The valence electrons in hydrogen, along with the other atoms in a molecule, determine its shape. In the case of water (H2O), the valence electrons in hydrogen participate in covalent bonding with the oxygen atom. Water adopts a V-shape, or bent shape, due to the arrangement of the electron pairs around the oxygen atom.
Water has two hydrogen atoms bonded to a central oxygen atom. The oxygen atom has six valence electrons, two of which are involved in the covalent bonding with the hydrogen atoms. The remaining four valence electrons in oxygen form two lone pairs, which repel the bonding electron pairs. This electron pair repulsion causes the hydrogen atoms to be pushed closer together, resulting in a bent or V-shaped molecular geometry.
The concept of electron pair repulsion can be extrapolated to predict the shapes of other molecules, with or without hydrogen involved. This concept is known as the valence shell electron pair repulsion (VSEPR) theory. According to VSEPR theory, electron pairs (whether bonding or lone pairs) around the central atom in a molecule repel each other, and the molecule adopts a shape that minimizes this repulsion.
In the VSEPR theory, the number of electron pairs around the central atom determines the molecular shape. The most common molecular shapes include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. The specific arrangement of electron pairs depends on the number of bonding and lone pairs.
For example, carbon dioxide (CO2) has a linear shape. It consists of a carbon atom double-bonded to two oxygen atoms. Each oxygen atom has two lone pairs of electrons. Since there are no lone pairs on the central carbon atom, the electron pairs are arranged in a straight line, resulting in a linear molecular geometry.
By considering the number of valence electrons and the arrangement of bonding and lone pairs around the central atom, you can apply the VSEPR theory to predict the molecular shape of various compounds, regardless of the presence of hydrogen.