The physical state and properties of a particular compound depend in large part on the type of ubraintv-jp.comical bonding it displays. Molecular compounds, sometimes called covalent compounds, display a wide range of physical properties due to the different types of intermolecular attractions such as different kinds of polar interactions. The melting and boiling points of molecular compounds are generally quite low compared to those of ionic compounds. This is because the energy required to disrupt the intermolecular forces between molecules is far less than the energy required to break the ionic bonds in a crystalline ionic compound (Figure \(\PageIndex{1}\)) . Ionic solids typically melt at high temperatures and boil at even higher temperatures. For example, sodium chloride melts at 801 °C and boils at 1413 °C. (As a comparison, the molecular compound water melts at 0 °C and boils at 100 °C.). The water solubility of molecular compounds is variable and depends primarily on the type of intermolecular forces involved.

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Figure \(\PageIndex{1}\) Interactions in Ionic and Covalent Solids.

(a) The positively and negatively charged ions in an ionic solid such as

sodium chloride (NaCl) are held together by strong electrostatic interactions.

(b) In this representation of the packing of methane (CH4) molecules in solid methane,

a prototypical molecular solid, the methane molecules are held together

in the solid only by relatively weak intermolecular forces, even though

the atoms within each methane molecule are held together by strong covalent bonds.

Since molecular compounds are composed of neutral molecules, their electrical conductivity is generally quite poor, whether in the solid or liquid state. In solid form, an ionic compound is not electrically conductive because its ions are unable to flow (“electricity” is the flow of charged particles). When molten, however, it can conduct electricity because its ions are able to move freely through the liquid (Figure \(\PageIndex{2}\); Video \(\PageIndex{1}\)).

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This figure shows three photos connected by right-facing arrows. The first shows a light bulb as part of a complex lab equipment setup. The light bulb is not lit. The second photo shows a substances being heated or set on fire. The third shows the light bulb again which is lit.