“Why Does Oxygen Reacting with Hydrogen Form Liquid Water, While Other Oxygen Family Elements Yield Gases?”

When oxygen reacts with hydrogen, it forms H2O, also known as water, which is a liquid. In contrast, when other elements in the oxygen family, such as sulfur, selenium, tellurium, and polonium, react with hydrogen, they form hydrides that are typically gases under normal conditions.

The formation of liquid water when oxygen reacts with hydrogen, as opposed to the production of gases when other elements in the oxygen family react with hydrogen, can be attributed to two main reasons, i.e. ‘Electronegativity’ and ‘Hydrogen Bonding’.

Consider water (H2O) and hydrogen sulfide (H2S). Both molecules consist of hydrogen and an element from the oxygen group (oxygen in water and sulfur in hydrogen sulfide), but they exhibit different behaviours due to their electronegativity. The former results in the formation of liquid i.e. Water, and the latter results in the formation of a gas called, Hydrogen Sulfide.

While elements within the same group of the periodic table (such as the oxygen group) typically share similar chemical behaviours due to their similar electronic configurations, there are also elements outside of the group that may exhibit similar properties due to certain shared characteristics. This is especially true for elements that are close to each other in the periodic table.

In the case of oxygen and its neighbouring elements, such as nitrogen, phosphorus, sulfur, and chlorine, there are indeed some shared chemical behaviours, despite them belonging to different groups. This is because they have similar outer electron configurations and can form similar types of chemical bonds.

Now let’s dive deeper into the factors influencing the formation of liquid H2O and Hydrides:

• Factors influencing the formation of Liquid H20:

What is a Hydrogen Bond: The partially positive hydrogen atom in one water molecule is attracted to the partially negative oxygen atom in another water molecule. This attraction is known as a hydrogen bond. It’s important to note that hydrogen bonds are not traditional chemical bonds where electrons are shared. Instead, they are weaker electrostatic attractions between molecules.

Role of Hydrogen Bonds: So, even though the oxygen atom in a water molecule is stable with a full valence shell, its lone pairs of electrons can still interact with other atoms or molecules, such as the hydrogen atoms in neighboring water molecules, through hydrogen bonding.

In a water molecule (H2O), the oxygen atom has six valence electrons: two are involved in covalent bonds with the hydrogen atoms, and the remaining four are arranged in two pairs of electrons that are not involved in bonding. These pairs of electrons are called lone pairs because they are not shared with any other atom.

When we say that the lone pairs of electrons on the oxygen atom of another water molecule are attracted to the positively charged hydrogen atom, we mean that these unshared pairs of electrons are drawn towards the partially positively charged hydrogen atom in another water molecule. This attraction is due to the electrostatic force between the opposite charges (the negative charge of the lone pair electrons and the positive charge of the hydrogen atom), which forms a hydrogen bond.

These hydrogen bonds are relatively weak compared to covalent bonds within the water molecule, but they are strong enough to affect the properties of water as a whole. In the liquid state, water molecules are in constant motion but are held together by hydrogen bonds, giving water its fluidity.

Hydrogen bonding also contributes to water’s unusually high boiling point, as a significant amount of energy is required to break these bonds. These hydrogen bonds between water molecules create cohesive forces, meaning they hold the water molecules together. As a result, water exists as a liquid at room temperature and standard pressure because the cohesive forces between water molecules are strong enough to overcome the thermal energy that would otherwise cause the molecules to separate and become a gas.

So, the high electronegativity of the oxygen atom and its ability to form hydrogen bonds is what results in the formation of liquid H2O.

• Factors influencing the formation of Hydrides/Gases:

The formation of hydrides or gases when other elements of the oxygen family bond with hydrogen can be explained by the following reasons:

Electronegativity: Other elements in the oxygen family, such as sulfur, selenium, tellurium, and polonium, have lower electronegativities than oxygen. As a result, they are less capable of attracting electrons and forming strong bonds with hydrogen. This means they attract electrons less strongly when bonding with hydrogen, resulting in weaker bonds between the elements. Weaker bonds make it easier for the resulting compound to break apart or exist as gases, leading to the formation of hydrides such as hydrogen sulfide (H2S) or hydrogen selenide (H2Se), which are gases at room temperature.

Intermolecular Forces: Intermolecular forces are interactions between molecules that determine their physical properties, such as boiling point, solubility, and state of matter.

When oxygen reacts with hydrogen to form water, strong hydrogen bonds are formed between the oxygen and hydrogen atoms within water molecules. These hydrogen bonds are a type of intermolecular force that results from the attraction between the positively charged hydrogen atoms and the negatively charged oxygen atoms.

On the other hand, when other elements of the oxygen family react with hydrogen to form hydrides, the resulting molecules typically have weaker intermolecular forces. These weaker forces may include van der Waals forces, such as dipole-dipole interactions and London dispersion forces. These intermolecular forces are generally less effective at holding molecules together compared to hydrogen bonds. As a result, hydrides formed by other oxygen family elements often have lower boiling points and are more likely to exist as gases at room temperature and standard pressure. This is because the weaker intermolecular forces are unable to overcome the thermal energy present, causing the molecules to separate and become a gas more easily.

To sum it up, liquid H20 forms due to the higher electronegativity of the oxygen atom and its ability to form hydrogen bonds. On the contrary, hydrides form due to the lower electronegativity of the other elements of the oxygen family and the presence of weaker intermolecular forces like dipole-dipole interactions and London dispersion forces.But there is one element of the oxygen family which is more electronegative than oxygen but its bonding with hydrogen does not result in the formation of a liquid, rather a gas called Hydrogen Fluoride. Find out more in our next blog.