Final answer:
Hydrogen, helium, lithium, beryllium, and boron are distinguished by their atomic numbers, which indicate the number of protons in the atom's nucleus. They are sequentially ordered in the periodic table and share similar chemical properties due to having the same number of valence electrons within their respective groups.
Step-by-step explanation:
Hydrogen (H), helium (He), lithium (Li), beryllium (Be), and boron (B) all have in common that they are differentiated by the number of protons in the nucleus of the atom. Hydrogen has one proton, helium has two, lithium has three, beryllium has four, and boron has five. This count of protons is known as the atomic number and is fundamental for identifying the element.
Furthermore, when looking at the periodic table, we can observe that these elements are some of the lightest and are organized sequentially in order of increasing atomic number. These elements, up to boron, are located in the first two rows, which corresponds to their shells being in the first or second energy level, and they have increasing numbers of electrons as we move from left to right within the row.
Additionally, elements in the same group (column) have similar chemical properties because they have the same number of valence electrons. Hydrogen, though commonly placed in Group 1, is not a metal like lithium, which is a member of the alkali metals. Beryllium is an alkaline earth metal with two valence electrons. This commonality in valence electrons across each group leads to similarities in chemical reactivity and bonding characteristics among the elements in the same group.