GRADE 7 TH : ELEMENTS, COMPOUNDS & MIXTURES

Atoms:

• Everything in this world is made of billions of atoms.

• The smallest atom known as hydrogen.

• Each atom is represented as a sphere having a diameter of 7 X 10-8 mm.

• Different elements have different diameter and different masses.

• Chemists use shorthand symbols to label the elements and their atoms.

• The symbols consists of 1, 2 or 3 letters.

• For example 1. C - Carbon 2. Cl – Chlorine

• Some symbols seems to have no relations with the name of the element. For

example: 1. Na – Sodium 2. Pb - Lead

• These symbols come from their Latin names.

Atomicity: the number of atoms present in the molecule of an element. For example, each molecule of oxygen (O2) is composed of two oxygen atoms. So atomicity of oxygen is 2.

Molecules:

• The atoms of same elements join together to form a molecule.

• The small group of atoms are called molecules.

• For example: when the atoms of elements hydrogen, oxygen, nitrogen, fluorine,

chlorine, bromine and iodine are each joined in pairs and are known as diatomic

molecules.

• but in phosphorus and sulfur the atoms are joined in larger numbers, 4 and 8

respectively (P4, S8)

• The gaseous elements helium, neon, argon, krypton, xenon and radon are composed

of separate and individual atoms.

• When an element exists as separate atoms, then the molecules are said to be

monatomic.

Valency: The valency of an element is the number of electrons an atom of the

element uses to combine with atoms of other elements - it is the combining power of

an atom of the element. In an atom, the valence electrons are the electrons that can

be used in combining with other atoms - these are the electrons in the orbits of the

outermost shell (also called valence shell).

Elements :

An element is a pure substance that cannot be separated into simpler substances by

physical or chemical means.

A pure substance is a substance in which there is only one type of particle.

Elements are pure substances so each element only contains one type of particle.

Example: Every particle in a 5g nugget of the element gold is like every other particle

of gold

Every element has a unique set of properties:

Each element can be identified by its properties.

Each element has its own characteristic properties.

Characteristic properties: properties that don’t depend on the amount of material

present in a sample of the element

Characteristic Properties of Elements:

Physical Properties

Boiling Point

Melting Point

Density

Chemical Properties

Reactivity with different substances

Elements are grouped into categories based on the properties they share.

Example: Iron, nickel, and cobalt are all shiny and conduct heat and electrical

current. They’ve been placed into a large group called metals with similar

elements.

Elements are divided into:

o Metals

o Non-metals

o Metalloids

o Noble gas

Major Categories of Elements:

1. Metals: shiny, good conductors of thermal energy and electric current,

malleable (can be hammered into thing sheets) and ductile (can be drawn into thin

wires). Elements in this category :Iron, Tin, Lead, Copper

2. Non-metals: dull, poor conductors of thermal energy and electric current,

brittle and unmalleable. Elements in this category: Neon, Bromine, Sulfur

3. Metalloids: have properties of both metals and non metals, some are shiny

while others are dull, some are good conductors while others are not. Elements in this

category : Silicon, Antimony, Boron

4. Noble gas: are chemically inert and do not react with elements and compounds.

Eg: Neon,krypton.

Compounds:

Are pure substance composed of two or more elements that are chemically

combined.

In order for elements to combine, they must react, or undergo a chemical

change, with one another.

Some Familiar Compounds:

Table Salt: Sodium and Chlorine

Water: Hydrogen and Oxygen

Sugar: Carbon, Hydrogen, and Oxygen

Carbon Dioxide: Carbon and Oxygen

Baking Soda: Sodium, Hydrogen, Carbon, and Oxygen

Compounds Have Unique Sets of Properties

Compounds have different properties from the elements that form it.

Ex: Table salt is made of sodium (which reacts violently with water) and

chlorine (which is poisonous).

Compounds Can Be Broken Down into Simpler Substances by:

Either broken down into elements through chemical changes. Or undergo

chemical changes and form simpler compounds

Compounds Cannot Be Broken Down by Physical Changes

Only way to break down a compound is through a CHEMICAL change.

Chemical formulae: A compound is a substance made up of a definite proportion of two or more elements. A chemical formula tells us the number of atoms of each element in a compound. It contains the symbols of the atoms of the elements present in the compound as well as how many there are for each element in the form of subscripts.

Mixtures:

Mixture is a combination of two or more substances that are not chemically

combined

Two or more materials form a mixture if they do not react to form a compound

Substance in a mixture keep their identities.

Mixtures can be physically separated.

Homogeneous mixture:

These are the types of mixtures in which the components mixed are uniformly

distributed throughout the mixture or in other words “the same throughout”.

We can observe only one phase of matter in a homogeneous mixture. Key

points regarding this type of mixtures:

Particles are distributed uniformly

We can’t judge a homogeneous mixture by just seeing it

Homogeneous mixtures are also called as solutions

Uniform composition

Example: rainwater, vinegar, etc.

Heterogenous mixture:

This is a type of mixture in which all the components are completely mixed

and all the particles can be seen under a microscope. We can easily identify

the components and more than one phase can be seen by naked eyes.

Key points regarding this type of mixtures:

Particles are distributed non uniformly

We can judge a heterogeneous mixture by just seeing it

Non-uniform composition

Example: seawater, etc.

Practice questions:

1. Define the following:

a. Atomicity

b. Atom

c. Molecule

d. Valency

2. What are the molecular formulae of the following compounds?

Hydrochloric acid, sulphuric acid, sodium chloride, glucose, methane.

3. State 4 differences between compounds & mixtures?

4. Give one example of each:

a. Solid-solid mixture

b. Solid-liquid mixture

c. Liquid-gas mixture

5. What are the characteristic properties of a pure substance? Why do we need them?

Worksheet 1:

Video links for better understanding:

https://www.youtube.com/watch?v=aq7L84Ylx34

https://www.youtube.com/watch?v=fQ_kiFRETGI

https://www.youtube.com/watch?v=j7690jLbvZk

https://www.youtube.com/watch?v=msSclkLW4Lk

TEXTUAL SHORT QUESTION-ANSWERS FOR CH-1,2,3

FOR CH3: ELEMENTS,COMPOUNDS AND MIXTURES:

1) What are non metals? Are non metals good conductors of heat and

electricity?

It is a chemical element that does not have the properties of a metal.No

they are not good conductors of heat and electricity.

2) Which elements are required to make sugar ? Also give the chemical

formula of sugar.

Elements required for making sugar are C,H ,O.The chemical formula for

sugar is C22H11O11.

3) “The properties of a compound are the same as the properties of the

elements that make up the compound”.is this statement correct.If not correct

it.

No. The properties of a compound are not the same as the properties of

the elements that make up the compound.

4) Name a few elements whose names start with Latin names.

Sodium (Natrium) ,Iron (Ferrum) ,Gold (Aurum) and Potassium (Kalium).

5) What do you mean by chemical symbols for elements ?

The abbreviation of the name of the element to its initial letter or

second letters is called symbol of element.

6) Write down the symbols for each of these elements.

i. Potassium K

ii. Sodium Na

iii. Chlorine Cl

iv. Gold Au

v. Silver Ag

vi. Aluminium Al

vii. Calcium Ca

viii. Cobalt Co

7) Write down the names of the elements /compounds for each of these

symbols.

i. Ca Calcium

ii. MgO Magnesium Oxide

iii. CuS Copper Sulphide

iv. P Phosphorous

v. S Sulphur

vi. Sb Antimony

vii. He Helium

viii. Zn Zinc

ix. ZnO Zinc Oxide

8) Write molecular formula for each of the following:

i. Chlorine Cl2

ii. Oxygen O2

iii. Common Salt NaCl

iv. Water H2O

v. Nitrogen N2

vi. Lead Sulphide PbS

vii. Hydrogen H2

viii. Calcium Oxide CaO

9) What do you understand by the term atomicity? Give the atomicity of

sulphur.

Atomicity is defined as the number of atoms present in one molecule of

an element. The atomicity of sulphur is 8.

10) Differentiate between Homogeneous and Heterogeneous mixture

Homogeneous Mixture Heterogeneous Mixture

It is the one in which there is perfect mixing of the components of the mixture.

It is a mixture that contains physically distinct parts, each with different properties.

FOR CH2: PHYSICAL AND CHEMICAL CHANGES

1. State whether rusting of iron is a physical or chemical change? How can you

prevent rusting of iron?

Ans. Rusting of iron is a chemical change. We can prevent rusting of iron by

painting, greasing and galvanizing.

2. Is an earthquake a periodic or non-periodic change? Give reasons. Also

mention some of the losses that occur due to earthquakes.

Ans. Earthquake is a non-periodic change because it occurs at any time. Due to

earthquake there is loss of property, mankind, health etc.

3. What are periodic and non-periodic changes? Is cyclonic weather a periodic

or non-periodic change?

Ans. Periodic change- Changes which are repeated after a fixed interval of time

are known as periodic changes.

Non-periodic changes- Changes which are not repeated at regular intervals and

occur irregularly are called non-periodic changes.

Cyclonic weather is a non-periodic change.

4. Is falling of leaves from a tree an irreversible change? Give reasons.

Ans. Yes, falling of leaves from a tree is an irreversible change because we

cannot put leaves back to the tree.

5. Give three characteristics of a physical change.

Ans. a) A physical change is temporary and easily reversible.

b) No new substances are formed during a physical change.

c) Heat may be given out or absorbed in a physical change.

6. Give three characteristics of a chemical change.

Ans. a) New substances, entirely with new properties are formed during a

chemical change.

b) Chemical change cannot be reversed to get the original substance.

c) Chemical changes are accompanied with change in energy in the form of

heat, light or sound.

FOR CH1:MATTER AND ITS COMPOSITION:

1. Why can’t we see atoms with naked eyes ?

• Atoms are too small in size, so it cannot be seen with naked eyes.

2. Can we change solid iron into liquid? If yes how?

• Yes, we can change solid iron into liquid by heating it.

3. Why can’t we compress the solid block of steel?

• The particles of solids are closely packed, intermolecular space is less

and intermolecular force of attraction is strongest. So we can’t compress the

solid block of steel.

4. Why gases do not have definite volume?

• The molecules of gases lie far apart from each other, intermolecular

space are very large and intermolecular force of attraction is weak .As a result

gases do not have definite volume.

5. Why can liquids flow whereas solids can be heaped ?

• In liquid molecules , particles are at greater distance. Intermolecular

forces between molecules is less. However in solids the bonds are very strong

and prevent molecules from moving around.