Molecules, Polyatomic Ions & Valency - Ninth Grade Science
With the exception of the noble gases—such as helium, neon, and argon—isolated individual atoms are chemically unstable and cannot exist independently under normal terrestrial conditions. To achieve electronic stability, atoms combine with one another, forming higher-order chemical entities known as molecules and ions. When non-metallic atoms share electrons, they form electrically neutral molecules; when atoms gain or lose electrons, they form electrically charged ions. Furthermore, groups of atoms can band together, carrying an overall electrical charge as polyatomic ions. Underlying all these chemical unions is a fundamental chemical capacity: valency, the combining power of an element. In this chapter, you will investigate how molecules and ions are constructed, differentiate homonuclear from heteronuclear molecules, analyze polyatomic ions, and master the concept of valency.
The Concept of a Molecule
A molecule is defined as the smallest particle of an element or a chemical compound that is capable of independent existence under ordinary conditions and retains all the characteristic physical and chemical properties of that substance.
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| Molecules of Matter |
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| 1. Molecules of Elements: Composed of ONLY ONE kind of atom. |
| Examples: O_2 (Diatomic), O_3 (Triatomic), P_4 (Tetratomic). |
| 2. Molecules of Compounds: Composed of TWO OR MORE different atoms. |
| Examples: H_2O (Water), NH_3 (Ammonia), CH_4 (Methane). |
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Molecules of Elements vs Molecules of Compounds
Molecules of Elements: Molecules of Compounds:
(Single atomic species) (Different atomic species bonded)
(O)-(O) (N)-(N) (H)-(O)-(H)
Oxygen (O_2) Nitrogen (N_2) Water (H_2O)
(P)-(P) (S)-(S)-(S)-(S) (H)
| | | | |
(P)-(P) (S)-(S)-(S)-(S) (H)-(C)-(H)
Phosphorus (P_4) Sulfur (S_8 ring) |
(H)
Methane (CH_4)
Atomicity: Quantifying Molecular Composition
Atomicity is defined as the total number of atoms constituting a single molecule of an element or compound:
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| Atomicity Class | Number of Atoms | Chemical Formula | Element Name |
+---------------------+-------------------+-------------------+--------------------+
| Monoatomic | 1 atom / molecule | He, Ne, Ar, Kr | Noble Gases |
| Diatomic | 2 atoms / molecule| H_2, N_2, O_2, Cl_2| Active Gases |
| Triatomic | 3 atoms / molecule| O_3 | Ozone |
| Tetratomic | 4 atoms / molecule| P_4 | White Phosphorus |
| Octatomic / Poly | 8 atoms / molecule| S_8 | Rhombic Sulfur |
+---------------------+-------------------+-------------------+--------------------+
Noble gases (helium, neon, argon) exist as single, unbonded atoms possessing stable outer electron shells; their atomicity is 1 (monoatomic). Oxygen gas exists as diatomic molecules ($ ext{O}_2$), while ozone gas exists as triatomic molecules ($ ext{O}_3$). Solid sulfur forms crown-shaped rings containing eight sulfur atoms ($ ext{S}_8$), making it polyatomic.
The Concept of Ions: Cations and Anions
When atoms interact, they can gain or lose valence electrons to achieve stable electron configurations, transforming neutral atoms into charged particles called ions.
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| Cation (Positive Ion) | Anion (Negative Ion) |
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| Formed by the LOSS of electrons | Formed by the GAIN of electrons |
| Net positive electrical charge | Net negative electrical charge |
| Typically formed by METALS | Typically formed by NON-METALS |
| Attracted to Cathode (-) | Attracted to Anode (+) |
| Examples: Na+, Mg2+, Al3+, Ca2+ | Examples: Cl-, O2-, N3-, S2- |
+------------------------------------+------------------------------------+
Formation of Ions via Electron Transfer
Sodium Atom (Na): 11 p+, 11 e- (Neutral) Chlorine Atom (Cl): 17 p+, 17 e- (Neutral)
- 1 electron (Loss) + 1 electron (Gain)
| |
v v
Sodium Cation (Na+): 11 p+, 10 e- Chloride Anion (Cl-): 17 p+, 18 e-
Net Charge: +1 Net Charge: -1
Monoatomic Ions
A monoatomic ion consists of a single atom that has acquired an electrical charge through electron gain or loss:
- A neutral sodium atom ($ ext{Na}$, 11 protons, 11 electrons) loses one electron to form a sodium cation: $ ext{Na} \rightarrow ext{Na}^+ + ext{e}^-$.
- A neutral chlorine atom ($ ext{Cl}$, 17 protons, 17 electrons) gains one electron to form a chloride anion: $ ext{Cl} + ext{e}^- \rightarrow ext{Cl}^-$.
- Electrostatic attraction between $ ext{Na}^+$ and $ ext{Cl}^-$ binds them into a three-dimensional crystal lattice of sodium chloride ($ ext{NaCl}$), an ionic compound.
Polyatomic Ions: Charged Molecular Clusters
A polyatomic ion is a tightly bound cluster of two or more covalently bonded atoms that acts as a single chemical unit and carries an overall net electrical charge.
The Architecture of a Polyatomic Ion
Ammonium Cation (NH_4+)
[ H ]
|
[ H ] - N - [ H ] +1 Charge Overall!
|
[ H ]
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| Polyatomic Ion Name | Chemical Formula | Charge | Classification |
+-----------------------+-------------------+----------+-------------------+
| Ammonium | NH_4^+ | +1 | Monovalent Cation |
| Hydroxide | OH^- | -1 | Monovalent Anion |
| Nitrate | NO_3^- | -1 | Monovalent Anion |
| Nitrite | NO_2^- | -1 | Monovalent Anion |
| Bicarbonate (Hydrogen | HCO_3^- | -1 | Monovalent Anion |
| carbonate) | | | |
| Carbonate | CO_3^2- | -2 | Divalent Anion |
| Sulfate | SO_4^2- | -2 | Divalent Anion |
| Sulfite | SO_3^2- | -2 | Divalent Anion |
| Phosphate | PO_4^3- | -3 | Trivalent Anion |
+-----------------------+-------------------+----------+-------------------+
In the sulfate ion ($ ext{SO}_4^{2-}$), one sulfur atom is bonded to four oxygen atoms, and the entire group carries an excess of two electrons, giving the cluster a net $-2$ charge. The polyatomic ion acts as an indivisible chemical entity during standard reactions.
The Concept of Valency: Combining Capacity
Valency is defined as the combining capacity of an atom or radical (polyatomic ion), representing the number of hydrogen atoms (or chlorine atoms) that can combine with or displace one atom of the element during chemical bonding.
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| Valency at a Glance |
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| Basis: The number of electrons an atom loses, gains, or shares to |
| achieve a stable noble gas octet (8 valence electrons). |
| Nature: Pure positive integer without an algebraic sign (1, 2, 3, 4). |
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Valency Based on Hydrogen and Oxygen
Historically, hydrogen was assigned a baseline valency of 1:
- In hydrogen chloride ($ ext{HCl}$), one chlorine atom bonds with one hydrogen atom; the valency of chlorine is 1 (monovalent).
- In water ($ ext{H}_2 ext{O}$), one oxygen atom bonds with two hydrogen atoms; the valency of oxygen is 2 (divalent).
- In ammonia ($ ext{NH}_3$), one nitrogen atom bonds with three hydrogen atoms; the valency of nitrogen is 3 (trivalent).
- In methane ($ ext{CH}_4$), one carbon atom bonds with four hydrogen atoms; the valency of carbon is 4 (tetravalent).
Determining Valency from Electronic Configuration
At the electronic level, valency is governed by the octet rule:
1. For metals with 1, 2, or 3 valence electrons: $ ext{Valency} = ext{Number of Valence Electrons}$. Sodium (configuration $2,8,1$) loses 1 electron, so its valency is 1. Magnesium ($2,8,2$) loses 2 electrons, so its valency is 2. Aluminum ($2,8,3$) loses 3 electrons, so its valency is 3.
2. For non-metals with 4, 5, 6, or 7 valence electrons: $ ext{Valency} = 8 - ext{Number of Valence Electrons}$. Oxygen ($2,6$) gains 2 electrons, so its valency is $8 - 6 = 2$. Nitrogen ($2,5$) gains 3 electrons, so its valency is $8 - 5 = 3$. Chlorine ($2,8,7$) gains 1 electron, so its valency is $8 - 7 = 1$. Carbon ($2,4$) shares 4 electrons, so its valency is $8 - 4 = 4$.
Table of Common Valencies and Ions
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| Valency Class | Valency = 1 | Valency = 2 | Valency = 3 |
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| Metal Cations | Na+, K+, Ag+, Cu+ | Mg2+, Ca2+, Zn2+, | Al3+, Fe3+ (Ferric)|
| | | Fe2+ (Ferrous) | |
| Non-Metal Anions | H-, F-, Cl-, Br- | O2-, S2- | N3-, P3- |
| Polyatomic Ions | NH_4+, OH-, NO_3-,| CO_3^2-, SO_4^2- | PO_4^3- |
| | HCO_3- | | |
+-------------------+-------------------+-------------------+--------------------+
Variable Valency
Certain transition metal elements exhibit more than one valency depending on reaction conditions, a phenomenon known as variable valency:
- Iron ($ ext{Fe}$) exhibits a valency of 2 in iron(II) compounds (ferrous, $ ext{Fe}^{2+}$) and a valency of 3 in iron(III) compounds (ferric, $ ext{Fe}^{3+}$).
- Copper ($ ext{Cu}$) exhibits a valency of 1 in copper(I) compounds (cuprous, $ ext{Cu}^+$) and a valency of 2 in copper(II) compounds (cupric, $ ext{Cu}^{2+}$).
Chapter Exercises
Exercise 1: Determine the atomicity and classify each of the following molecular entities as homonuclear (molecules of an element) or heteronuclear (molecules of a compound): molecular nitrogen gas ($ ext{N}*2$), carbon dioxide ($ ext{CO}_2$), ozone gas ($ ext{O}_3$), sulfuric acid ($ ext{H}_2 ext{SO}_4$), white phosphorus ($ ext{P}_4$), and glucose ($ ext{C}_6 ext{H}*{12} ext{O}_6$).
Exercise 2: Write the chemical formula, state the magnitude of the net electrical charge, and determine the valency for each of the following polyatomic ions: ammonium, sulfate, nitrate, phosphate, and hydroxide.
Exercise 3: An element X has an atomic number of 12, and an element Y has an atomic number of 17. Write the electronic configuration for each element, determine the valency of element X and element Y, and state whether each element forms a cation or an anion when reacting together.
Solutions and Explanations
Solution 1: Molecular nitrogen ($ ext{N}*2$) has an atomicity of 2 (diatomic) and is homonuclear; carbon dioxide ($ ext{CO}_2$) has an atomicity of $1 + 2 = 3$ (triatomic) and is heteronuclear; ozone gas ($ ext{O}_3$) has an atomicity of 3 (triatomic) and is homonuclear; sulfuric acid ($ ext{H}_2 ext{SO}_4$) has an atomicity of $2 + 1 + 4 = 7$ (polyatomic) and is heteronuclear; white phosphorus ($ ext{P}_4$) has an atomicity of 4 (tetratomic) and is homonuclear; and glucose ($ ext{C}_6 ext{H}*{12} ext{O}_6$) has an atomicity of $6 + 12 + 6 = 24$ (polyatomic) and is heteronuclear.
Solution 2: The chemical formulas, net electrical charges, and valencies are: Ammonium: formula is $ ext{NH}_4^+$, net charge is $+1$, valency is 1 (monovalent cation). Sulfate: formula is $ ext{SO}_4^{2-}$, net charge is $-2$, valency is 2 (divalent anion). Nitrate: formula is $ ext{NO}_3^-$, net charge is $-1$, valency is 1 (monovalent anion). Phosphate: formula is $ ext{PO}_4^{3-}$, net charge is $-3$, valency is 3 (trivalent anion). Hydroxide: formula is $ ext{OH}^-$, net charge is $-1$, valency is 1 (monovalent anion).
Solution 3: Element X has atomic number 12, giving the electronic configuration $2, 8, 2$. Because it has two valence electrons, it achieves a stable octet by losing two electrons, meaning its valency is 2, and it forms a divalent cation ($ ext{X}^{2+}$). Element Y has atomic number 17, giving the electronic configuration $2, 8, 7$. Because it has seven valence electrons, it achieves a stable octet by gaining one electron ($8 - 7 = 1$), meaning its valency is 1, and it forms a monovalent anion ($ ext{Y}^-$). When they react, element X transfers two electrons to two atoms of element Y, forming the ionic compound $ ext{XY}_2$ (magnesium chloride, $ ext{MgCl}_2$).