Why Iron Is Written Fe and Not Ir
Learn what makes a substance pure, how element symbols are derived including the Latin ones, how to classify metals, non-metals, metalloids and noble gases, and what defines a compound.
Why is iron written as Fe and not Ir?
Because its symbol comes from the Latin name ferrum, not the English one. Several familiar metals were named in Latin long before English chemistry existed, and their symbols kept those older roots.
Knowing which elements do this saves marks every time you write a formula. This page covers everything in the ICSE Class 7 Chemistry chapter's first part: pure and impure substances, elements and their symbols, the classification of elements, and compounds.
Knowing which elements do this saves marks every time you write a formula. This page covers everything in the ICSE Class 7 Chemistry chapter's first part: pure and impure substances, elements and their symbols, the classification of elements, and compounds.
What makes a substance pure, and what is an element?
A pure substance is made of only one kind of particle throughout, with a fixed composition and fixed properties such as melting point. An impure substance contains more than one kind.
Pure substances are of two types — elements and compounds. Everything else is a mixture.
An element is a pure substance made of only one kind of atom, and it cannot be split into simpler substances by any chemical means. There are over a hundred of them, and they are the building blocks of everything else.
Examples: hydrogen, oxygen, nitrogen, carbon, sulphur, iron, copper, gold, aluminium, zinc.
The word pure is used differently in a chemistry lab and in a shop. Packaged "pure" ghee or "pure" honey is pure in the everyday sense of unadulterated, but chemically it is a mixture of many substances. In chemistry only a single element or a single compound counts as pure.
That is also why distilled water is chemically pure while mineral water is not, despite both being drinkable and clean. Purity here is about composition, not about cleanliness.
Pure substances are of two types — elements and compounds. Everything else is a mixture.
An element is a pure substance made of only one kind of atom, and it cannot be split into simpler substances by any chemical means. There are over a hundred of them, and they are the building blocks of everything else.
Examples: hydrogen, oxygen, nitrogen, carbon, sulphur, iron, copper, gold, aluminium, zinc.
The word pure is used differently in a chemistry lab and in a shop. Packaged "pure" ghee or "pure" honey is pure in the everyday sense of unadulterated, but chemically it is a mixture of many substances. In chemistry only a single element or a single compound counts as pure.
That is also why distilled water is chemically pure while mineral water is not, despite both being drinkable and clean. Purity here is about composition, not about cleanliness.
How are chemical symbols written?
A symbol is a short form for an element, following three rules.
Rule 1 — first letter, capitalised. H for hydrogen, O for oxygen, N for nitrogen, C for carbon, S for sulphur, P for phosphorus, B for boron, F for fluorine, I for iodine.
Rule 2 — first letter plus one more, the second in small case. Since several elements begin with the same letter, a second letter is added: Ca for calcium, Cl for chlorine, Co for cobalt, Cu for copper, Mg for magnesium, Mn for manganese, Al for aluminium, Zn for zinc, Ba for barium, He for helium.
Rule 3 — from the Latin name. These must simply be learnt:
- Fe — iron, from ferrum
- Cu — copper, from cuprum
- Na — sodium, from natrium
- K — potassium, from kalium
- Ag — silver, from argentum
- Au — gold, from aurum
- Pb — lead, from plumbum
- Sn — tin, from stannum
- Hg — mercury, from hydrargyrum
The capitalisation rule is not decoration, and getting it wrong changes the meaning. Co is the element cobalt, while CO is the compound carbon monoxide — two capitals mean two elements. Similarly Ni is nickel and NI is meaningless.
So always write the first letter capital and any second letter small.
Rule 1 — first letter, capitalised. H for hydrogen, O for oxygen, N for nitrogen, C for carbon, S for sulphur, P for phosphorus, B for boron, F for fluorine, I for iodine.
Rule 2 — first letter plus one more, the second in small case. Since several elements begin with the same letter, a second letter is added: Ca for calcium, Cl for chlorine, Co for cobalt, Cu for copper, Mg for magnesium, Mn for manganese, Al for aluminium, Zn for zinc, Ba for barium, He for helium.
Rule 3 — from the Latin name. These must simply be learnt:
- Fe — iron, from ferrum
- Cu — copper, from cuprum
- Na — sodium, from natrium
- K — potassium, from kalium
- Ag — silver, from argentum
- Au — gold, from aurum
- Pb — lead, from plumbum
- Sn — tin, from stannum
- Hg — mercury, from hydrargyrum
The capitalisation rule is not decoration, and getting it wrong changes the meaning. Co is the element cobalt, while CO is the compound carbon monoxide — two capitals mean two elements. Similarly Ni is nickel and NI is meaningless.
So always write the first letter capital and any second letter small.
How are elements classified?
Elements fall into four groups by their properties.
Metals — lustrous, good conductors of heat and electricity, malleable (can be beaten into sheets), ductile (can be drawn into wires), and sonorous. Iron, copper, aluminium, gold, silver, zinc, sodium. All are solid at room temperature except mercury, which is a liquid.
Non-metals — dull, poor conductors, brittle when solid. Carbon, sulphur, phosphorus, oxygen, nitrogen, chlorine, iodine. They occur in all three states: solid (carbon, sulphur, iodine), liquid (bromine), and gas (oxygen, nitrogen, chlorine, hydrogen).
Metalloids — show properties of both metals and non-metals. Silicon, germanium, arsenic, antimony, boron. Silicon is used in electronic chips precisely because it conducts partly.
Noble gases — chemically almost completely unreactive, which is why they are also called inert gases. Helium, neon, argon, krypton, xenon, radon. All are gases.
Everyday uses follow the properties: copper wires for conduction, gold beaten into thin foil for malleability, helium in balloons because it is light and unreactive, and neon in glowing display tubes.
Two exceptions carry marks on their own. Mercury is the metal that is liquid at room temperature, and bromine is the non-metal that is liquid. Also note that graphite, a form of carbon, conducts electricity even though it is a non-metal.
Metals — lustrous, good conductors of heat and electricity, malleable (can be beaten into sheets), ductile (can be drawn into wires), and sonorous. Iron, copper, aluminium, gold, silver, zinc, sodium. All are solid at room temperature except mercury, which is a liquid.
Non-metals — dull, poor conductors, brittle when solid. Carbon, sulphur, phosphorus, oxygen, nitrogen, chlorine, iodine. They occur in all three states: solid (carbon, sulphur, iodine), liquid (bromine), and gas (oxygen, nitrogen, chlorine, hydrogen).
Metalloids — show properties of both metals and non-metals. Silicon, germanium, arsenic, antimony, boron. Silicon is used in electronic chips precisely because it conducts partly.
Noble gases — chemically almost completely unreactive, which is why they are also called inert gases. Helium, neon, argon, krypton, xenon, radon. All are gases.
Everyday uses follow the properties: copper wires for conduction, gold beaten into thin foil for malleability, helium in balloons because it is light and unreactive, and neon in glowing display tubes.
Two exceptions carry marks on their own. Mercury is the metal that is liquid at room temperature, and bromine is the non-metal that is liquid. Also note that graphite, a form of carbon, conducts electricity even though it is a non-metal.
What is a compound, and how is it different from its elements?
A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass.
Its characteristics are:
- The constituent elements are present in a fixed ratio by mass — water is always 1 part hydrogen to 8 parts oxygen by mass, wherever it comes from.
- It is homogeneous, the same throughout.
- Its properties are completely different from those of its elements.
- Its elements cannot be separated by physical methods, only by chemical means.
- Energy is absorbed or released during its formation.
- It has a fixed melting point and boiling point.
The standard examples make the third point vividly. Water () is formed from hydrogen, which burns, and oxygen, which supports burning — yet water puts fires out. Common salt () is formed from sodium, a metal so reactive it must be stored under kerosene, and chlorine, a poisonous gas — yet salt is safe to eat every day. Carbon dioxide () is formed from solid carbon and a gas that supports burning, yet it is used in fire extinguishers.
That loss of the original properties is the surest sign of a compound, and it is what separates a compound from a mixture. Salt dissolved in water keeps tasting salty because that is a mixture; sodium combined with chlorine does not behave like sodium at all.
Its characteristics are:
- The constituent elements are present in a fixed ratio by mass — water is always 1 part hydrogen to 8 parts oxygen by mass, wherever it comes from.
- It is homogeneous, the same throughout.
- Its properties are completely different from those of its elements.
- Its elements cannot be separated by physical methods, only by chemical means.
- Energy is absorbed or released during its formation.
- It has a fixed melting point and boiling point.
The standard examples make the third point vividly. Water () is formed from hydrogen, which burns, and oxygen, which supports burning — yet water puts fires out. Common salt () is formed from sodium, a metal so reactive it must be stored under kerosene, and chlorine, a poisonous gas — yet salt is safe to eat every day. Carbon dioxide () is formed from solid carbon and a gas that supports burning, yet it is used in fire extinguishers.
That loss of the original properties is the surest sign of a compound, and it is what separates a compound from a mixture. Salt dissolved in water keeps tasting salty because that is a mixture; sodium combined with chlorine does not behave like sodium at all.
Exam tip
Exam tip: capital and small letters in symbols
Symbol questions look easy and lose marks quietly, so fix the habits.
Write the first letter capital and the second small, always.
Learn the nine Latin-derived symbols as a block — Fe, Cu, Na, K, Ag, Au, Pb, Sn, Hg — because these are the ones most often asked and impossible to guess from English.
For classification questions, give a property as well as the label: copper is a metal because it is lustrous, malleable and a good conductor of electricity.
Remember the two liquid exceptions, mercury among metals and bromine among non-metals, and that graphite conducts.
And when asked how a compound differs from its elements, name the elements' properties and the compound's. The contrast is the answer.
Write the first letter capital and the second small, always.
CO and Co are different substances, and an examiner cannot award a mark for a symbol that names something else.Learn the nine Latin-derived symbols as a block — Fe, Cu, Na, K, Ag, Au, Pb, Sn, Hg — because these are the ones most often asked and impossible to guess from English.
For classification questions, give a property as well as the label: copper is a metal because it is lustrous, malleable and a good conductor of electricity.
Remember the two liquid exceptions, mercury among metals and bromine among non-metals, and that graphite conducts.
And when asked how a compound differs from its elements, name the elements' properties and the compound's. The contrast is the answer.
Did you know
Why does salt made from a violent metal and a poisonous gas taste safe?
Because in a compound the elements no longer exist as themselves.
Sodium reacts fiercely with water, and chlorine is choking to breathe. But when they combine chemically, each atom's arrangement changes, and what results — sodium chloride — has its own properties belonging to the new substance alone.
That is the difference between combining and merely mixing. A mixture of sodium and chlorine would still be dangerous; the compound is the salt on your dining table.
Sodium reacts fiercely with water, and chlorine is choking to breathe. But when they combine chemically, each atom's arrangement changes, and what results — sodium chloride — has its own properties belonging to the new substance alone.
That is the difference between combining and merely mixing. A mixture of sodium and chlorine would still be dangerous; the compound is the salt on your dining table.
Key takeaways
Elements, symbols and compounds: quick revision
- A pure substance has one kind of particle and is either an element or a compound; everyday "pure" foods are chemically mixtures.
- An element contains only one kind of atom and cannot be split by chemical means.
- Symbols use the first letter capitalised, sometimes with a second small letter — so Co is cobalt while CO is carbon monoxide.
- Nine symbols come from Latin names: Fe, Cu, Na, K, Ag, Au, Pb, Sn and Hg.
- Elements are metals, non-metals, metalloids or noble gases; mercury is the liquid metal, bromine the liquid non-metal, and graphite a conducting non-metal.
- A compound has its elements combined chemically in a fixed proportion by mass, is homogeneous, cannot be separated physically, and has properties entirely unlike its elements.
You will remember all of this far better after answering five questions on it than after reading it twice.
- An element contains only one kind of atom and cannot be split by chemical means.
- Symbols use the first letter capitalised, sometimes with a second small letter — so Co is cobalt while CO is carbon monoxide.
- Nine symbols come from Latin names: Fe, Cu, Na, K, Ag, Au, Pb, Sn and Hg.
- Elements are metals, non-metals, metalloids or noble gases; mercury is the liquid metal, bromine the liquid non-metal, and graphite a conducting non-metal.
- A compound has its elements combined chemically in a fixed proportion by mass, is homogeneous, cannot be separated physically, and has properties entirely unlike its elements.
You will remember all of this far better after answering five questions on it than after reading it twice.