Do isomers have weak covalent bonds as compared to normal hydrocarbons (alkanes)?

Isomers are species of the same chemical formula, but DIFFERENT connectivity. The subject is usually broached in organic chemistry, because carbon chemistry offers great opportunity for both #"structural"# and #"geometric isomerism"#.

The formula #C_4H_10# is the smallest alkane molecule that can support isomerism; we can have differences in the connectvity in the carbon chain to give 2 structural isomers, #"n-butane"#, #H_3C-CH_2CH_2CH_3#, and #"isobutane"#, #H_3C-CH(CH_3)CH_3#. Are these both #C_4H_10# molecules? As the alkane gets longer, the opportunity for isomerism gets much larger. Note that these are different compounds, with different physical constants, though with the same molecular formula.

In both of the given isomers, #C-C# and #C-H# bonds, are individually strong and are comparable.

Geometric isomerism is a further refinement of isomerism. Geometric isomers have of course the same formula (because they are isomers!), and the same CONNECTIVITY but different GEOMETRY. You will have to seek illustrations of examples in your text.