Elements, Compounds, Symbols and Formulae

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

Reason — All pure substance have a definite set of property, which includes their physical state, colour and composition but it is not same for all the substances.

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

Reason — Sugar has the chemical formula C₁₂H₂₂O₁₁, which signifies that it is made up of carbon (C), hydrogen (H) and oxygen (O).

Sulphur and carbon are:

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

Answer

non-metals

Reason — Sulphur and carbon are non-metals with dull surfaces that do not shine.

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

Reason — The most abundant elements in the universe are hydrogen and helium.

The symbol of which of the following elements is derived from its Latin name ?

1. Cobalt
2. Copper
3. Calcium
4. Chlorine

Answer

Copper

Reason — Among the following elements the symbol of copper i.e., Cu is derived from its latin name "Cuprum".

The metal which is not a good conductor of electricity is:

1. silver
2. aluminium
3. copper
4. tungsten

Answer

tungsten

Reason — Metals are good conductors of electricity excepting tungsten, which is a poor conductor of electricity.

The symbol of sodium metal is:

1. S
2. Sn
3. Na
4. Ne

Answer

Na

Reason — The symbol of sodium metal is Na is derived from "Natrium".

From the list given below, select the correct substance which is most suitable to the given statements

(oxygen, diamond, zinc, graphite, gold)

(a) A metal which is brittle.

(b) A non-metal which is a good conductor of electricity.

(c) The hardest naturally occurring substance.

(d) The most ductile metal.

(e) A gaseous non-metal.

Answer

(a) Zinc

(b) Graphite

(c) Diamond

(d) Gold

(e) Oxygen

Classify the following substances into elements and compounds.

Mercury, sulphur, sugar, water, sand, gold, carbon, oxygen, alcohol, iron, marble, baking soda.

Answer

1. Elements — Mercury, sulphur, gold, carbon, oxygen, iron
2. Compounds — Sugar, water, sand, alcohol, marble, baking soda

Give the symbols of: carbon, calcium, copper, chlorine, cobalt, argon, silicon, aluminium.

Answer

1. Carbon — C
2. Calcium — Ca
3. Copper — Cu
4. Chlorine — Cl
5. Cobalt — Co
6. Argon — Ar
7. Silicon — Si
8. Aluminium — Al

Give the names of the elements represented by the following symbols:

(a) B

(b) He

(c) Si

(d) Be

(e) Li

(f) P

(g) K

(h) F

Answer

(a) B — Boron

(b) He — Helium

(c) Si — Silicon

(d) Be — Beryllium

(e) Li — Lithium

(f) P — Phosphorus

(g) K — Potassium

(h) F — Fluorine

Define a pure substance.

Answer

A substance which has a definite composition and consistent properties throughout is called a pure substance.

Name the types of pure substances you know.

Answer

Pure substances are of two types,

1. Elements
2. Compounds

Define:

(a) Elements

(b) Compounds.

Answer

(a) An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process.

(b) Compounds are pure substances formed by the chemical combination of two or more elements in a definite proportion by mass.

Give two examples for each of the following:

(a) Non-metals which are solids

(b) Metals which are soft

(e) Non-metals which are lustrous

(d) Elements which are liquids

(e) Inert gases

Answer

(a) Non-metals which are solids:

1. Sulphur(S)
2. Phosphorus (P)

(b) Metals which are soft:

1. Sodium (Na)
2. Potassium (K)

(c) Non-metals which are lustrous:

1. Iodine (I)
2. Carbon (C) in the form of diamond

(d) Elements which are liquids:

1. Mercury (Hg) — Metal
2. Bromine (Br₂) — Non metal

(e) Inert gases:

1. Helium (He)
2. Neon (Ne)

Name the elements present in the following compounds.

(a) Sugar

(b) Ammonia

(c) Marble

(d) Washing soda

Answer

(a) Sugar — Carbon, hydrogen and oxygen

(b) Ammonia — Nitrogen and hydrogen

(c) Marble — Calcium, carbon and oxygen

(d) Washing soda — Sodium, carbon and oxygen

Give three differences between metals and non-metals

Answer

The differences between metals and non-metals are given below:

Give three differences between elements and compounds.

Answer

The differences between elements and compounds are given below:

Name the elements which form water. State three characteristics of water to justify that it is a compound.

Answer

Hydrogen and Oxygen combine to form water.
Three characteristics of water are :

1. Water consist of two elements, hydrogen and oxygen.
2. In the formation of water, hydrogen and oxygen always combine in a fixed proportion i.e., two hydrogen atoms and one oxygen atom.
3. The properties of water are entirely different from those of its constituent elements. Hydrogen and oxygen are gases but water is a liquid under normal conditions.

State four important characteristics of compounds.

Answer

1. A compound consists of two or more elements.
2. The constituting elements of a compound are always same, combined in a definite proportion in every sample of that compound, i.e. from whichever source it is obtained.
3. A compound is pure and homogeneous and have same properties throughout its bulk.
4. The physical and chemical properties of a compound differ from those of its constituent elements.

How is sodium chloride different from its constituent elements, sodium and chlorine in its properties? Justify.

Answer

The compound "sodium chloride" made up of sodium and chlorine. Sodium is a metal that is stored in kerosene oil as it reacts very fast with air and water. Chlorine is a reactive greenish yellow gas which is poisonous.
When these two elements combine chemically they form common salt, a non poisonous white solid substance that we use in our food to add taste and to obtain nutrition.

(a) What are the four types of elements ? Name them.

(b) Give two examples of each type of the elements you have mentioned above.

Answer

(a) Four types of elements are :

1. Metals
2. Non-metals
3. Metalloids
4. Nobel (Inert) gases

(b) Examples :

1. Metals — Zinc, Lead
2. Non-metals — Carbon, Hydrogen
3. Metalloids — Boron, Silicon
4. Nobel (Inert) gases — Neon, Argon

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

Reason — When atoms of two or more different elements combine, they form a molecule of a compound. Whereas, molecule of element forms when two or more atoms of the same element combine.

The compound used as common salt is:

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

Answer

sodium chloride

Reason — The compound used as common salt is sodium chloride made up of sodium and chlorine.

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

Reason — Sand or silica is a compound of silicon and oxygen in the ratio of 1 : 2 .

The atomicity of ozone is:

1. 2
2. 3
3. 4
4. 8

Answer

3

Reason — The number of atoms in a molecule of an element is called its atomicity. Ozone (O₃) is made of three oxygen atoms, hence the atomicity of ozone is three.

The molecular formula of ammonia is:

1. NO
2. NH₃
3. NO₂
4. N₂O

Answer

NH₃

Reason — Ammonia is made up of nitrogen and hydrogen. The valency of nitrogen is 3 and valency of hydrogen is 1.

$$ \overset{\phantom{3}{3}}{\text{N}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{H}} \Rightarrow \underset{\phantom{1}{1}}{\text{N}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{3}}{\text{H}} \\[1em] \text{N}\text{H}_3 $$

The combining power of magnesium and oxygen is the same, i.e. 2. What will be the molecular formula of magnesium oxide ?

1. Mg₂O₂
2. MgO₂
3. Mg₂O
4. MgO

Answer

MgO

Reason — The combining power of magnesium and oxygen is the same, i.e. 2.

$$ \overset{\phantom{2}{2}}{\text{Mg}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{2}{2}}{\text{Mg}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{2}}{\text{O}} \\[1em] \text{Mg}_2\text{O}_2 \Rightarrow \text{Mg}\text{O} $$

When the subscript numbers are same, we don't write them. So, the molecular formula of magnesium oxide is MgO.

Define:

(a) Atom

(b) Molecule

(c) Atomicity

(d) Formula

Answer

(a) An atom can be defined as the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have independent existence.

(b) A molecule can be defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has independent existence. They are divisible into atoms.

(c) The number of atoms in a molecule of an element is called its atomicity.

(d) A formula (plural: formulae) is a short way of representing the molecule of an element or a compound. It indicates the types and numbers of atoms present in a molecule.

State the difference between the following:

(a) 2H and H₂

(b) H₂O and 3Н₂О ?

Answer

(a) 2H represents two atoms of hydrogen while H₂ represents one molecule of hydrogen.

(b) H₂O indicates that water molecule is formed by the combination of two elements i.e., Hydrogen and Oxygen. The number of hydrogen atom is two and that of oxygen atom is one.
If we write 3Н₂О it represents three molecules of water. It contains six atoms of hydrogen and three atoms of oxygen.

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

Answer

1. Calcium oxide — CaO
2. Hydrogen sulphide — H₂S
3. Lead sulphide — PbS
4. Zinc oxide — ZnO
5. Carbon dioxide — CO₂

Give two examples each of compounds existing in the following states:

(a) Solid

(b) Liquid

(c) Gaseous

Answer

(a) Solid :

1. Common salt (Sodium chloride-NaCl)
2. Sodium hydroxide (NaOH)

(b) Liquid :

1. Water (H₂O)
2. Hydrochloric acid (HCl)

(c) Gaseous :

1. Carbon dioxide (CO₂)
2. Ammonia (NH₃)

State two conditions necessary to write the molecular formula of a compound.

Answer

Two conditions necessary to write the molecular formula of a compound are :

1. The symbols of elements.
2. The valency of that element i.e. its combining power.

State the number of atoms of each element, present in

(a) C₆H₁₂O₆

(b) H₂SO₄

(c) HNO₃

(d) CaCO₃

Also name these compounds.

Answer

(a) C₆H₁₂O₆ — Glucose

1. Carbon (C): 6 atoms
2. Hydrogen (H): 12 atoms
3. Oxygen (O): 6 atoms

(b) H₂SO₄ — Sulphuric acid

1. Hydrogen (H): 2 atoms
2. Sulfur (S): 1 atom
3. Oxygen (O): 4 atoms

(c) HNO₃ — Nitric acid

1. Hydrogen (H): 1 atom
2. Nitrogen (N): 1 atom
3. Oxygen (O): 3 atoms

(d) CaCO₃ — Calcium carbonate

1. Calcium (Ca): 1 atom
2. Carbon (C): 1 atom
3. Oxygen (O): 3 atoms

Why are symbols and formulae of substances important?

Answer

The symbols and formulae of substance gives information of element present in the compound and number of each kind of atom in one molecule of the compound. They also allow us to represent chemical reactions using equations.

Mention three gaseous elements and write their molecular formulae.

Answer

Three gaseous elements along with their molecular formulae are given below:

1. Hydrogen – H₂
2. Oxygen – O₂
3. Nitrogen – N₂

State the information obtained from the formula of a compound.

Answer

A formula gives us the following information about a compound.

1. Types of elements present in the compound.
2. Number of each kind of atoms in one molecule of the compound.
3. Ratio of different types of atoms present in the molecules.

Gold is used to make jewellery because it is:

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

Answer

lustrous and attractive

Reason — The gold is used to make jewellery because it is lustrous. It looks shiny and attractive and do not tarnish in air.

Brass and bronze are:

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

Answer

mixtures

Reason — Brass is a mixture of copper and zinc and bronze is a mixtures of copper and tin.

The hardest naturally occuring substance is:

1. graphite
2. iron
3. diamond
4. coal

Answer

diamond

Reason — Diamond is a form of carbon, it is hardest naturally occuring substance. Hence, it is used to cut glass.

The inert gas filled in electric bulbs is:

1. Helium
2. Argon
3. Neon
4. Both (1) and (3)

Answer

Both (2) and (3)

Reason — Nobel gases (Inert gases) like argon and neon are filled in electric bulbs due to their inert nature. They do not react with the tungsten filament of the bulb and prevents it from destruction.

Which of the following metals is used to make heavy tools and machines?

1. Iron
2. Aluminium
3. Copper
4. Silver

Answer

Iron

Reason — Iron is strong and easily available. Therefore, it is used to make heavy tools and machines.

Fill in the blanks:

(a) ............... refers to the number of atoms in the molecule of an element.

(b) The most abundant element in the earth's crust is ............... .

(c) A metal which is a liquid at room temperature is ............... .

(d) The most abundant element in the atmosphere is ............... .

(e) A metal which is a poor conductor of electricity is ............... .

(f) A diatomic gaseous element is ............... .

(g) A liquid non-metal is ............... .

(h) Elements that do not react chemically are ............... .

(i) The Latin name of iron is ............... .

(j) The horizontal rows in the Periodic table are called ............... .

(k) The vertical columns in the Periodic table are called ............... .

Answer

(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 lead.

(f) A diatomic gaseous element is oxygen(O₂).

(g) A liquid non-metal is bromine.

(h) Elements that do not react chemically are nobel gases.

(i) The Latin name of iron is ferrum.

(j) The horizontal rows in the Periodic table are called periods.

(k) The vertical columns in the Periodic table are called groups.

Indicate whether the following statements are true or false.

(a) A compound is made up of just one kind of atom. ...............

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

(c) Metals can be polished. ...............

(d) Elements are made up of compounds. ...............

(e) All elements are artificially prepared. ...............

(f) Molecules can exist independently. ...............

(g) Molecules combine to form atoms. ...............

(h) Noble gases are highly reactive. ...............

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

Answer

(a) False
Correct Statement — A compound is made of two or more different kinds of atoms.

(b) True

(c) True

(d) False
Correct Statement — Compounds are made up of elements.

(e) False
Correct Statement — Most elements occur naturally, only a few are synthetic.

(f) True

(g) False
Correct Statement — Atoms combine to form molecules.

(h) False
Correct Statement — Noble gases are inert, i.e., non-reactive under normal conditions.

(i) True

Match the columns:

Answer

Circle the odd one out on the basis of the property given in the brackets.

(a) hydrogen, nitrogen, chlorine, phosphorus (state)

(b) sodium, potassium, iron, lithium (hardness)

(c) graphite, diamond, iodine, coal (forms of a same element)

(d) helium, neon, argon, bromine (reactivity)

Answer

(a) Phosphorus
Reason — Phosphorus is present in solid state under normal conditions, whereas, hydrogen, nitrogen, and chlorine are gases at room temperature.

(b) Iron
Reason — Iron is hard metal, while, sodium, potassium and lithium are soft metals.

(c) Iodine
Reason — Coal, diamond and graphite are all different forms of carbon but iodine is a different element.

(d) Bromine
Reason — Bromine is a reactive element. While, helium, neon and argon are non-reactive nobel gases.

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) Coal, diamond and graphite.

(b) Diamond

(c) Copper and aluminium

(d) Gold and platinum

(e) Wood or Plastic

Give one use of each of the following substances:

(a) Iron

(b) Brass

(c) Coal

Answer

(a) Iron is used to make heavy tools and machines.

(b) Brass is used to make utensils, door knobs, handles, etc.

(c) Coal is used as fuel.

Give reason:

A frying pan is made up of steel but its handle is made up of wood.

Answer

A frying pan is made up of steel but its handle is made up of wood or plastic. This is because steel is a good conductor of heat and it allows heat to pass to the food kept in the pan for cooking while wood and plastic being bad-conductors or insulators do not get too hot to burn our hand.

Give reason:

Graphite is used to make lead of the pencils.

Answer

Graphite can mark the paper black. So, it is used to make the lead of pencils.

Give reason:

Argon is filled in electric bulbs.

Answer

Argon is filled in electric bulbs due to its inert nature. It does not react with the tungsten filament of the bulb and prevents it from destruction.

Give reason:

Copper and aluminium are used to make electric wires.

Answer

Copper and aluminium are good conductors of heat and electricity. They can be drawn into wires and beaten into sheets. Therefore, they are used to make electric wires.

What do you understand by the statement — 'Metals are ductile and malleable? Give one example of each.

Answer

Metal are ductile, means they can be drawn or stretched into thin wires and malleable means they can be beaten into thin sheets.

Example of a ductile metal : Copper is highly ductile and is commonly used in electrical wiring.

Example of a malleable metal : Aluminium is malleable and is used to make foil and packaging materials.

(a) What do you understand by 'Periodic Table'.

(b) A part of the Periodic table is given below with the symbols of some elements. Write the symbols of the missing elements.

Answer

(a) Periodic Table is a systematic arrangement of elements in a tabular form arranged into rows ("periods") and columns ("groups"), for our convenient study.

(b)