Names and Formulae of Substances Revision Notes for Grade 10 NSC Matric Physical Sciences

Names and Formulae of Substances

Introduction to chemical naming

Just like people need names to identify them, chemical substances also need names so scientists can communicate effectively. Think of it like your friends having both full names (like substance names) and nicknames (like chemical formulae). Without proper names, we would have no way to tell others which chemical we're talking about.

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The analogy between human names and chemical identification helps us understand why systematic naming is so crucial in chemistry. Just as confusion would arise if everyone had the same name, chemistry requires precise identification systems.

Chemical substances have two ways of being identified:

Chemical names - the full descriptive names
Chemical formulae - the shorthand symbols using element symbols

Elements are easy to name because we use the names from the periodic table. Mixtures can be described with simple words. However, compounds require special naming rules because they contain multiple elements combined together.

Guidelines for naming compounds

When naming chemical compounds, there are important rules to follow that ensure consistency and clarity in scientific communication:

1. Include element names

The compound name will always include the names of all elements that are part of the compound, but these names are slightly modified.

Examples:

Iron ( $Fe$ ) + sulfur ( $S$ ) = iron sulfide ( $FeS$ )
Potassium ( $K$ ) + bromine ( $Br$ ) = potassium bromide ( $KBr$ )
Sodium ( $Na$ ) + chlorine ( $Cl$ ) = sodium chloride ( $NaCl$ )

2. Order of elements

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In a compound, the element that appears on the left of the Periodic Table is written first in the compound name. This rule is essential for consistent naming across all chemical compounds.

For example, in $NaCl$ , sodium is a Group 1 element (left side) and chlorine is in Group 7 (right side), so sodium comes first.

3. Chemical formulae

The symbols of elements can be used to create chemical formulae. These show the ratio of elements in each compound using a 1:1 ratio in simple cases.

Examples:

$FeS$ (1 iron atom : 1 sulfur atom)
$NaCl$ (1 sodium atom : 1 chlorine atom)
$H_2O$ (2 hydrogen atoms : 1 oxygen atom)

4. Ions in compounds

A compound may contain ions, which are atoms that have lost or gained electrons. Ions can be simple (one element) or compound (several elements combined).

Types of ions

Understanding the two main types of ions is fundamental to chemical naming and formula writing.

Cations (positive ions)

Cations are positively charged ions that form when atoms lose electrons. Here are the most important ones to memorise:

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The term "cation" comes from the Greek word meaning "to go down" - these ions move toward the negative electrode (cathode) in electrolysis.

Ion Name	Formula	Ion Name	Formula	Ion Name	Formula
Hydrogen	$H^+$	Lithium	$Li^+$	Sodium	$Na^+$
Potassium	$K^+$	Silver	$Ag^+$	Mercury (I)	$Hg^+$
Copper (I)	$Cu^+$	Ammonium	$NH_4^+$	Beryllium	$Be^{2+}$
Magnesium	$Mg^{2+}$	Calcium	$Ca^{2+}$	Barium	$Ba^{2+}$
Tin (II)	$Sn^{2+}$	Lead (II)	$Pb^{2+}$	Chromium (II)	$Cr^{2+}$
Manganese (II)	$Mn^{2+}$	Iron (II)	$Fe^{2+}$	Cobalt (II)	$Co^{2+}$
Nickel	$Ni^{2+}$	Copper (II)	$Cu^{2+}$	Zinc	$Zn^{2+}$
Aluminium	$Al^{3+}$	Chromium (III)	$Cr^{3+}$	Iron (III)	$Fe^{3+}$
Cobalt (III)	$Co^{3+}$	Chromium (VI)	$Cr^{6+}$	Manganese (VII)	$Mn^{7+}$

Anions (negative ions)

Anions are negatively charged ions that form when atoms gain electrons:

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The term "anion" comes from the Greek word meaning "to go up" - these ions move toward the positive electrode (anode) in electrolysis.

Ion Name	Formula	Ion Name	Formula
Fluoride	$F^-$	Oxide	$O^{2-}$
Chloride	$Cl^-$	Peroxide	$O_2^{2-}$
Bromide	$Br^-$	Carbonate	$CO_3^{2-}$
Iodide	$I^-$	Sulfide	$S^{2-}$
Hydroxide	$OH^-$	Sulfite	$SO_3^{2-}$
Nitrite	$NO_2^-$	Sulfate	$SO_4^{2-}$
Nitrate	$NO_3^-$	Thiosulfate	$S_2O_3^{2-}$
Hydrogen carbonate	$HCO_3^-$	Chromate	$CrO_4^{2-}$
Hydrogen sulfite	$HSO_3^-$	Dichromate	$Cr_2O_7^{2-}$
Hydrogen sulfate	$HSO_4^-$	Manganate	$MnO_4^{2-}$
Dihydrogen phosphate	$H_2PO_4^-$	Oxalate	$(COO)_2^{2-}/C_2O_4^{2-}$
Hypochlorite	$ClO^-$	Hydrogen phosphate	$HPO_4^{2-}$
Chlorate	$ClO_3^-$	Nitride	$N^{3-}$
Permanganate	$MnO_4^-$	Phosphate	$PO_4^{3-}$
Acetate (ethanoate)	$CH_3COO^-$	Phosphide	$P^{3-}$

Common covalent compounds

Some compounds have special names that don't follow the regular naming rules. These covalent compounds are often known by their common names:

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These common names have been used for centuries in chemistry and are so well-established that they continue to be used instead of systematic names. For example, we say "water" instead of "dihydrogen monoxide."

Name	Formula
Water	$H_2O$
Sulfuric acid	$H_2SO_4$
Ethane	$C_2H_6$
Nitric acid	$HNO_3$
Hydrochloric acid	$HCl$
Methane	$CH_4$
Ammonia	$NH_3$

Using prefixes in naming

Prefixes help us describe the ratio of elements in compounds. These are especially useful for non-metals and provide precise information about molecular composition:

Important prefixes:

Mono = one (1)
Di = two (2)
Tri = three (3)

Examples:

CO (carbon monoxide) - one oxygen atom for every carbon atom
NO₂ (nitrogen dioxide) - two oxygen atoms for every nitrogen atom
SO₃ (sulfur trioxide) - three oxygen atoms for every sulfur atom

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For metals, we use Roman numerals (I, II, III, IV) in brackets to show the charge instead of prefixes. This is because metals form ionic compounds, not covalent ones.

Writing chemical formulae - worked examples

Understanding how to write chemical formulae correctly requires systematic practice with the charge balancing method.

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Worked Example: Writing the formula for sodium fluoride

Step 1: Identify the ions involved

Sodium ion: $Na^+$
Fluoride ion: $F^-$

Step 2: Write down the charges

Sodium has a +1 charge
Fluoride has a -1 charge

Step 3: Find the right combination

For every positive charge, we need a negative charge
+1 from sodium cancels out -1 from fluoride
They combine in a 1:1 ratio

Step 4: Write the formula $NaF$

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Worked Example: Writing the formula for magnesium chloride

Step 1: List the ions involved

$Mg^{2+}$ and $Cl^-$

Step 2: Find the right combination

Magnesium has a +2 charge
Chloride has a -1 charge
We need two chloride ions to balance one magnesium ion
The ratio is 1:2

Step 3: Write the formula $MgCl_2$

Tip: You can use a cross method:

Write $Mg^{2+}$ crossed with 2, $Cl^-$ crossed with 1
This gives you $Mg_1Cl_2$ , which simplifies to $MgCl_2$

Exam tips for formulae writing

Success in chemical formula writing requires consistent application of key principles and thorough preparation:

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Essential Exam Strategies:

Always balance charges - the total positive charge must equal the total negative charge
Learn common ions - memorise the ion tables as they appear frequently in exams
Check your ratios - make sure you've used the simplest whole number ratio
Practice prefixes - know mono-, di-, tri- for covalent compounds
Roman numerals - use these for metals that can have different charges

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Key Points to Remember:

Chemical names help scientists communicate about substances effectively
Compound names include the names of all elements, with the left-side periodic table element first
Cations are positive ions, anions are negative ions
Chemical formulae use element symbols to show the ratio of atoms in compounds
Charge balance is essential - total positive charges must equal total negative charges
Common compounds like water ( $H_2O$ ) and ammonia ( $NH_3$ ) have special names to memorise

Names and Formulae of Substances (Grade 10 NSC Matric Physical Sciences): Revision Notes