Elements, Compounds, Symbols and Formulae

This chapter introduces the basic building blocks of matter. It explains that elements are the purest substances, made up of only one kind of atom (think of gold or oxygen). These elements are represented by symbols, which are short abbreviations (like ‘Au’ for gold or ‘O’ for oxygen).

Then, the chapter moves on to compounds, which are formed when two or more different elements chemically combine in a fixed ratio (water, H2O, is a classic example).

So, in a nutshell, Chapter 4 lays the groundwork for understanding chemistry by explaining how we represent the simplest substances (elements) and how they combine to form more complex substances (compounds) using symbols and formulae.

EXERCISE-I

Question 1.
Classify the following substances into elements and compounds.
Mercury, sulphur, sugar, water, sand, gold, coal, oxygen, alcohol.

Ans. Element: Mercury, sulphur, gold, coal, oxygen.

Compound : Sugar, water, sand, alcohol.

Question 2.

Give the symbols of: Carbon, calcium, copper, chlorine, cobalt, argon.

Answer:Carbon is C      

 Chlorine is Cl

Calcium is Ca     

Cobalt is Co

Copper is Cu      

 Argon is Ar

Question 3.

Define a pure substance. How many types of pure substances do you know ?

Answer:Pure substances are matter with a consistent makeup and unique characteristics. They’re either:

• Elements: The fundamental building blocks of matter, like oxygen, gold, or carbon – you can’t break them down further chemically.
• Compounds: Combinations of two or more different elements chemically joined in a specific ratio, such as water, salt, or sugar – you can break them down into simpler substances through chemical reactions.

Question 4.

Define : (a) Elements (b) Compounds.

Name the particles from which elements and compounds are made of.

Answer:(a) Elements:You can’t break them down into anything simpler using chemistry. Some elements hang out as single atoms (like helium), while others pair up to form molecules (like the oxygen we breathe, O₂).

(b) Compounds: These are formed when two or more different elements chemically join together in a specific, unchanging recipe.. Compounds can exist as molecules (like water, H₂O) or as networks of charged particles called ions (like table salt, NaCl).

Question 5.

Give two examples for each of the following :

(a) Metals

(b) Non-metals

(c) Metalloids

(d) Noble gases

Answer:(a) Metals:

• Iron (Fe)
• Copper (Cu)

(b) Non-metals:

• Oxygen (O)
• Sulfur (S)

(c) Metalloids:

• Silicon (Si)
• Germanium (Ge)

(d) Noble gases:

• Helium (He)
• Neon (Ne)

Question 6.

Name the elements which form water. How will you justify that water is a compound

Answer:hydrogen and oxygen get together and chemically link up. This isn’t like just tossing sand and sugar in a bowl; they actually form new connections. Because of this chemical bond, they lock into a specific mass ratio – for every bit of hydrogen, you always get about eight bits of oxygen in water.

What’s really cool is that this new combination results in something totally different. You wouldn’t want to drink flammable hydrogen or breathe pure oxygen all the time, but put them together just right, and you get life-sustaining water! So, the unique properties of water compared to its elements are a clear sign it’s a brand-new substance – a compound.

Question 7.

Give three differences between metals and nonmetals.

Answer:Metals are like superhighways for heat and electricity, letting them flow easily, while non-metals act more like roadblocks. 

Also, most metals are solid at room temperature, like the iron in a bridge or a gold ring, except for liquid mercury. Non-metals are more diverse; you can find them as solids like charcoal, liquids like bromine, or gases like the air we breathe (oxygen and nitrogen).

Finally, metals are usually easy to shape – you can hammer them into sheets or draw them into wires. Imagine shaping copper into intricate designs or pulling it into thin electrical wires. Non-metals, on the other hand, tend to be brittle and will snap if you try to bend or stretch them.

Question 8.

How is sodium chloride different from its constituent elements, sodium and chlorine ?

Answer:Sodium chloride, common table salt, exhibits drastically different properties compared to the reactive metal sodium and the poisonous gas chlorine. ¹ Sodium is a soft, silvery metal that violently reacts with water, while chlorine is a greenish-yellow gas with a pungent odor. When they chemically combine, they form sodium chloride, a stable, white crystalline solid essential for life. ² This transformation occurs because sodium readily loses an electron to become a positively charged ion (Na+), and chlorine  gain an electron to become a negatively charged ion (Cl−). These oppositely charged ions then strongly attract each other, forming an ionic bond in the sodium chloride crystal lattice. ³ This new arrangement results in a compound with entirely new characteristics, such as its ability to dissolve in water and its salty taste, neither of which are properties of the original elements

Question 9.

State four important characteristics of compounds.

Answer:Think of compounds as precise recipes where the ingredient list (the elements) is always the same, like water being consistently H₂O. When these elements combine, they undergo a complete makeover, their individual characteristics fading away to create something entirely new – just like how reactive sodium and chlorine become everyday table salt. These elements aren’t just mixed; they’re tightly held together by chemical bonds that need a proper chemical reaction to break. Finally, each compound has its own unique set of characteristics, like its melting point or how it interacts with other substances, making it identifiable – its own special fingerprint!

Question 10.
Give two examples for each of the following :

(a) Non-metals which are solids
(b) Metals which are soft
(c) Non-metals which are lustrous
(d) Elements which are liquids.
(e) Inert gases
(f) Metalloids

Answer:(a) Non-metals which are solids:

• Sulfur
• Phosphorus

(b) Metals which are soft:

• Sodium
• Potassium

(c) Non-metals which are lustrous:

• Iodine
• Graphite

(d) Elements which are liquids:

• Mercury
• Bromine

(e) Inert gases:

• Helium
• Neon

(f) Metalloids:

• Silicon
• Germanium

Question 11.
Name the elements present(a) Sugar
(b) Ammonia
(c) Marble
(d) Washing soda

Answer:a) Sugar: Carbon, Hydrogen, Oxygen

(b) Ammonia: Nitrogen, Hydrogen

(c) Marble: Calcium, Carbon, Oxygen

(d) Washing soda: Sodium, Carbon, Oxygen

Question 12.

What is the proportion of elements present in the following compounds?

(a) H2O

(b) CO2

(c) CaO

(d) NO2

Answer:(a) H₂O (Water):

• Hydrogen (H): 2 parts
• Oxygen (O): 1 part

So, in water, the proportion of hydrogen to oxygen is 2:1.

(b) CO₂ (Carbon Dioxide):

• Carbon (C): 1 part
• Oxygen (O): 2 parts

The proportion of carbon to oxygen in carbon dioxide is 1:2.

(c) CaO (Calcium Oxide):

• Calcium (Ca): 1 part
• Oxygen (O): 1 part

In calcium oxide, the proportion of calcium to oxygen is 1:1.

(d) NO₂ (Nitrogen Dioxide):

• Nitrogen (N): 1 part
• Oxygen (O): 2 parts

Therefore, the proportion of nitrogen to oxygen in nitrogen dioxide is 1:2.

Question 13.

Name two compounds which dissolve in water.

Answer:Sugar and salt.

EXERCISE-II

(ATOMS & MOLECULES)

Question 1.
Define:

(a) Atom
(b) Molecule
(c) Atomicity
(d) Formula

Answer:(a) Atom: The tiniest fundamental unit of an element, indivisible by chemical means.

(b) Molecule: A group of two or more atoms chemically bonded.

(c) Atomicity: The number of atoms in a single molecule (e.g., O₂ has 2, O₃ has 3).

(d) Formula: A symbolic representation of a molecule’s composition (e.g., H₂O).

Question 2.

Why are symbols and formulae of substances important?

Answer:Symbols and formulae are crucial for substances because they act like a universal shorthand. Instead of writing out the full name of a chemical compound like “dihydrogen monoxide,” we can simply use its formula, H2O. This makes communication in chemistry much faster and clearer, especially when dealing with complex reactions or large amounts of information.

Think of it like using abbreviations in everyday language. They save time and space while still conveying the same meaning. Chemical symbols and formulae do the same for scientists worldwide, ensuring everyone is on the same page when discussing substances and their properties.

Question 3.

Mention three gaseous elements and write their molecular formulae.

Answer: Hydrogen: H2​

Nitrogen: N2​

Oxygen: O2​

Question 4.

State the information obtained from the formula of a compound.

Answer:The types of atoms present in the compound. For example, H2​O tells us it contains hydrogen (H) and oxygen (O) atoms.

The ratio in which these atoms are combined. In H2​O, there are two hydrogen atoms for every one oxygen atom.

Question 5.

What is meant by

(a) 2H and H2

(b) H20 and 3H2O ?

Answer:(a) 2H is two atoms of hydrogen. H2 is one molecule of hydrogen gas.

(b) H20 represents one molecule of water. 3H20 represents 3 molecules of water.

Question 6.

State the number of atoms of each kind, present in

(a) C6 H12O6

(b) H2SO4

(c) HNO3

(d) CaCO3

Also name these compounds.

Answer:(a) C6H12O6

• Carbon (C): 6 atoms
  
• Hydrogen (H): 12 atoms
  
• Oxygen (O): 6 atoms
  
  Name: Glucose (a type of sugar)
  

(b) H2SO4

• Hydrogen (H): 2 atoms
  
• Sulfur (S): 1 atom
  
• Oxygen (O): 4 atoms
  
  Name: Sulfuric acid
  

(c) HNO3

• Hydrogen (H): 1 atom
  
• Nitrogen (N): 1 atom
  
• Oxygen (O): 3 atoms
  
  Name: Nitric acid
  

(d) CaCO3

• Calcium (Ca): 1 atom
  
• Carbon (C): 1 atom
  
• Oxygen (O): 3 atoms
  
  Name: Calcium carbonate (found in things like chalk and limestone)

Question 7.

Write the molecular formulae of compounds calcium oxide, hydrogen sulphide, carbon monoxide and lead sulphide.

Answer:Calcium oxide: CaO

Hydrogen sulfide: H2​S

Carbon monoxide: CO

Lead sulfide: PbS

EXERCISE-III

Question 1.
Name:
(a) Three different forms of carbon.
(b) A form of carbon used as a gem.
(c) Two substances used to make electric wires.
(d) Two substances used to make jewellery.
(e) A substance used as an insulator.

Answer:a) Diamond, graphite, and fullerene. 

(b) Diamond.

 (c) Copper and aluminum. 

(d) Gold and silver. 

(e) Rubber.

Question 2.
Give one use of each of the following substances :

(a) Iron
(b) Brass
(c) Coal

Answer:(a) Iron: Tough material for building strong bridges and buildings.

 (b) Brass: Shiny metal used for decorations and musical instruments. 

(c) Coal: Fuel for making electricity in power plants

Question 3.
Give reason:

(a) A frying pan is made up of steel but its handle is made up of wood.
(b) Graphite is used to make the lead of the pencils.
(c) Argon is filled with electric bulbs.

Answer:(a) Steel’s quick heating makes it perfect for frying pans, while wood’s slow heating keeps handles cool.

(b) Graphite’s softness and mark-making ability make it ideal for pencil leads.

(c) Argon’s non-reactive nature prevents hot bulb filaments from burning out, extending their life.

Question 4.
Answer the following questions:

(a) Why are copper and aluminium used to make electric wires?
(b) What do you understand by the statement: ‘metals are ductile and malleable’?
(e) Give the advantages of using symbols instead of names of elements or compounds.
(d) When iron is mixed with sulphur at room temperature, it does not form a compound. Why?
(e) Find the atomicity of the Following molecules:

1. calcium chloride
2. aluminium sulphide
3. acetic acid
4. dinitrogen oxide
5. carbon monoxide

Answer:(a) Copper and aluminum are top choices for electric wires because they’re excellent at conducting electricity and can be easily drawn into wires.

(b) Ductility in metals means they can be stretched into thin wires, while malleability means they can be hammered into sheets.

(c) Using symbols for elements and compounds is super useful for writing short and clear chemical formulas and equations that everyone in the science world understands.

(d) Iron and sulfur don’t react at room temperature because they need more energy to break their existing bonds and form new ones. Heating them up usually does the trick.

(e) Here’s the atomicity breakdown:

• Calcium chloride (CaCl2​): 3
• Aluminum sulfide (Al2S3​): 5
• Acetic acid (CH3​COOH): 8
• Dinitrogen oxide (N2​O): 3
• Carbon monoxide (CO): 2

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) Atomicity refers to the number of atoms in the molecule of an element.
(b) The most abundant element in the earth’s crust is oxygen.
(c) A metal which is a liquid at room temperature is mercury.
(d) The most abundant element in the atmosphere is nitrogen.
(e) A metal which is a poor conductor of electricity is tungsten.
(f) A diatomic gaseous element is oxygen.
(g) A liquid non-metal is bromine.

2. Match the columns

3. Indicate whether the following statements are true or false.

(a) A compound is made up of just one kind of atom.
Answer. False
Correct : A compound made up of two or more elements is a fixed proportion by mass.

(b) Metals reflect light and are good conductors of electricity.
Answer. True

(c) Metals can be polished.
Answer. True

(d) Elements are made up of compounds.
Answer. False
Correct : Elements are made up of atoms.

(e) All elements are artificially prepared.
Answer. False
Correct: All elements are made up of a limited number of basic substances.

(f) Molecules can exist independently.
Answer. True

(g) Molecules combine to form atoms.
Answer. False
Correct : Atoms combine to form molecules.

(h) Noble gases are highly reactive.
Answer. False
Correct: Noble gases are non-reactive.

(i) Ozone is a triatomic molecule.
Answer. True

MULTIPLE CHOICE QUESTIONS

Tick (√) the correct alternative from the choice given for the following statements :

1. All pure substances have

1. the same physical state .
2. the same colour
3. the same composition
4. a definite set of properties

Answer. a definite set of properties

2. Sugar is a compound which consists of the elements

1. carbon and hydrogen
2. hydrogen and oxygen
3. carbon, hydrogen and oxygen
4. hydrogen, carbon and sulphur

Answer. carbon, hydrogen and oxygen

3. Atoms of different kinds combine to form molecules of

1. an element
2. a compound
3. a mixture
4. all of the above

Answer. a compound

4. Sulphur and carbon are

1. metals
2. non-metals
3. metalloids
4. noble gases

Answer. non-metals

5. Gold is used to make jewellery because

1. it is dull
2. lustrous and attractive
3. highly reactive
4. very cheap

Answer.lustrous and attractive 

6. The most abundant elements in the universe are

1. neon and argon
2. hydrogen and helium
3. aluminium and copper
4. oxygen and nitrogen

Answer. hydrogen and helium

7. The compound used as common salt is

1. sodium chloride
2. calcium chloride
3. sodium oxide
4. hydrogen chloride

Answer.sodium chloride

 8. Brass and bronze are

1. elements
2. mixtures
3. compounds
4. all of the above

Answer. mixtures

9. Sand is a compound of

1. silicon and nitrogen
2. silicon and oxygen
3. oxygen and sulphur
4. none of the above

Answer. silicon and oxygen

10. From the list given below select the correct substance which is most suitable to the statements given : (oxygen, diamond, zinc, graphite, gold)

1. A metal which is brittle.
2. A non-metal which is a good conductor of electricity.
3. The hardest naturally occurring substance.
4. The most ductile metal.
5. A gaseous non-metal.

Answer.(a) Zinc

(b) Graphite

(c) Diamond

(e) Oxygen