Unit 2: Chemical Bonding
Understanding how atoms combine to form the diverse materials around us.
2.4 Chemical Bonding
Chemical bonding is the attractive force that holds atoms, ions, or molecules together to form more stable chemical species. Atoms bond to achieve a more stable electron configuration, typically by having a full outermost electron shell, often following the octet rule (eight valence electrons). This stability is achieved by either gaining, losing, or sharing electrons. There are three main types of primary(Inter-atomic) bonding:
- Ionic Bonding
- Covalent Bonding
- Mettalic Bonding
2.5 Ionic Bonding
Ionic bonding occurs when there is a transfer of one or more electrons from an electropositive atom (typically a metal with low ionization energy) to an electronegative atom (typically a non-metal with high electron affinity). This transfer is driven by a significant difference in electronegativity between the two atoms.
The metal atom loses electrons to become a positively charged ion called a cation, while the non-metal atom gains those electrons to become a negatively charged ion called an anion. An ionic bond is the strong electrostatic force of attraction between these oppositely charged ions.
The formation of sodium chloride (NaCl) is a classic example. A sodium atom (Na) transfers its single valence electron to a chlorine atom (Cl).
$$ Na \rightarrow Na^+ + e^- $$ $$ Cl + e^- \rightarrow Cl^- $$This creates a sodium cation ($Na^+$) and a chloride anion ($Cl^-$). The strong attraction between them forms the ionic bond. This can be shown with a Lewis dot structure:
Solved Examples:
-
Show the formation of magnesium oxide (MgO) using Lewis dot
structures.
Solution: Magnesium (Group 2) has two valence electrons, and Oxygen (Group 6) has six. Mg transfers its two electrons to O. Mg becomes $Mg^{2+}$ and O becomes $O^{2-}$. The oppositely charged ions attract.
-
How many protons and electrons are in a calcium ion
($Ca^{2+}$)?
Solution: Calcium (Ca) has an atomic number of 20, so it always has 20 protons. A neutral Ca atom has 20 electrons. The $2+$ charge indicates it has lost 2 electrons. Therefore, the ion has $20 - 2 = 18$ electrons. -
Explain why potassium (K) and fluorine (F) are likely to form an ionic
bond.
Solution: Potassium (K) is in Group 1, making it a highly electropositive metal that readily loses one electron. Fluorine (F) is in Group 7 and is the most electronegative element, readily gaining one electron. The large difference in electronegativity leads to the transfer of an electron from K to F, forming an ionic bond. -
What is the chemical formula for the compound formed between aluminum (Al)
and oxygen
(O)?
Solution: Aluminum (Group 13) forms an $Al^{3+}$ ion. Oxygen (Group 16) forms an $O^{2-}$ ion. To form a neutral compound, the total positive charge must equal the total negative charge. The lowest common multiple of 3 and 2 is 6. We need two $Al^{3+}$ ions ($2 \times +3 = +6$) and three $O^{2-}$ ions ($3 \times -2 = -6$). The formula is $Al_2O_3$. -
Predict the ion formed by sulfur (S) and give its correct name.
Solution: Sulfur (S) is in Group 16, like oxygen. It needs to gain two electrons to achieve a full outer shell. It forms the anion $S^{2-}$. Simple negative ions are named by taking the root of the element name and adding "-ide". Therefore, the ion is named the sulfide ion.
2.6 Covalent Bonding
When two electronegative atoms (typically non-metals) with similar electronegativities interact, neither is strong enough to completely remove electrons from the other. Instead, they achieve stability by sharing electrons.
Atoms can share one, two, or three pairs of electrons, forming single, double, or triple bonds, respectively.
Dative (Coordinate) Covalent Bonding
A dative covalent bond is a special type where both electrons in the shared pair come from the same atom. This occurs when one atom has a lone pair of electrons and another atom has an empty orbital. For example, the ammonium ion ($NH_4^+$) is formed when the lone pair on the nitrogen atom in ammonia ($NH_3$) forms a bond with a hydrogen ion ($H^+$), which has no electrons.
Solved Examples:
-
Draw the Lewis dot structure for methane ($CH_4$).
Solution: Carbon (Group 14) has 4 valence electrons and each Hydrogen has 1. Carbon shares one electron with each of the four hydrogen atoms, forming four single covalent bonds.
-
Explain the difference between the C-C bond in ethane ($C_2H_6$) and ethene
($C_2H_4$).
Solution: In ethane, the two carbon atoms are joined by a single bond, meaning they share one pair of electrons. In ethene, the two carbon atoms are joined by a double bond, sharing two pairs of electrons. This makes the bond in ethene stronger and shorter. -
Show the formation of the hydronium ion ($H_3O^+$) from water ($H_2O$) and a
hydrogen ion
($H^+$).
Solution: The oxygen atom in a water molecule has two lone pairs of electrons. It can donate one of these lone pairs to form a dative covalent bond with a hydrogen ion ($H^+$), which has an empty orbital.
-
Draw the Lewis structure for a nitrogen molecule ($N_2$) and identify the
type of
bond.
Solution: Each nitrogen atom (Group 15) has 5 valence electrons and needs 3 more to complete its octet. The two nitrogen atoms share three pairs of electrons, forming a triple bond.
-
Explain why two hydrogen atoms form a covalent bond, not an ionic
bond.
Solution: Both hydrogen atoms have identical, intermediate electronegativity. Neither atom is strong enough to pull the electron away from the other completely. Therefore, they share their electrons to form a stable $H_2$ molecule, which is a covalent bond.
2.7 Metallic Bonding
When two electropositive atoms (metals) come together, both have a weak hold on their valence electrons. Instead of transferring or sharing, they release their valence electrons into a shared pool.
This model explains the classic properties of metals. The mobile electrons allow metals to conduct electricity and heat, while the non-directional nature of the bonding allows the layers of ions to slide past one another, making metals malleable (can be hammered into sheets) and ductile (can be drawn into wires).
Solved Examples:
-
Draw a simple 2D diagram to illustrate metallic bonding in sodium
(Na).
Solution: The diagram should show a regular lattice of $Na^+$ cations surrounded by a sea of delocalised electrons (e⁻). Each sodium atom contributes one electron to the sea.
-
Using the concept of metallic bonding, explain why metals are good
conductors of
electricity.
Solution: Metals have delocalised electrons that are not tied to any single atom and are free to move. When a voltage is applied, these mobile electrons can flow towards the positive terminal, creating an electric current. -
Why is the melting point of magnesium (Mg) significantly higher than that of
sodium
(Na)?
Solution: There are two main reasons: 1) A magnesium atom contributes two valence electrons to the delocalised sea, compared to sodium's one. This results in more "glue" holding the ions together. 2) The magnesium ion is $Mg^{2+}$, which has a greater positive charge than $Na^+$. This leads to a stronger electrostatic attraction between the cations and the electron sea, requiring more energy to break the bonds. -
What is an alloy? Give a common example.
Solution: An alloy is a mixture containing at least one metal. They are not compounds because they do not have a fixed composition. A common example is steel, which is an alloy primarily of iron and carbon. -
Predict the type of bond that would form between two calcium (Ca) atoms and
explain your
reasoning.
Solution: Two calcium atoms would form a metallic bond. Calcium is an electropositive metal from Group 2. Like other metals, it will release its valence electrons to form a lattice of $Ca^{2+}$ cations surrounded by a sea of delocalised electrons.
Knowledge Check (20 Questions)
