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Cyclopentane, C₅H₁₀ , , , all bond angles ~109^o, but it is NOT planar, pentagon is slightly bent.

Interpreting the fragmentation pattern of the mass spectrum of cyclopentane

[M]⁺ is the molecular ion peak (M) with an m/z of 70 corresponding to [C₅H₁₀]⁺, the original cyclopentane molecule minus an electron, []⁺.

The small M+1 peak at m/z 71, corresponds to an ionised cyclopentane molecule with one ¹³C atom in it i.e. an ionised cyclopentane molecule of formula ¹³C¹²C₄H₁₀

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of cyclopentane.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of cyclopentane

Equations to explain the most abundant ion peaks of cyclopentane

Formation of m/z 42 ion: []⁺  ===>  [C₃H₆]⁺  +  C₂H₄

The m/z 42 ion is the base peak ion, the most abundant and 'stable' ion fragment.

This equates to a loss of two CH₂ units from a broken cyclopentane ring.

Formation of m/z 71 ion by proton loss: []⁺  ===>  [C₅H₉]⁺  +   H

and then does this rapidly break down losing a CH₂ group to give the prominent m/z 55 ion?

 [C₅H₉]⁺  ===>  [C₄H₇]⁺  +  CH₂