Maharashtra Board Class 8 Science Chapter 7 Metals and Nonmetals PDF Download

7. Metals And Nonmetals

All the objects or materials in the world are made from elements, compounds or their mixtures. Scientists classified all the elements into three general types which are metals, nonmetals and metalloids.

Metals

Gold, silver, iron, copper, aluminium, magnesium, calcium, sodium, platinum are a few metals. Metals have lustre. The metals are hard. Wire or sheet can be made from metal. Metals are good conductors of heat and electricity. Metals lose their valence electrons to produce positively charged ions, that is, cations.

Physical Properties Of Metals

1. Physical State: Under ordinary temperature metals stay in solid state. However metals like mercury and gallium are exception, which are in liquid state even at room temperature.

2. Lustre: Take copper vessels at your home. Scrub them with lemon and rinse with water. Observe the lustre before and after cleaning. Light gets reflected from the cleaned or freshly cut surface of metal and the metal looks lustrous.

3. Hardness: Generally metals are hard, not soft. Exception: Sodium and potassium are soft and can be easily cut by a knife.

4. Ductility: Have you ever gone to a goldsmith's shop? Have you seen the goldsmith making a wire of gold or silver? When a metal is pulled through a hole its wire is formed. This property of metal is called ductility.

5. Malleability: Take an iron nail. Place it on a platform and keep on hammering it. After sometime you will see a thin sheet forming. This property is called malleability of metals.

6. Conduction Of Heat: Take a copper plate. Fix some wax at one of its ends. Heat the other end and observe what happens. Discuss with teacher. Metals are good conductors of heat. Silver, copper and aluminium are the best conductors of heat.

7. Conduction Of Electricity: Which metals are used to make electrical wires? Metals are good conductors of electricity. Lead is an exception, which is neither a good conductor of heat nor a good conductor of electricity.

8. Density: Metals have high density Sodium, potassium and lithium are exception, having lower density then water. The density of lithium is only 0.53 g/cc.

9. Melting Point And Boiling Point: Generally metals have high melting points and boiling points. Exceptions: Hg, Ga, Na, K.

10. Sonority: What is the metal that your school bell is made of? How does a bell function? Metals are sonorous. They produce sound on striking.

You might have seen a doctor's pressure gauge to measure blood pressure, during your visit to a dispensary, accompanying your relative. In that, you might have seen a grey coloured liquid in a glass tube. What is that metal?

Teacher's Note

Metals are all around us in everyday life. For example, the spoon you eat with, the iron gate at your home, and the silver ring on your finger are all made of metals. These metals are shiny and strong.

Exam Trick

Remember: Most metals are hard and shiny. But sodium and potassium are soft and can be cut like butter. Gold and silver can be stretched into wires and hammered into sheets - this is very important for exams.

Points To Remember

Metals are shiny and hard materials that conduct heat and electricity well.
Most metals are solid at normal temperature, except mercury and gallium.
Ductility means metals can be pulled into thin wires.
Malleability means metals can be hammered into thin sheets.
Metals make a sound when you hit them.

Nonmetals

Carbon, Sulphur, Phosphorus are a few nonmetals. Generally nonmetals are brittle and nonlustrous.

Physical Properties Of Nonmetals

1. Physical State: At ordinary temperature nonmetals occur as solids, liquids and gases. For example: Solids: C, S, P; Liquids: Br₂; Gases: H₂, N₂, O₂

2. Lustre: Nonmetals do not have lustre, except diamond and iodine crystals. Some nonmetals are colourless while others have different colours. What is the colour of carbon in the form of coal? Sulphur is yellow and bromine is brown.

3. Brittleness: Take coal and hammer it. See what happens to it. Solid nonmetals are brittle. Some nonmetals are soft. Diamond (an allotrope of carbon) is an exception, which is the hardest natural substance.

4. Ductility And Malleability: Non metals are neither ductile nor malleable.

5. Conduction Of Heat And Electricity: Non metals are bad conductors of heat and electricity, except graphite (an allotrope of carbon) which is a very good conductor of electricity.

6. Density: Nonmetals have low densities.

7. Melting And Boiling Point: Nonmetals have low melting and boiling points. Exceptions: the solid nonmetals carbon and boron melt at high temperature.

Teacher's Note

Nonmetals are materials that are not shiny and are easy to break. For example, the coal you use, the sulphur in matches, and the oxygen in the air are all nonmetals. They do not conduct electricity or heat well.

Exam Trick

Remember: Nonmetals are the opposite of metals. They are dull, brittle, and do not conduct electricity. But graphite is special - it is a nonmetal that conducts electricity very well!

Points To Remember

Nonmetals are dull and can be brittle or soft.
Nonmetals do not conduct heat or electricity well (except graphite).
Nonmetals can be solids, liquids, or gases at room temperature.
They cannot be stretched into wires or hammered into sheets.
Diamond is a hard nonmetal, but most nonmetals are not hard.

Metalloids

Some elements such as arsenic (As), Silicon (Si), Germanium (Ge), Antimony (Sb) have properties which are intermediate between metals and nonmetals. Such elements are called metalloids.

Chemical Properties Of Metals

a. Electronic Configuration: Electronic configuration is the basis of chemical behaviour of elements. Majority of metals have upto three electrons in their outermost shell.

b. Formation Of Ions: Metals have a tendency to lose their valence electrons to form positively charged ions, that is, cations.

Na \(\rightarrow\) Na⁺ + 1e⁻
(2,8,1) (2,8)
Sodium Sodium ion

Mg \(\rightarrow\) Mg²⁺ + 2e⁻
(2,8,2) (2,8)
Magnesium Magnesium ion

Al \(\rightarrow\) Al³⁺ + 3e⁻
(2,8,3) (2,8)
Aluminium Aluminium ion

c. Reaction With Oxygen: Metals combine with oxygen to form their oxides.

Metal + Oxygen \(\rightarrow\) Metal oxide

The metal oxides are basic in nature. Metal oxides react with an acid to form salt and water.

Metal oxide + Acid \(\rightarrow\) Salt + Water

Teacher's Note

When you leave an iron nail in water for some days, it becomes rusty and red. This is because iron metal reacts with oxygen in the air and water. This is a chemical property of metals.

Exam Trick

Remember: All metals lose electrons to form positive ions. The number of electrons they lose depends on the number of electrons in their outermost shell. This is why sodium loses 1 electron, but aluminium loses 3 electrons.

Points To Remember

Metals lose electrons to form positive ions called cations.
Metal oxides are basic in nature.
When metals react with acids, they form salt and hydrogen gas.
Metals react with oxygen to form oxides.
Some metals react with water and steam to form hydroxides and hydrogen gas.

Chemical Properties Of Nonmetals

a. Electronic Configuration: Most of the nonmetals have 4 to 7 electrons in their valence shells.

b. Formation Of Ions: Non metals have a tendency to accept electrons in their valence shell to form negatively charged ions called anions.

Cl + 1e⁻ \(\rightarrow\) Cl⁻
(2, 8, 7) (2, 8, 8)
Chlorine Chloride ion

O + 2e⁻ \(\rightarrow\) O²⁻
(2, 6) (2, 8)
Oxygen Oxide ion

N + 3e⁻ \(\rightarrow\) N³⁻
(2, 5) (2, 8)
Nitrogen Nitride ion

c. Reaction With Oxygen: Nonmetals combine with oxygen to form their oxides.

Nonmetal + oxygen \(\rightarrow\) Nonmetal oxide

The oxides of nonmetals are acidic in nature. They react with bases to form soluble salt and water.

C + O₂ \(\rightarrow\) CO₂

CO₂ + 2NaOH \(\rightarrow\) Na₂CO₃ + H₂O

The oxides of nonmetals react with water to form an acid.

CO₂ + H₂O \(\rightarrow\) H₂CO₃ Carbonic acid

SO₂ + H₂O \(\rightarrow\) H₂SO₃ Sulphurous acid

SO₃ + H₂O \(\rightarrow\) H₂SO₄ Sulphuric acid

d. Nonmetals Do Not React With Dilute Acids.

Teacher's Note

When you burn wood, carbon in the wood combines with oxygen to form carbon dioxide gas. This carbon dioxide can make limewater turn milky white. This shows that nonmetals react with oxygen too.

Exam Trick

Remember: Nonmetals accept electrons to form negative ions called anions. This is the opposite of metals. Also, nonmetal oxides are acidic, while metal oxides are basic.

Points To Remember

Nonmetals accept electrons to form negative ions called anions.
Nonmetal oxides are acidic in nature.
Nonmetals do not react with dilute acids.
When nonmetal oxides react with water, they form acids.
Nonmetals react with oxygen to form oxides that are acidic.

This is a preview of the first 3 pages. To get the complete book, click below.