Are all linear molecules polar

In chemistry, polarity refers to the dipole-dipole intermolecular forces between the slightly positively-charged end of one molecule to the negative end of another or the same molecule. Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. For example, water is thought to be polar because of the unequal sharing of its electrons. However, methane is considered non-polar because the carbon shares the hydrogen atoms uniformly. A compound is composed of one or more chemical bonds between different atoms. The polarity of each bond within the compound may determine the overall polarity of the compound: how polar or non-polar it is.

A polar molecule may be polar as a result of polar bonds or as a result of an asymmetric arrangement of non-polar bonds and non bonding pairs of electrons.

Example 1. A polar molecule by virtue of polar bonds (bonds which have unequal sharing of electrons between the two atoms involved in bonding), e.g hydrogen fluoride, HF, where the bonding pair of electron is displaced towards the more electronegative fluorine atom.

Example 2. In ammonia, NH3, the three N-H bonds have only a slight polarity (towards the more electronegative nitrogen atom), however the lone pair of electrons (pointing towards the fourth apex of the approximate tetrahedron, (VSEPR) is electron rich and results in a powerful dipole across the whole ammonia molecule.

A non-polar compound may be non polar because there is (almost) no polarity in the bonds or because of the symmetrical arrangement of polar bonds.

Example 3. Methane, CH4 The four C-H bonds, arranged tetrahedrally around the carbon atom, has very little polarity in the bonds and so there is no dipole in the molecule.

Example 4. BF3, boron trifluoride has a trigonal planar arrangement of three polar bonds at 120o This results in no overall dipole in the molecule.

Overall: Linear molecules=always polar