Final answer:
The question seeks to identify elements with an atomic number less than 8 and an atomic mass greater than 8.1 u. These elements include lithium, beryllium, boron, carbon, and nitrogen.
Step-by-step explanation:
The question is asking about a chemical element with an atomic number less than 8 and an atomic mass greater than 8.1 atomic mass units (u). In the periodic table, elements are ordered according to their atomic number, which is the number of protons in their nucleus, and atomic mass represents the average weight of the atoms, primarily composed of the sum of protons and neutrons in the atom's nucleons.
Atomic mass unit (u) is a standard unit of mass that quantifies mass on an atomic or molecular scale, specifically defined as one twelfth of the mass of a carbon-12 atom.
This definition implies that elements with an atomic number (Z) less than 8 include hydrogen (Z=1), helium (Z=2), lithium (Z=3), beryllium (Z=4), boron (Z=5), carbon (Z=6), and nitrogen (Z=7). Among these elements, those with an atomic mass greater than 8.1 u are lithium, beryllium, boron, carbon, and nitrogen because they have mass numbers greater than 8. Helium has a mass number of 4, which does not meet the criteria, while hydrogen's most abundant isotope has a mass number of 1.