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Anna Kowalski

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calendar_month2025-12-16

Valency: The Rulebook for Chemical Partnerships

Understanding the simple number that dictates how elements combine to form molecules.

Summary: Valency is the fundamental combining capacity of an element, representing the number of chemical bonds it can form with other atoms. It is a key concept that connects the world of atoms to the vast universe of compounds we see around us. This article explores the valency definition, its historical development from atomic theory, and how it relates to an element's position on the periodic table. We will learn how to determine valency from simple chemical formulas and use it to predict the formation of common molecules like water ($H_2O$) and sodium chloride ($NaCl$).

What Exactly is Valency?

Imagine atoms as individuals with a certain number of "hands" they can use to hold hands with other atoms. Valency tells us how many "hands" an atom of a particular element has available for bonding. Formally, it is defined as the number of hydrogen atoms, or twice the number of oxygen atoms, that one atom of an element can combine with or displace in a chemical reaction.

For example, in water ($H_2O$), one oxygen atom combines with two hydrogen atoms. Therefore, the valency of oxygen is 2. In methane ($CH_4$), one carbon atom combines with four hydrogen atoms, so the valency of carbon is 4. Valency is usually represented by a simple positive whole number (e.g., 1, 2, 3, 4). This number is central to writing chemical formulas correctly.

Quick Rule of Thumb: The valency of a non-metal can often be found by noting the number of hydrogen atoms it combines with. The valency of a metal can be found by noting the number of chlorine atoms it combines with.

The Atomic Secret Behind Valency: Electrons

Why does hydrogen have a valency of 1 and oxygen a valency of 2? The answer lies in the arrangement of electrons around the nucleus. The electrons in the outermost shell of an atom are called valence electrons. These are the key players in chemical bonding.

Atoms tend to be most stable when their outermost shell is completely filled, a state known as having a "noble gas configuration." Valency is determined by how many electrons an atom needs to gain, lose, or share to achieve this stable state.

Losing Electrons (Metals): Metals like sodium ($Na$) have 1 or 2 valence electrons. It's easier for them to lose these electrons to achieve a full outer shell. The valency of such a metal is equal to the number of electrons it loses. Sodium loses 1 electron to form $Na^+$, so its valency is 1.
Gaining or Sharing Electrons (Non-metals): Non-metals like oxygen have 6 or 7 valence electrons. It's easier for them to gain or share electrons to complete their outer shell. Oxygen needs 2 electrons, so its valency is 2.

Element	Symbol	Valence Electrons	Common Valency	How it Achieves Stability
Sodium	$Na$	1	1	Loses 1 electron
Magnesium	$Mg$	2	2	Loses 2 electrons
Chlorine	$Cl$	7	1	Gains 1 electron
Oxygen	$O$	6	2	Gains or shares 2 electrons
Carbon	$C$	4	4	Shares 4 electrons

Valency's Map: The Periodic Table

The periodic table is a chemist's roadmap, and valency follows clear trends across it. For main group elements^[1], the group number often reveals the number of valence electrons. This gives a strong hint about their maximum valency.

Group 1 (Alkali Metals): Elements like Lithium ($Li$), Sodium ($Na$), Potassium ($K$) have 1 valence electron and a consistent valency of 1.
Group 2 (Alkaline Earth Metals): Elements like Magnesium ($Mg$), Calcium ($Ca$) have 2 valence electrons and a valency of 2.
Groups 13-18: For non-metals, the story is slightly different. Their valency is often equal to 8 minus the group number. This gives the number of electrons they need to gain. For example, Oxygen is in Group 16. 8 - 16 = -2, but we take the absolute value, so its valency is 2. Chlorine (Group 17): 8 - 17 = -1, so its valency is 1.

Some elements, however, can exhibit more than one valency. This is called variable valency. A classic example is Iron ($Fe$), which can have a valency of 2 (as in Ferrous oxide, $FeO$) or 3 (as in Ferric oxide, $Fe_2O_3$). Copper and sulfur are other common examples.

Putting Valency to Work: Writing Chemical Formulas

This is where valency becomes a practical and essential tool. Knowing the valencies of elements allows us to predict how they will combine and write the correct chemical formula. The guiding principle is that chemical compounds are electrically neutral. The total positive valency must equal the total negative valency.

The Criss-Cross Method: A simple way to write formulas. Write the symbols of the elements with their valencies as superscripts. Then, criss-cross these numbers to become subscripts (but omit '1' and reduce to the simplest ratio).

Example for Magnesium Chloride: $Mg^{2}$ and $Cl^{1}$ → Criss-cross the 2 and 1 → $Mg_1Cl_2$ → Simplify to $MgCl_2$.

Let's build a molecule from scratch. If we want aluminum to bond with oxygen, we follow these steps:

Identify valencies: Aluminum (Al) has a valency of 3. Oxygen (O) has a valency of 2.
Find the smallest whole numbers where the total positive charge equals the total negative charge.
Positive: Al (3+) × ?
Negative: O (2-) × ?
The least common multiple of 3 and 2 is 6. So we need 2 Al atoms (2 × 3+ = 6+) and 3 O atoms (3 × 2- = 6-).
Write the formula with these numbers as subscripts: $Al_2O_3$.

This is aluminum oxide, the main component of sapphires and rubies!

Valency in Action: From Salt to Sugar

Let's look at practical examples where valency explains everyday substances.

Table Salt (Sodium Chloride): Sodium (Na, valency 1) and Chlorine (Cl, valency 1) combine in a perfect 1:1 ratio to form NaCl. Each sodium atom donates one electron to a chlorine atom, satisfying both their needs.

Water (H₂O): The quintessential example. Hydrogen (H, valency 1) and Oxygen (O, valency 2) combine. It takes two hydrogen atoms, each offering one "bonding hand," to satisfy oxygen's need for two. This gives H₂O.

Calcium Carbonate (Chalk, Marble): Calcium (Ca, valency 2) and the carbonate ion (CO₃, valency 2) combine in a 1:1 ratio to form CaCO₃. The carbonate ion itself is a group of atoms with an overall valency of 2, demonstrating that ions^[2] and radicals^[3] also possess valency.

Glucose (Sugar): $C_6H_{12}O_6$. This looks complex, but valency still holds. Each Carbon (valency 4) forms four bonds, each Oxygen (valency 2) forms two, and each Hydrogen (valency 1) forms one. The entire structure is a network where these valency rules are perfectly satisfied.

Valency vs. Oxidation Number: A Crucial Distinction

As you advance in chemistry, you will encounter the term oxidation number (or oxidation state). It is easy to confuse it with valency, but they are different concepts.

Valency is a simple whole number representing bonding capacity. It is always positive and does not consider the type of bond (ionic or covalent).

Oxidation Number is a theoretical charge assigned to an atom, assuming all bonds are 100% ionic. It can be positive, negative, zero, or even a fraction. In water (H₂O), oxygen has a valency of 2 but an oxidation number of -2. Hydrogen has a valency of 1 but an oxidation number of +1.

For many simple ions, the magnitude of the oxidation number equals the valency (e.g., Na⁺ has oxidation state +1 and valency 1; O^2- has oxidation state -2 and valency 2). However, in covalent molecules like methane (CH₄), carbon has a valency of 4 but an oxidation number of -4.

Conclusion: Valency is the foundational concept that brings order to the chemical universe. It provides the rulebook for how elements connect, from the simplicity of table salt to the complexity of DNA. By understanding valency's connection to electron structure and its predictable patterns on the periodic table, we unlock the ability to decipher chemical formulas and predict the existence of new compounds. While modern chemistry uses more advanced concepts like oxidation states and molecular orbital theory, valency remains the essential first step in the journey of understanding chemical bonding.

Important Questions

Q1: Can the valency of an element be zero?
Yes. Elements that already have a completely filled outer electron shell are chemically inert and have virtually no tendency to form bonds. The noble gases like Helium (He), Neon (Ne), and Argon (Ar) are said to have a valency of zero.

Q2: Why do some elements have variable valency?
Variable valency occurs because some elements can lose different numbers of electrons from their valence shell, or sometimes from a penultimate shell, depending on the reaction conditions. Transition metals^[4] are famous for this. For example, Iron can lose two electrons from its 4s orbital to form Fe²⁺, or lose an additional electron from its 3d orbital to form the more stable Fe³⁺.

Q3: Is valency the same thing as the number of bonds we see in a structural formula?
For most simple molecules, yes. In a structural formula, a single line represents a bond, and the number of lines connected to an atom usually equals its valency. In a water molecule (H-O-H), oxygen has two bonds (lines), matching its valency of 2. However, in cases of coordinate covalent bonds^[5], the structural count might visually exceed the simple valency number, but the fundamental combining capacity is still governed by electron availability.

Footnote

^[1] Main group elements: The elements in groups 1, 2, and 13 through 18 of the periodic table. Their chemical properties are largely determined by the electrons in their outermost (valence) shell.
^[2] Ion: An atom or molecule that has gained or lost one or more electrons, giving it a net positive or negative electrical charge (e.g., Na⁺, Cl^-, SO₄^2-).
^[3] Radical (or Polyatomic ion): A group of atoms that are bonded together and carry an overall net charge, behaving as a single unit in chemical reactions (e.g., Carbonate $CO_3^{2-}$, Sulfate $SO_4^{2-}$).
^[4] Transition metals: The metallic elements found in the central block of the periodic table (Groups 3-12). They are characterized by having partially filled d-orbitals and often exhibit multiple oxidation states (variable valency).
^[5] Coordinate covalent bond: A type of covalent bond where both shared electrons in the bond come from the same atom.

#IGCSE #Chemical Bonding #Valence Electrons #Periodic Table #Chemical Formula #Oxidation State

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