Write formulas for and name the binary hydrogen compounds of the second-period elements (Li to F). Describe how the physical and chemical properties of these compounds change from left to right across the period.

Understanding how to write chemical formulas for binary hydrogen compounds is fundamental in chemistry. A binary compound is one made up of two different elements. In this case, we focus on those that contain hydrogen and one other element from the second period of the periodic table, which ranges from lithium to fluorine. To properly write the chemical formulas for these compounds, we must recognize that hydrogen (H) has a valency of 1, meaning it can form one bond.

Starting with lithium (Li), which also has a valency of 1, we combine it with hydrogen to form lithium hydride, represented as \(LiH\). Beryllium (Be) has a valency of 2, leading to the formula \(BeH_2\) for beryllium hydride. This pattern continues, considering the valency of each subsequent element. For boron (B), carbon (C), nitrogen (N), oxygen (O), and fluorine (F), their hydrides are represented as \(BH_3\), \(CH_4\), \(NH_3\), \(H_2O\), and \(HF\), respectively.

It's crucial for students to remember the valency of hydrogen when pairing it with other elements to ensure that the resulting chemical formula is balanced and correct.

The periodic table is not just a list of elements but a grid of patterns known as trends. These trends help predict several properties of elements including their reactivity and the nature of compounds they form with hydrogen. As we consider the second-period elements from lithium (Li) to fluorine (F), we observe a trend from metals to nonmetals. This transition reflects in the type of binary hydrogen compounds formed.

For instance, lithium and beryllium form ionic compounds with hydrogen, since they are metals. These ionic compounds have high melting points and are good conductors of electricity in molten state or when dissolved in water. Moving right, from boron to fluorine, the elements become increasingly nonmetallic, and thus, the compounds they form with hydrogen, such as \(BH_3\), \(CH_4\), \(NH_3\), and \(HF\), are covalent. Covalent bonding leads to a variety of molecular compounds with different physical and chemical properties, primarily influenced by the number of valence electrons and electronegativity of the elements involved.

The physical and chemical properties of binary hydrogen compounds vary significantly across the second period of the periodic table. Starting with lithium hydride (\(LiH\)), which is an ionic compound, it has a high melting point and, as with beryllium hydride (\(BeH_2\)), is a good electrical conductor in its liquid form. This ionic character decreases as we move to compounds like borane (\(BH_3\)), methane (\(CH_4\)), and ammonia (\(NH_3\)), which are all covalent.

The polarity and boiling points of these compounds increase along the period. Polarity refers to the separation of electric charge leading to a molecule having an electric dipole, with a partial positive charge on one side and a partial negative charge on the other. Fluorine, with its high electronegativity, forms hydrogen fluoride (\(HF\)), which is highly polar and has a higher boiling point compared to the other compounds mentioned. Students should observe the connection between electronegativity, bond formation, and compound properties to better understand the behavior of these compounds.