In the figure above, I've illustrated the molecular orbitals and it's immediatly obvious that they are no longer nicely symmetric as they've been all along. The bonding orbital bulges toward the hydrogen and the antibonding orbital bulges slightly on the Li side. As a result, the hydrogen end of the moelcule would be slightly negative and the lithium end would be slightly positive. This situation is called a polar bond in which the electrons in the bond are being shared, but not equally shared. In almost every case in which a bond is formed between two different atoms the resulting bond will be polar.
Electronegativity only has meaning in a bond.
The table below presents the electronegativities for the main group elements. Generally, the electronegativity increases moving left to right across a row, and decreases going down the table. Notice that this trend is violated by the Group 13 metals for which the electronegativity drops from B to Al as expected, but hen rises slightly going down to Tl. This effect is due to the intervention of the d electrons and other effects that come into play with very large atoms. The transition metals are not presented in this chart to conserve room, but their values range from 1.0 to about 2.4.