Symbols, Valency, Radicals and Formulae — Notes

A symbol is a short sign, one capital letter or a capital followed by a small letter, that stands for an element.

Some symbols come from the Latin names of the elements.

• Qualitative meaning: a symbol names the element.
• Quantitative meaning: a symbol stands for one atom of the element, and for its relative atomic mass (for example C: one atom of carbon, 12 u).

Cobalt is Co, but CO is the compound carbon monoxide: the second letter must be small.

Valency is the combining power of an atom: the number of hydrogen atoms (or atoms that can replace hydrogen) that one atom combines with or displaces. Hydrogen has valency 1.

Valence electrons are the electrons in the outermost shell. They decide the valency.

• Metals lose their valence electrons: valency = number of valence electrons (1, 2 or 3).
• Non-metals gain electrons to complete the octet: valency = 8 − number of valence electrons (4 valence electrons gives valency 4).
• A full outer shell (a duplet in helium, an octet in neon and argon) means valency 0.

Some metals have variable valency: iron is 2 in iron(II) and 3 in iron(III); copper is 1 or 2. The Roman number in the name gives the valency. Class IX only needs the common values.

A radical is an atom or a group of atoms that carries a charge and behaves as one unit in chemical reactions.

• Positive (basic) radicals are formed by metals, and also ammonium: sodium Na⁺, potassium K⁺, silver Ag⁺, ammonium NH₄⁺, magnesium Mg²⁺, calcium Ca²⁺, zinc Zn²⁺, copper(II) Cu²⁺, iron(II) Fe²⁺, lead(II) Pb²⁺, barium Ba²⁺, aluminium Al³⁺, iron(III) Fe³⁺.
• Negative (acid) radicals are formed by non-metals or groups with oxygen: chloride Cl⁻, nitrate NO₃⁻, hydroxide OH⁻, hydrogencarbonate HCO₃⁻, oxide O²⁻, sulphide S²⁻, sulphate SO₄²⁻, sulphite SO₃²⁻, carbonate CO₃²⁻, nitride N³⁻, phosphate PO₄³⁻.
• A simple radical is a single charged atom (Na⁺, Cl⁻, O²⁻). A compound radical is a group of atoms with one charge (NH₄⁺, SO₄²⁻, OH⁻).
• The valency of an acid radical is the number of hydrogen ions the acid gives: $\ce{H2SO4 -> 2H+ + SO4^2-}$, so sulphate has valency 2.

Books differ on whether hydroxide is listed with the acid radicals; it is a negative radical, and here it is listed with them for convenience.

This topic is about free radicals in the air, not the charged radicals of formulae.

• A free radical is a very reactive atom or fragment of a molecule with an unpaired electron. It forms when a bond breaks and each part keeps one electron, for example when sunlight breaks a chlorine molecule into two chlorine atoms.
• In polluted air, sunlight and heat act on vehicle and factory fumes and produce such radicals, which react at once with other molecules.

Effects on living organisms

• They irritate the eyes, nose, throat and lungs.
• They can damage cells and tissues in the body.
• They help form smog, and they harm plants and reduce crop growth.

Ways to reduce them

• Cut emissions from vehicles and industries; fit catalytic converters; use cleaner fuels.
• Stop burning rubbish, stubble and plastics in the open.
• Stop the use of chlorofluorocarbons; plant more trees.

Only the idea and the effects are asked at Class IX; the detailed reactions belong to higher classes.

A compound is neutral, so the positive and negative charges cancel. The criss-cross method uses valencies to write the formula.

1. Write the positive radical first, then the negative one.
2. Write the valency of each above its symbol.
3. Cross over: the valency of one becomes the subscript of the other.
4. Divide both subscripts by their common factor; do not write a subscript of 1.
5. Put a compound radical in brackets if its subscript is more than 1.

Naming: the positive part first, then the negative part. One non-metal gets the ending -ide (chloride, oxide, sulphide, nitride). A group with oxygen keeps its name (sulphate, sulphite, nitrate, carbonate, phosphate). Variable valency goes in brackets: FeCl₂ is iron(II) chloride, FeCl₃ is iron(III) chloride. The valency of a metal can also be found back from a formula, for example in Fe₂O₃ three oxygen atoms carry 6 negative charges shared by two iron atoms.