Chemical Valency Table
The essential chart for Writing Formulas. Instantly find the combining capacity of any element, understand Variable Valency, and master the Criss-Cross method.
The Vocabulary of Bonding
If chemistry is a language, atoms are the letters and molecules are the words. Valency is the grammar rule that tells you how many letters can fit together.
In simple terms, Valency is the Combining Capacity of an atom. It represents the number of "arms" an atom has to hold onto other atoms. Carbon has 4 arms, Oxygen has 2, and Hydrogen has 1.
How to Write Formulas: The "Criss-Cross" Method
Want to write the formula for Aluminium Oxide? Just swap their valency numbers!
Why do some elements change? (Variable Valency)
You'll notice in the table that metals like Iron (Fe) and Copper (Cu) have multiple numbers (e.g., 2, 3). These are called Transition Metals.
Unlike simple elements, transition metals have a "backyard" of electrons (the d-orbital) that they can dip into. Under certain conditions, Iron might give away 2 electrons (becoming Fe²⁺), and in others, it might give away 3 (becoming Fe³⁺). This is why rust (Iron-III Oxide) is red, but other iron compounds can be green or black!
Quick Memorization
- 1Group 1 (Alkalis): Always 1 (Li, Na, K)
- 2Group 2 (Earths): Always 2 (Mg, Ca, Ba)
- 3Group 17 (Halogens): Usually 1 (F, Cl, Br, I)
- 4Noble Gases: Always 0 (They are happy!)
Old Names vs New
Frequently Asked Questions
What is the valency of Iron (Fe)?
Iron is a transition metal with variable valency. It typically exhibits a valency of 2 (in Ferrous compounds like FeO) or 3 (in Ferric compounds like Fe₂O₃).
What is the "Criss-Cross" method?
The Criss-Cross method is a technique for writing chemical formulas. You write the valency of the first element above the second element, and vice versa. For example, Aluminium (3) and Oxygen (2) cross over to form Al₂O₃.
Why do noble gases have a valency of 0?
Noble gases like Helium, Neon, and Argon have full outer electron shells (stable octet). They do not need to lose, gain, or share electrons to become stable, so their combining capacity (valency) is zero.
What is the difference between Valency and Oxidation State?
Valency is a simple whole number representing combining capacity (e.g., 2). Oxidation State is a signed number indicating charge (e.g., -2 or +2). While often numerically similar, oxidation state includes direction of electron transfer.
Can valency be negative?
Strictly speaking, valency is just a combining "capacity" and is traditionally a positive whole number. Oxidation states, however, can be positive or negative.
What is the valency of Carbon?
Carbon has a valency of 4 (Tetravalent). This allows it to form four covalent bonds, which is why it is the backbone of organic chemistry.
How do I memorize variable valencies?
Suffixes help! The lower valency usually ends in "-ous" (Cuprous = 1) and the higher one ends in "-ic" (Cupric = 2). However, modern IUPAC naming uses Roman numerals directly, like Copper(I) and Copper(II).
Is valency equal to the group number?
For Groups 1, 2, and 13, valency equals the group number (1, 2, 3). For Groups 14-18, valency usually equals "8 minus the group number" (e.g., Group 17 halogens have valency 8-7=1).
What determines an element's valency?
Valency is determined by the number of valence electrons—the electrons in the outermost shell of the atom. Atoms want 8 electrons (octet) in this shell to be stable.
What is the valency of Gold?
Gold (Aurum) typically shows a valency of 1 (Aurous) or 3 (Auric), though 3 is more common in stable compounds.