What causes cycloalkanes to have two less hydrogen atoms than similar alkanes that are open chain ones?

Because it's a ring; the carbon atoms are limited to four bonds each and it takes away two H atoms when the ring is closed.

The reason cycloalkanes have two less hydrogen atoms than similar open chain alkanes is due to a concept called ring strain.

To understand why this happens, we need to consider the structure of cycloalkanes. A cycloalkane is a closed ring of carbon atoms bonded to hydrogen atoms. In contrast, an open chain alkane, also known as an acyclic alkane, consists of a straight or branched chain of carbon atoms bonded to hydrogen atoms.

In cycloalkanes, the carbon atoms in the ring are bonded to each other and to hydrogen atoms. However, due to the cyclic nature of the molecule, each carbon atom in the ring is also bonded to two adjacent carbon atoms. This introduces a geometric constraint on the molecule, as the carbon atoms in the ring are forced to deviate from their preferred tetrahedral bond angle of 109.5 degrees.

This deviation from the ideal bond angle leads to a phenomenon known as ring strain. Ring strain occurs because the bond angles and bond lengths in cycloalkanes are distorted, which results in increased energy for the molecule. To minimize this strain, cycloalkanes adopt puckered or distorted conformations.

To alleviate ring strain, cycloalkanes adopt a variety of conformations, such as chair, boat, or twist-boat conformations. In these conformations, some carbon atoms are forced closer together, increasing the repulsion between their electron clouds. To reduce this repulsion, cycloalkanes eliminate two hydrogens from the ring, creating what is known as a "ring" or "cyclic" alkene.

By removing two hydrogens, the cycloalkane reduces the steric interactions and minimizes the overall ring strain. This leads to a more stable molecule. Consequently, cycloalkanes have two less hydrogen atoms compared to their open chain counterparts.

In summary, cycloalkanes have two less hydrogen atoms than open chain alkanes due to the presence of ring strain caused by the cyclic nature of their structure. Removing two hydrogens helps to alleviate this strain and stabilize the molecule.