Hello, and welcome to today's chemistry lesson. We are diving into one of the most fundamental topics in chemistry: Chemical Bonding. By the end of this lesson, you will understand why atoms combine, the three main types of chemical bonds, and how to represent them using electron dot structures.
Let us begin with a simple truth: every atom in nature seeks stability. For atoms, stability means achieving the electron arrangement of a noble gas. Helium is stable with just two electrons in its outermost shell — this is called a duplet. All other noble gases — neon, argon, krypton, xenon, and radon — are stable with eight electrons in their outermost shell. This is called an octet.
Atoms with incomplete outer shells are reactive. To become stable, they redistribute their valence electrons through three possible methods. First, by transferring electrons from one atom to another, forming an electrovalent or ionic bond. Second, by sharing electrons between atoms, forming a covalent bond. And third, when the shared electron pair comes entirely from one atom, we get a coordinate or dative bond.
A chemical bond is defined as the force of attraction between any two atoms in a molecule, which maintains stability.
Let us explore electrovalent bonding in detail. This occurs when a metal atom transfers one or more electrons to a non-metal atom. The metal loses electrons and becomes a positively charged ion, called a cation. The non-metal gains electrons and becomes a negatively charged ion, called an anion.
Consider sodium chloride, NaCl. Sodium has the electronic configuration 2, 8, 1. It readily loses one electron to achieve the stable configuration of neon, becoming Na⁺. Chlorine, with configuration 2, 8, 7, gains that electron to complete its octet, becoming Cl⁻. The electrostatic attraction between Na⁺ and Cl⁻ forms the ionic bond.
In magnesium chloride, MgCl₂, magnesium loses two electrons to become Mg²⁺. Since each chlorine atom can accept only one electron, two chlorine atoms are needed, forming two Cl⁻ ions. The formula is therefore MgCl₂.
Calcium oxide, CaO, forms when calcium loses two electrons to oxygen. Calcium becomes Ca²⁺ and oxygen becomes O²⁻. Only one oxygen atom is needed to accept both electrons, giving the formula CaO.
Three conditions favour electrovalent bond formation. Low ionisation potential of the metal, so it loses electrons easily. High electron affinity of the non-metal, so it gains electrons readily. And a large electronegativity difference between the combining atoms. Generally, bonds between metals and non-metals are ionic.
The formation of ionic compounds involves a redox process. The metal undergoes oxidation by losing electrons, while the non-metal undergoes reduction by gaining electrons. Oxidation and reduction always occur simultaneously.
Now, let us turn to covalent bonding. When two non-metal atoms combine, neither can afford to lose electrons. Instead, they share electron pairs to achieve stable configurations.
A single covalent bond involves sharing one electron pair. In hydrogen, H₂, each atom contributes one electron, forming a shared pair that completes the duplet for both. In chlorine, Cl₂, each atom shares one electron to complete its octet.
A double covalent bond involves two shared pairs. In oxygen, O₂, two oxygen atoms share two pairs, written as O double bond O. Carbon dioxide, CO₂, contains two double bonds: O double bond C double bond O.
A triple covalent bond involves three shared pairs. In nitrogen, N₂, two nitrogen atoms share three pairs, forming a triple bond.
The covalency of an atom equals the number of electrons it shares. Hydrogen has covalency one, oxygen two, nitrogen three, and carbon four.
Covalent compounds can be non-polar or polar. Non-polar compounds have equal sharing of electrons, occurring between identical atoms like H₂, Cl₂, or O₂. Some symmetrical molecules like methane, CH₄, and carbon tetrachloride, CCl₄, are also non-polar despite having different atoms.
Polar covalent compounds have unequal electron sharing due to electronegativity differences. In hydrogen chloride, HCl, chlorine pulls the shared pair closer, developing a slight negative charge on itself and a slight positive charge on hydrogen. Such molecules are called dipoles. Water, H₂O, and ammonia, NH₃, are also polar.
Greater electronegativity difference means greater polarity. When the difference exceeds 1.7, the bond becomes essentially ionic.
Finally, we come to coordinate bonding. This is a special type of covalent bond where both electrons in the shared pair come from the same atom.
The donor atom must have a lone pair — electrons not involved in bonding. The acceptor atom must be electron-deficient.
Consider the hydronium ion, H₃O⁺. Water has two lone pairs on its oxygen atom. When a hydrogen ion, H⁺, approaches, oxygen donates one lone pair to form a coordinate bond. The resulting ion has three hydrogen atoms bonded to oxygen: two by regular covalent bonds and one by a coordinate bond.
The ammonium ion, NH₄⁺, forms similarly. Ammonia, NH₃, has a lone pair on nitrogen. This lone pair is donated to a hydrogen ion, creating NH₄⁺. Once formed, all four nitrogen-hydrogen bonds become identical.
Ammonium chloride beautifully demonstrates all three bond types. Within the ammonium ion, there are covalent and coordinate bonds. Between NH₄⁺ and Cl⁻, there is an ionic bond.
Let us compare electrovalent and covalent compounds.
Electrovalent compounds are hard crystalline solids with high melting and boiling points. They conduct electricity when molten or dissolved in water, because their ions become mobile. They are soluble in polar solvents like water but insoluble in organic solvents. They react rapidly in aqueous solutions due to free ions.
Covalent compounds are typically gases, liquids, or soft solids with low melting and boiling points. They do not conduct electricity, as they lack free ions. Non-polar covalent compounds are insoluble in water but dissolve in organic solvents. Polar covalent compounds like HCl ionise in water and can conduct electricity.
Here are your key takeaways from this lesson.
First, atoms combine to achieve stable noble gas configurations through duplet or octet completion.
Second, electrovalent bonds form by electron transfer between metals and non-metals, creating ions held by electrostatic attraction.
Third, covalent bonds form by electron sharing between non-metals, and can be single, double, or triple depending on the number of shared pairs.
Fourth, coordinate bonds are special covalent bonds where one atom donates both electrons in the shared pair.
Fifth, electronegativity differences determine bond polarity: zero difference gives non-polar bonds, small differences give polar covalent bonds, and large differences above 1.7 give ionic bonds.
Sixth, electrovalent compounds conduct electricity when molten or aqueous due to mobile ions, while most covalent compounds do not conduct electricity.
That brings us to the end of our lesson on Chemical Bonding. You have learned how atoms achieve stability, the three types of chemical bonds, and how to distinguish between different kinds of compounds. Keep practising electron dot structures, and remember: chemistry is all about understanding how the smallest particles build our material world. Stay curious, and I will see you in the next lesson.