Write the formulas of the binary hydride for the second-period elements LiH to HF. Comment on the change from ionic to covalent character of these compounds. Note that beryllium behaves differently from the rest of the Group 2 A metals (see Section 8.6 ).

Understanding the difference between ionic and covalent bonds is fundamental in chemistry. Ionic bonds occur when one atom donates an electron to another, creating ions that adhere due to opposite charges. In contrast, covalent bonds involve the sharing of electrons between two atoms. Typically, metals will form ionic bonds with nonmetals, as is the case with certain binary hydrides like lithium hydride (LiH), where lithium donates its single electron to hydrogen.

Covalent bonds, on the other hand, typically occur between nonmetals. As we see with molecules such as water (H₂O), where oxygen shares its electrons with hydrogen atoms. In binary hydrides across the second period of the periodic table, we observe a transition from ionic to covalent character as we move from left (lithium) to right (fluorine), mirroring the change in bond type from LiH's ionic nature to HF's covalent bonding.

Electronegativity is a measure of an atom's ability to attract and hold onto electrons when forming a chemical bond. Across the periodic table, electronegativity tends to increase from left to right and decrease as you go down a group. This trend is crucial in predicting the type of bond that will form between atoms.

In the context of binary hydrides, as you move from lithium to fluorine in the second period, the electronegativity increases. Lithium has a much lower electronegativity compared to hydrogen, resulting in the ionic bond of LiH. Beryllium follows suit, although it adopts a slightly different formula (BeH₂). From boron onwards, however, the elements have higher electronegativities, favoring covalent bonding, as seen in BH, CH₄, NH₃, H₂O, and finally HF.

Chemical formulas provide a way of representing the atoms and the number of each kind of atom in a chemical compound. For binary hydrides, which consist of hydrogen and one other element, the formula generally follows the pattern MH or MH₂. The number of hydrogen atoms will vary depending on the element's group number and whether the compound follows the ionic or covalent bonding pattern.

For example, alkali metals like lithium will form hydrides with the formula LiH. Beryllium is an exception in its group and forms BeH₂, reflecting its unique properties compared to other Group 2A metals. As we progress to carbon and beyond, we see an increase in the number of hydrogen atoms, correlating with the number of electrons available for bonding in each element, leading to formulas like CH₄ (methane) and NH₃ (ammonia). The correct depiction of these formulas is vital for understanding the composition and properties of the compounds.