Chemistry Lesson # 4 Molecular Compounds

Ionic vs. Molecular Bonds Recall that in ionic bonding, a positive and negative ion come together and are called a salt, or ionic compound. This involves the transfer of electrons from a metal to a non-metal. When working with only non-metals, a different type of bond forms, called a molecular, or covalent bond. A molecular or covalent bond is a shared pair of electrons held between two non-metal atoms that hold the atoms together in a stable molecule.

Diatomic Molecules When two of the same non-metal are covalently bonded together, it is called a molecular element, or diatomic molecule (di = two). Chlorine is an example of a molecular element – in nature it exists as a pair: Cl2. Other molecules that can form diatomic bonds are: H, O, N, F, Cl, Br, I – the "hockey stick and puck" on the periodic table.

Properties of Molecular Compounds When two or more different non-metals are covalently bonded together, it is called a molecular compound. Molecular compounds tend to be soft, they do not conduct electricity, and they have low melting points (which is why sugar so easily melts and turns black when on a stove).

Naming Molecular Compounds It is much easier to determine names and formulas for molecular compounds, because there is no “criss cross rule” involved. You simply “name them like you see them.” This means, that if given the formula, you can determine the name from the number of each element present, and vice versa. Before this, you need the prefixes associated with each number.

Prefixes 1 – mono 6 – hexa 2 – di 7 – hepta 3 – tri 8 – octa 4 – tetra 9 – nona 5 – penta 10 – deca

Examples C3H8 Diphosphorus Pentoxide NO Silicon Tetrafluoride SBr6 Nitrogen Trihydride

Drawing Molecular Compounds Similar to ionic compounds, you must draw the Lewis structure for each atom, and determine how many of each atom you need to make the compound stable. The only difference is that the electrons are shared (circled) versus transferred (arrow). You also need to determine the central atom – this is always the non-metal that has the most bonding (lone) electrons.

Examples Carbon + Hydrogen Nitrogen + Fluorine

Examples (continued) Silicon + Oxygen Sulfur + Chlorine

Videos Ionic vs. Covalent to “Dancing Queen” Ionic vs. Covalent “Love”