Elements
¶ What is an element?
An element is a pure substance that cannot be split up into two or more simpler substances by chemical processes or by electricity.
¶ Give examples of elements and explain your choice.
Copper, carbon, hydrogen and oxygen are elements because they cannot be further broken down into simpler substances.
¶ All known elements and their symbols are recorded in a table known as the Periodic Table.
Chemical symbols of elements
¶ Chemists use chemical symbols to represent elements.
¶ Each element has a unique symbol consisting of one or two letters.
o In the table below, give the symbol of some common elements.
Element | Symbol | Element | Symbol |
Calcium | Ca | Mercury | Hg |
Carbon | C | Neon | Ne |
Hydrogen | H | Silicon | Si |
Iron | Fe | Sodium | Na |
Classification of Elements (Metals and Non-metals)
¶ Elements can be classified into two major groups: metals and non-metals.
¶ Iron is a metal. Oxygen is a non-metal. How are we able to differentiate between a metallic element and a non-metal element?
¶ The answer is by making use of the Periodic Table. The elements in the Periodic Table are arranged from metallic to non-metallic elements.
Þ In the table below write down 6 main differences between metals and non-metals.
Metals | Non-metals |
Shiny appearance
| Dull appearance |
Malleable (can be hammered into different shapes), Ductile (into wires) | Brittle if solid |
Solid at r.t.p | Gases, volatile liquids or solids with low m.p at r.t.p |
High melting and boiling points | Low melting points |
Good conductors of heat
| Poor conductors of heat |
Good conductors of electricity
| Poor conductors of electricity |
Atoms and molecules
¶ Elements are made up of tiny particles called atoms.
¶ The atoms of one element are not the same as the atoms of another element.
¶ Do all elements exist as atoms?
No. Helium, neon, argon, krypton, xenon and radon are the only elements that exist as individual atoms. They are called monotomic elements.
¶ Monotomic elements means that their atoms are not joined together chemically.
¶ Metals also exist as atoms but large numbers of their atoms are arranged in a special way to form giant structures.
¶ Most non-metals exist as molecules.
¶ What is a molecule?
A molecule is a group of two or more atoms that are chemically combined (joined together).
¶ For example, a hydrogen molecule is made up of two hydrogen atoms chemically combined. Chemists use the symbol H2 to represent a hydrogen molecule. A molecule of ozone is formed when three atoms of oxygen are chemically combined.
¶ Molecules formed by the combination of two atoms are called diatomic molecules.
¶ Molecules consisting of three atoms are called triatomic molecules
.
¶ Molecules that are formed by the combination of four or more atoms are called polyatomic molecules.
*Important Note*:
You should make sure that you understand the difference between N2 and 2N. N2 represents a molecule of nitrogen; 2N represents two atoms of nitrogen.
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COMPOUNDS
¶ What is a compound?
A compound is a pure substance that contains two or more elements chemically combined.
¶ When the element magnesium burns in the element oxygen, a brilliant white flame is seen and a new substance, magnesium oxide, is obtained. The new substance, magnesium oxide, is called a compound.
Þ Write a word and a chemical equation for the reaction between magnesium and oxygen.
Magnesium + oxygen à magnesium oxide
2Mg(s) + O2(g) ---> 2MgO(s)
Þ In the table below, list the elements the common compounds contain.
Compound | Elements present |
Common salt (sodium chloride) | Sodium, chlorine |
Carbon dioxide | Carbon, oxygen |
Marble (Calcium carbonate) | Calcium, carbon, oxygen |
Copper(II) sulphate | Copper, sulphur, oxygen |
Hydrogen Chloride | Chlorine, hydrogen |
Naming Compounds
¶ Compounds are named according to some general rules. Use the rules below to help you identify or name compounds.
1) A compound made up of two elements has a name that ends in –ide.
Example: sodium chloride- made up of the elements sodium and chlorine
Zinc oxide –made up of the elements zinc and oxygen
Carbon dioxide – made up of the elements carbon and oxygen.
2) A compound that contains hydroxide ions, OH- (a negatively charged ion made up of oxygen and hydrogen) is named a hydroxide.
Example: Potassium hydroxide- contains potassium ions and hydroxide ions
3) A compound that contains a negatively charged polyatomic ion containing oxygen usually has a name ending in -ate.
Example: Copper (II) sulphate- contains four oxygen atoms in one sulphate ion.
Sodium nitrate- contains three oxygen atoms in one nitrate ion.
Fixed Composition of Compounds
¶ A compound is made up of different elements chemically combined in a fixed ratio.
¶ For example, water (H2O) is a compound made only by joining together two atoms of hydrogen to one atom of oxygen.
¶ That is the ratio of hydrogen atoms to oxygen atoms in water is always 2:1.
¶ You have learnt that the smallest particle of an element that can exist independently is the atom. So, what is the smallest particle of a compound?
A compound is made up of at least two elements. Thus, the smallest particle of a compound must contain at least two atoms.
¶ Therefore, the smallest particle of a compound that can exist independently is therefore the molecule.
¶ Compounds made up of ions are called ionic compounds.
Chemical Formula of a Compound
¶ A compound can be represented by a chemical formula.
¶ The chemical formula of a compound is written by putting together the chemical symbols of the elements that make up the compound.
¶ The chemical formula states
a) the types of atoms (i.e. elements) present in the compound.
b) the ratio of the different atoms present in the compound.
Look at the chemical formula of water. What does the chemical formula states?
Hydrogen and oxygen atoms are present in the compound.
The ratio of hydrogen to oxygen is 2:1.
¶ Some chemical formulae contain brackets. For example, the formula of lead (II) nitrate is Pb(NO3)2.
o How then is the ratio of lead, nitrogen and oxygen atoms in lead (II) nitrate calculated?
1 lead atom
2 nitrogen atoms
6 oxygen atoms
Thus the ratio of atoms in lead (II) nitrate is lead:nitrogen:oxygen =1:2;6.
Decomposition of Compounds
¶ Heat can be used to form compounds.
¶ Heat can also be used to break down compounds into elements or simpler compounds. Such a chemical reaction is called thermal decomposition.
¶ For example, when mercury (II) oxide is heated strongly, it decomposes to give the elements mercury and oxygen.
¶ Besides using heat, compounds can also be broken down into simpler substances by using electricity.
*Important Note*:
Note the difference between the chemical symbols- Co for Cobalt and CO for carbon monoxide Mn is manganese and Mg is magnesium.
Note also how fluorine is spelled. |
MIXTURES
¶ What are mixtures?
Mixtures are formed when two substances are added together without chemical bonds being formed.
¶ Give examples of mixtures.
Muddy water and air because air is made up of gases such as nitrogen and oxygen mixed together.
¶ Mixtures can be made up of elements and compounds. The components of a mixture are not fixed. They can be present in any ratio.
¶ The figure below shows the arrangement of particles in
a) a mixture of two elements
b) a mixture of two compounds.
c) a mixture of one element and one compound.
Þ What are the differences between a mixture and a compound?
| Mixture | Compound |
Separation
| Separated by physical methods, e.g. filtration | Separated by chemical reactions by using electricity. |
Properties
| Chemical properties are the same as their components. | Physical and chemical properties are different from their components. |
Energy Change | No chemical takes place when a mixture is formed. | A chemical reaction takes place when a compound is formed. |
Composition
| Can be mixed in any proportion. | Always combined in a fixed proportion (by mass)
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Alloy
¶ What is an alloy?
An alloy is a mixture of metals with other elements (usually metals but sometimes non-metals such as carbon).
¶ Alloys are widely used and in great demand because they tend to be stronger than pure metals.
¶ The table below shows some common alloys and their compositions.
Alloy | Composition (Percentage of element by mass) |
Steel | Iron (99%), Carbon(1%) |
Stainless Steel | Iron(73%), Chromium(18%), Nickel(8%), Carbon(1%) |
Brass | Copper(66-70%), Zinc(30-34%) |
Bronze | Copper(90%), Tin(10%) |
Duralumin | Aluminium(95%), Copper(4%), Magnesium(1%) |
Solder | Lead(67%), Tin(33%) |
Alloys have chemical properties similar to those of the elements they contain but they have different physical properties.