<Tr> <Th> Atomic #Name </Th> <Td> </Td> <Td> He </Td> </Tr> <P> The first period contains fewer elements than any other, with only two, hydrogen and helium . They therefore do not follow the octet rule . Chemically, helium behaves as a noble gas, and thus is taken to be part of the 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 . </P> <Ul> <Li> Hydrogen (H) is the most abundant of the chemical elements, constituting roughly 75% of the universe's elemental mass . Ionized hydrogen is just a proton . Stars in the main sequence are mainly composed of hydrogen in its plasma state . Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons such as methane . Hydrogen can form compounds with most elements and is present in water and most organic compounds . </Li> <Li> Helium (He) exists only as a gas except in extreme conditions . It is the second - lightest element and is the second-most abundant in the universe . Most helium was formed during the Big Bang, but new helium is created through nuclear fusion of hydrogen in stars . On Earth, helium is relatively rare, only occurring as a byproduct of the natural decay of some radioactive elements . Such' radiogenic' helium is trapped within natural gas in concentrations of up to seven percent by volume . </Li> </Ul> <Li> Hydrogen (H) is the most abundant of the chemical elements, constituting roughly 75% of the universe's elemental mass . Ionized hydrogen is just a proton . Stars in the main sequence are mainly composed of hydrogen in its plasma state . Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons such as methane . Hydrogen can form compounds with most elements and is present in water and most organic compounds . </Li>

There are groups and periods in the periodic table