Question

Explain the difference between a covalent bond formed between two atoms of the same element and a covalent bond formed between atoms of two different elements.

Short answer

In a covalent bond between atoms of the same element, their electronegativity values are equal, leading to equal sharing of electrons and forming a nonpolar covalent bond (e.g., H_2, O_2). In a covalent bond between atoms of two different elements, electronegativity values are usually different, resulting in unequal sharing of electrons and creating a polar covalent bond (e.g., H_2O). Thus, the main differences are electronegativity, polarity, and the resulting properties such as boiling and melting points or intermolecular forces.

Step-by-step solution

Introduction to Covalent Bonds

A covalent bond is formed when two atoms share electrons between them in order to achieve a stable electron configuration, usually with a completed outer valence shell. This kind of bonding typically occurs between nonmetal atoms that have similar electronegativity values.

Covalent Bonds Between Same Elements

When two atoms of the same element are bonded together in a covalent bond, their electronegativity values are equal. As a result, the electrons are shared equally between the two atoms, and the bond is called nonpolar covalent bond. Examples of this type of bond include diatomic molecules like H_2, O_2, and Cl_2.

Covalent Bonds Between Different Elements

In a covalent bond between atoms of two different elements, the electronegativity of the atoms is usually not equal. This results in the electrons being more attracted to one atom than the other. Consequently, the electrons are shared unequally between the two atoms, creating a polar covalent bond. An example of this type of bond is the bond between hydrogen and oxygen in a water molecule (H_2O).

Differences Between the Two Types of Covalent Bonds

1. Electronegativity: In a covalent bond between atoms of the same element, the electronegativity values are equal, while in a covalent bond between atoms of two different elements, the electronegativity values are usually different. 2. Polarity: The covalent bond between atoms of the same element is nonpolar due to equal sharing of electrons, while the covalent bond between atoms of two different elements is often polar due to unequal sharing of electrons. 3. Properties: Molecules with polar covalent bonds often have different physical and chemical properties compared to molecules with nonpolar covalent bonds due to their polarity. Polar molecules tend to have higher boiling and melting points, and also show stronger intermolecular forces compared to nonpolar molecules.

Key concepts

Electronegativity

Electronegativity is a critical concept in understanding why atoms bond in the way they do. It refers to the tendency of an atom to attract shared electrons in a covalent bond. Atoms with high electronegativity pull electrons closer to themselves. On the periodic table, electronegativity values generally increase as you move from left to right and bottom to top.

For example, in water (H₂O), oxygen has a higher electronegativity than hydrogen, which means it pulls the shared electrons closer to itself. Understanding the relative electronegativity of different elements is essential for predicting the type of bond they will form: a nonpolar covalent bond, a polar covalent bond, or even an ionic bond if the difference is very high.

Polarity

Polarity in chemistry refers to the distribution of electrical charge around atoms, molecules, or chemical groups. Polarity is directly related to the concept of electronegativity. If two atoms with different electronegativities form a bond, the shared electrons are more likely to be found closer to the atom with the higher electronegativity, creating a polar covalent bond.

Polar molecules, like water, have a partial negative charge near the more electronegative atom and a partial positive charge near the less electronegative atom. This uneven distribution of charge leads to unique properties, such as the tendency of polar substances to dissolve well in water (hydrophilicity), and the occurrence of dipole-dipole interactions between molecules.

Nonpolar Covalent Bond

A nonpolar covalent bond is one where the electrons are shared equally between two atoms. This occurs when the atoms involved have equal or very similar electronegativity values. Molecules such as diatomic nitrogen (N₂) or oxygen (O₂) are perfect examples as the atoms are identical, and thus share electrons equitably.

Nonpolar molecules often exhibit properties such as being non-reactive and having lower melting and boiling points compared to polar molecules. They are also hydrophobic, which means they do not mix well with water, because there is no charge separation to facilitate interaction with water molecules.

Polar Covalent Bond

Polar covalent bonds arise when two atoms with different electronegativities share electrons unequally. The result is a molecule with a slightly positive charge on one end and a slightly negative charge on the other end, known as a dipole. For instance, the water molecule (H₂O) is polar because the oxygen atom is more electronegative than the hydrogen atoms, drawing the shared electrons more to its side.

Polar covalent bonds give rise to molecules with distinctive physical and chemical properties. These molecules can dissolve in water due to their charge polarity, they can align in an electric field, and they tend to have higher melting and boiling points. Polar covalent bonds are the reason we see a wide variety of behaviors in different molecular compounds.