<Tr> <Th> Group </Th> <Th> </Th> <Th> </Th> <Th> 13 </Th> <Th> 14 </Th> <Th> 15 </Th> <Th> 16 </Th> <Th> 17 </Th> <Th> 18 </Th> </Tr> <Tr> <Th> Atomic #Name </Th> <Td> Li </Td> <Td> Be </Td> <Td> 5 </Td> <Td> 6 </Td> <Td> 7 </Td> <Td> 8 O </Td> <Td> 9 </Td> <Td> 10 Ne </Td> </Tr> <P> Period 2 elements involve the 2s and 2p orbitals . They include the biologically most essential elements besides hydrogen: carbon, nitrogen, and oxygen . </P> <Ul> <Li> Lithium (Li) is the lightest metal and the least dense solid element . In its non-ionized state it is one of the most reactive elements, and so is only ever found naturally in compounds . It is the heaviest primordial element forged in large quantities during the Big Bang . </Li> <Li> Beryllium (Be) has one of the highest melting points of all the light metals . Small amounts of beryllium were synthesised during the Big Bang, although most of it decayed or reacted further within stars to create larger nuclei, like carbon, nitrogen or oxygen . Beryllium is classified by the International Agency for Research on Cancer as a group 1 carcinogen . Between 1% and 15% of people are sensitive to beryllium and may develop an inflammatory reaction in their respiratory system and skin, called chronic beryllium disease . </Li> <Li> Boron (B) does not occur naturally as a free element, but in compounds such as borates . It is an essential plant micronutrient, required for cell wall strength and development, cell division, seed and fruit development, sugar transport and hormone development, though high levels are toxic . </Li> <Li> Carbon (C) is the fourth-most abundant element in the universe by mass after hydrogen, helium and oxygen and is the second-most abundant element in the human body by mass after oxygen, the third-most abundant by number of atoms . There are an almost infinite number of compounds that contain carbon due to carbon's ability to form long stable chains of C--C bonds . All organic compounds, those essential for life, contain at least one atom of carbon; combined with hydrogen, oxygen, nitrogen, sulfur, and phosphorus, carbon is the basis of every important biological compound . </Li> <Li> Nitrogen (N) is found mainly as mostly inert diatomic gas, N, which makes up 78% of the Earth's atmosphere by volume . It is an essential component of proteins and therefore of life . </Li> <Li> Oxygen (O) comprising 21% of the atmosphere by volume and is required for respiration by all (or nearly all) animals, as well as being the principal component of water . Oxygen is the third-most abundant element in the universe, and oxygen compounds dominate the Earth's crust . </Li> <Li> Fluorine (F) is the most reactive element in its non-ionized state, and so is never found that way in nature . </Li> <Li> Neon (Ne) is a noble gas used in neon lighting . </Li> </Ul>

The elements c n and o are contained in