Lesson 4: Chemical Reactivity, Ion Formation, Ionic and Covalent Bonds
The chemical reactivity of any element is directly governed by the count of valence electrons inhabiting its outermost energy level. When this valence level is completely filled with the stable octet of eight electrons (or two for Helium), the element is chemically inert and does not enter chemical reactions under ambient conditions, exemplified by the noble gases. Conversely, when the outermost level is incomplete, the atom is chemically active, striving to attain octet stability by losing, gaining, or sharing electrons with partner atoms. Metallic atoms possess fewer than four valence electrons and tend to lose them during chemical reactions, forming positive ions (cations) carrying positive charges equal to the lost electrons. Nonmetallic atoms possess more than four valence electrons and tend to gain electrons, forming negative ions (anions). Electrostatic attraction between positive cations and negative anions establishes ionic bonds, yielding ionic compounds such as sodium chloride. In contrast, nonmetals share electron pairs to construct covalent bonds, categorized into single, double, and triple bonds according to ministerial curriculum directives.
1. Metallic Behavior & Positive Ion Formation
Metals contain 1, 2, or 3 electrons in their valence level (< 4):
- Reason for Loss: To reach the stable electronic configuration of the nearest preceding noble gas.
- Positive Charge: Positive because the number of nuclear protons becomes greater than the number of orbital electrons.
- Sodium Example:
₁₁Na (2,8,1) ⇐ loses 1e⁻ ⇐ Na⁺ (2,8)carrying 1 positive charge. - Magnesium Example:
₁₂Mg (2,8,2) ⇐ loses 2e⁻ ⇐ Mg²⁺ (2,8)carrying 2 positive charges.
2. Nonmetallic Behavior & Negative Ion Formation
Nonmetals contain 5, 6, or 7 electrons in their valence level (> 4):
- Reason for Gain: To complete their valence shell with 8 electrons, mimicking the nearest succeeding noble gas.
- Negative Charge: Negative because the number of orbital electrons exceeds the number of nuclear protons.
- Chlorine Example:
₁₇Cl (2,8,7) + 1e⁻ ⇐ Cl⁻ (2,8,8)carrying 1 negative charge. - Oxygen Example:
₈O (2,6) + 2e⁻ ⇐ O²⁻ (2,8)carrying 2 negative charges.
3. The Ionic Bond & Its Distinct Characteristics
A chemical bond originating from electrostatic attraction between a positive metallic cation and a negative nonmetallic anion:
- Produces Compound Molecules Exclusively: An ionic bond cannot occur between two atoms of the same element due to electrostatic repulsion, always producing compound molecules (e.g., Table salt NaCl, Magnesium oxide MgO).
- No Physical Existence: It is merely electrostatic attraction between opposite electrical charges.
4. The Covalent Bond & Its Three Types
A bond formed between nonmetallic atoms by mutually sharing electron pairs:
| Bond Type | Shared Electron Pairs | Elemental Examples | Compound Examples |
|---|---|---|---|
| Single Covalent (-) | 1 pair (1 electron from each atom) | Hydrogen (H - H) | Water (H - O - H), HCl |
| Double Covalent (=) | 2 pairs (2 electrons from each atom) | Oxygen (O = O) | Carbon dioxide (O = C = O) |
| Triple Covalent (≡) | 3 pairs (3 electrons from each atom) | Nitrogen (N ≡ N) | Carbon monoxide |
Covalent bonding produces both element molecules (H2, O2, N2) and compound molecules (H2O, HCl).
Problem 1: Formation of Sodium Chloride (NaCl)
Question: Show how the ionic bond in sodium chloride NaCl is established.
• Sodium atom ₁₁Na (2,8,1) loses 1 valence electron to become Na⁺ (2,8).
• Chlorine atom ₁₇Cl (2,8,7) gains this electron to become Cl⁻ (2,8,8).
• Electrostatic attraction pulls Na⁺ and Cl⁻ together, forming ionic NaCl.
Problem 2: Bonding in Oxygen Molecule (O2)
Question: Explain the bond type in an oxygen molecule O2 (Oxygen Z = 8).
• Oxygen configuration: ₈O (2,6). Each atom requires 2 electrons to complete its valence shell.
• Each atom contributes 2 electrons, sharing 2 pairs: Double covalent bond (O = O).
Problem 3: Bonding in Nitrogen Molecule (N2)
Question: Deduce the bond type in nitrogen gas N2 (Nitrogen Z = 7).
• Configuration of ₇N: K = 2, L = 5.
• Each atom needs 3 electrons. Both atoms share 3 electron pairs: Triple covalent bond (N ≡ N).
Problem 4: Bonds in Water Molecule (H2O)
Question: How many and what types of covalent bonds exist in a water molecule?
• Oxygen shares 1 electron with the first hydrogen (single covalent bond).
• Oxygen shares another electron with the second hydrogen (second single covalent bond).
• A water molecule contains two single covalent bonds (H - O - H).
Problem 5: Why Ionic Bonds Produce Compounds Only
Question: Give reason: An ionic bond results in compound molecules only and never element molecules.
• Because it only arises between atoms of different elements (a metal that loses electrons and a nonmetal that gains them). It cannot occur between identical atoms due to like charges repelling.
Problem 6: Comparing Sodium Atom and Ion
Question: Compare a sodium atom ₁₁Na and a sodium ion Na⁺ regarding proton count and energy levels.
• Both contain 11 protons.
• Sodium atom has 3 energy levels (2,8,1), whereas sodium ion has only 2 levels (2,8) due to losing its outermost electron.
Problem 7: Magnesium Oxide (MgO)
Question: Explain the bond type in magnesium oxide MgO (Magnesium Z = 12, Oxygen Z = 8).
• Mg (2,8,2) loses 2e⁻ ⇐ Mg²⁺.
• O (2,6) gains 2e⁻ ⇐ O²⁻.
• Electrostatic attraction creates an ionic bond.
Problem 8: Deducing Ion Charge
Question: A nonmetallic atom has 16 protons in its nucleus. What is the charge and formula of its ion?
• Configuration: (2,8,6) Sulfur S.
• Needs 2 electrons to complete 8 ⇐ gains 2e⁻.
• Ion charge is negative divalent: S²⁻.
Tap any card to reveal its meaning, or tap the audio icon to listen:
Click on an option to check your answer and view the detailed explanation: