Elements, Compounds, Symbols and Formulae

a definite set of properties

Reason — All pure substances have a definite set of properties. These properties include physical state, colour and composition, but they are not the same for all substances.
Pure substances can exist in different states - solid, liquid or gas. Different pure substances have different colours. Each pure substance has a fixed composition, but it is not the same as that of other substances.

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).
Carbon and hydrogen describe compounds like hydrocarbons, not sugar. Hydrogen and oxygen form water. Sugar doesn't contain sulphur.

non-metals

Reason — Sulphur and carbon are non-metals with dull surfaces that do not shine.
Sulphur and carbon do not show metallic properties like conductivity or malleability. Metalloids have properties of both metals and non-metals (e.g., silicon). Noble gases (like helium, neon) are inert gases, unlike sulphur and carbon.

hydrogen and helium

Reason — The most abundant elements in the universe are hydrogen and helium.
Neon and argon are noble gases, but not the most abundant in the universe. Aluminium and copper are metals found mainly on Earth, not abundant in the universe. Oxygen and nitrogen are abundant in Earth’s atmosphere, not in the entire universe.

Copper

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

tungsten

Reason — Metals are good conductors of electricity excepting tungsten, which is a poor conductor of electricity.
Silver is the best conductor of electricity. Aluminium is a good conductor and commonly used in power cables. Copper is a very good conductor, widely used in wiring.

Na

Reason — The symbol of sodium metal is Na, derived from its Latin name "Natrium".
S is sulphur.
Sn is tin.
Ne is neon.

(a) Zinc

(b) Graphite

(c) Diamond

(d) Gold

(e) Oxygen

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

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

(a) B — Boron

(b) He — Helium

(c) Si — Silicon

(d) Be — Beryllium

(e) Li — Lithium

(f) P — Phosphorus

(g) K — Potassium

(h) F — Fluorine

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

Pure substances are of two types,

1. Elements
2. Compounds

(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.

(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)

(a) Sugar — Carbon, hydrogen and oxygen

(b) Ammonia — Nitrogen and hydrogen

(c) Marble — Calcium, carbon and oxygen

(d) Washing soda — Sodium, carbon and oxygen

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

The differences between elements and compounds are given below:

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

1. Water consists 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.

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 has the same properties throughout its bulk.
4. The physical and chemical properties of a compound differ from those of its constituent elements.

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) Four types of elements are :

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

(b) Examples :

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

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.
Molecules of an element are made of same kind of atoms. In mixtures, substances are not chemically combined.

sodium chloride

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

silicon and oxygen

Reason — Sand or silica is a compound of silicon and oxygen in the ratio of 1 : 2 .
Calcium chloride is used for de-icing and drying agents, not as table salt. Sodium oxide is a reactive compound, not used as salt. Hydrogen chloride is a gas that forms hydrochloric acid in water.

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.

NH₃

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

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

MgO

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

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

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

(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.

(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.

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

(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)

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.

(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. Sulphur (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

The symbols and formulae of substances give information about the elements present in the compound and the number of each kind of atom in one molecule of the compound. They also allow us to represent chemical reactions using equations.

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

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

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.

lustrous and attractive

Reason — Gold is used to make jewellery because it is lustrous and attractive. It looks shiny and does not tarnish easily in air.

mixtures

Reason — Brass is a mixture of copper and zinc and bronze is a mixture of copper and tin.
Brass and bronze are not single elements; they are made of more than one metal. So, they can't be elements. Compounds have a fixed chemical formula, but alloys do not.

diamond

Reason — Diamond is a form of carbon, it is hardest naturally occuring substance. Hence, it is used to cut glass.
Graphite is soft and slippery (used in pencils). Iron is strong, but not the hardest natural substance. Coal is soft and brittle.

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.
Helium is an inert gas but is not commonly used in electric bulbs due to cost and suitability.

Iron

Reason — Iron is strong and easily available. Therefore, it is used to make heavy tools and machines.
Aluminium is light and used in aircraft, utensils, and packaging, not heavy machines. Copper is used mainly for electrical wiring due to good conductivity. Silver is expensive and used in jewellery and electronics, not heavy machinery.

Both A and R are true and R is the correct explanation of A.

Reason — A is true as pure substances have a fixed composition and definite properties. Both elements and compounds fit this definition.
R is true as an element contains only one type of atom, and a compound contains only one type of molecule.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Both A and R are true and R is not the correct explanation of A.

Reason — A is true as a chemical formula is like a symbolic representation of a molecule. The formula directly gives the exact number of each type of atom.
R is true as carbon and oxygen elements combine to form a molecule of carbon dioxide.

Therefore, Both A and R are true and R is not the correct explanation of A is the correct option.

Both A and R are true and R is the correct explanation of A.

Reason — A is true as Pencil “lead” is actually made of graphite, not lead metal.
R is true as Graphite is soft and slippery, so it leaves a black mark on paper when rubbed.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

A is false and R is true.

Reason — A is false as most metals actually have high melting and boiling points.
R is true as non-metals are generally dull (not shiny) and brittle (break easily).

Therefore, A is false and R is true is the correct option.

(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 noble 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.

(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

(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 noble gases.

(a) Coal, diamond and graphite.

(b) Diamond

(c) Copper and aluminium

(d) Gold and platinum

(e) Wood or Plastic

(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.

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.

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

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.

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.

(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)

Metals are ductile, which means they can be drawn or stretched into thin wires. Metals are malleable, which 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) Copper is a metal.

(b) Copper is used for making electrical wires because it is a good conductor of electricity and is ductile (can be drawn into thin wires).

(c) Silver is more ductile and a better conductor of electricity than copper, it is not used to make wires because it is expensive.

(d) It is necessary to cover metal wires with plastic because plastic is a bad conductor of electricity. It acts as an insulator and protects us from electric shock by preventing direct contact with the live metal wire.

(e) Plastic is a compound. It is also a bad conductor of electricity and acts as an insulator.