column have similar chemical and physical properties, reflecting the periodic law. For example, the halogens lie in the second-last column (group 17) and share similar properties, such as high reactivity and the tendency to gain one electron to arrive at a noble-gas electronic configuration.
In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However, in the d-block (lanthanides), trends across periods become significant, and in the f-block elements (actinides) show a high degree of similarity across periods.
There are currently 7 complete periods in the periodic table, comprising the 118 known elements. Any new elements will be placed into an eighth period. Inside the periodic table, elements are generally color-coded below by their block for convenience.
The first period contains the fewest elements from the table, with only two, hydrogen and helium. They do not follow the octet rule, but rather a duplet rule. Chemically, helium behaves like a noble gas, and thus is often placed in group 18 elements. However, in terms of its nuclear structure it belongs to the s-block, and is therefore sometimes classified as a group 2 element, or simultaneously both 2 and 18. Hydrogen readily loses and gains an electron, and so behaves chemically as both a group 1 and a group 17 element.
Period 2 elements involve the 2s and 2p orbitals. Elements contained in this period are: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon. At standard conditions, the first four elements are solid and the other four are gasses.
Period 3 elements occur in nature and have at least one stable isotope. Elements in this period are: sodium, magnesium, aluminium, phosphorus, sulfur, chlorine and argon. They all display an empty d orbital, which alters their oxidation states.
Period 4 contains most of the biologically essential metallic elements and is the first period in the d-block with the lighter transition metals. Elements in this period are: potassium, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, arsenic, selenium, bromine and krypton. Of all the elements from this period, bromine is the first element liquid at room temperature, while krypton is the first of the noble gas elements that can form true chemical compounds at standard conditions.
Period 5 has the same number of elements as period 4 and follows the same general structure but with one more post transition metal and one fewer nonmetal. Elements in period 5 are: rubidium, strontium, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, antimony, tellurium and xenon. Period 5 also includes technetium, the lightest exclusively radioactive element.
Period 6 is the first period to include the f-block, with the lanthanides, and includes the heaviest stable elements. Elements in this period are: caesium, barium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, thallium, lead, bismuth, polonium, astatine and radon.
All elements of period 7 are radioactive. Elements in this period are: francium, radium, actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, curium, einsteinium, fermium, mendelevium, nobelium, lawrencium, rutherfordium, dubnium, seaborgium, bohrium, hassium, meitnerium, darmstadtium, roentgenium, copernicium, nihonium, flerovium, moscovium, livermorium, tennessine and oganesson. This period contains the heaviest element which occurs naturally on Earth, plutonium. All of the subsequent elements in the period have been synthesized artificially. Whilst five of these (from americium to einsteinium) are now available in macroscopic quantities, most are extremely rare, having only been prepared in microgram amounts or less. Some of the later elements have only ever been identified in laboratories in quantities of a few atoms at a time.
Beyond period 7
No element of the 8th period has yet been synthesized. A g-block is predicted. It is not clear if all elements predicted for the eighth period are in fact physically possible. There may therefore be no 9th period.