ICSC Chemistry Class 6

Topic - Elements, Compounds and Mixtures

Todays topic for ICSC Chemistry Class 6 is Elements, Compounds and Mixtures.

ICSC Chemistry Class 6

What is matter?

Anything that has mass and occupies space is known as matter. Matter consists of living and non living things. Living things mainly consist of plants and animals while non living things mainly consist of materials and substances.

Classification of matter

Element - An element consists of same type of atoms and is a pure substance. We can't break it or convert it into any other substance by any physical or chemical reaction. The smallest unit of an element is an atom which may or may not have an independent existence. For example - oxygen, hydrogen, nitrogen etc. Elements are divided into metals, non-metals, metalloids and inert gases.

Metals - Properties of metals

1.Metals are generally hard solids. Except - Sodium.

2.Metals possess lustre or shine. For example - Gold, Silver etc.

3.Metals are generally malleable i.e they can be converted or beaten into sheets. For example - Iron.

4.Metals are ductile. For example - Copper.

5.Metals generally have high melting and boiling points.Except - Mercury which is a liquid at room temperature.

Non-metals - Properties of non-metals

1.Non-metals are either gases or soft metals. Except - Bromine which is a liquid at room temperature.

2.Non metals don't have any shine or lustre.Except -Iodine

3.Non-metals are non-malleable.

4.Non-metals are non-ductile.

5.Non-metals have low boiling and melting points.

Metalloids - These are the elements that possess properties of both metals and non-metals. Except - Boron

Inert or noble gases - These occur in traces.They don't show any chemical reaction with other elements as they are highly unreactive. For example - Helium, Argon, Neon, Krypton,Xenon and Radon.

Symbols of elements

A symbol is generally the short form of an element that tells us about its name.

The scientist named Berzelius suggested the following criteria for the symbols to be chosen for the elements.

1.First letter of the name of some elements was chosen as their symbols. The symbol should be in capital letter.

For example - Oxygen (O) ,Carbon (C), Nitrogen (N).

2.If the first letter is same in two elements such as Argon and Aluminium then first two letters in the name of the element were chosen as its symbol.The first letter should be capital in the symbol while the second letter should be small.For example - Aluminium (Al), Argon (Ar), Cobalt (Co)

3.Symbols were also derived from the latin names of the elements.For example - Natrium is the latin name of sodium and its symbol is Na. Cuprum is the latin name of Copper and its symbol is Cu.

Compounds

When two or more elements combine chemically in a definite proportion they form a compound. A compound is a pure substance. For example -

C + O2 ➡ CO2

1 atom of 2 atoms of Carbon

Carbon Oxygen dioxide

The properties of a compound differ from the elements from which is made up of. The elements of a compound lose their individual properties.

H2 + O ➡ H2O

2 atoms 1 atom Water

of hydrogen of oxygen

Hydrogen is combustible (that catches fire easily) but doesn't support combustion (that allows the substances to. Oxygen is not combustible but it supports combustion. But the properties of water differ from both hydrogen and oxygen as it is neither combustible nor supports combustion. So the elements lose their individual properties during the formation of compounds.

We can't separate the elements of a mixture by any physical method.

Energy is absorbed or released during the formation of a compound.

Atom

The word 'atom' is derived from the Greek word 'atomos' which means non-cuttable.The smallest particle of an element is known as an atom. All the properties of an atom are same as that of the element.An atom may or may not exist independently. For example - O, Ne, H etc.

Characteristics of an atom

1.All the atoms present in an element are same or identical.

2.The atoms of an element are different from that of the other element.

3.Most of the atoms found in the elements are highly reactive.

4.They undergo chemical reactions.

Molecule

A molecule is made up of two or more atoms and has all the properties of that compound or element.

Characteristics of a molecule

1.Molecules can be formed by combination of atoms of same or different elements. For example - O2, HCl etc.

2.Atoms of different elements are combined in a fixed ratio.

3.The atoms present in a molecule are held together by a chemical bond.

4.They don't undergo any chemical reaction.

5.Molecules of all the substances are represented by chemical formulas. For eg - O2, CO2 etc.

Molecules are further of two types -

1.Molecules of elements

2.Molecules of compounds

Molecules of Elements

1.They are made up of atoms of same type of elements. For example - O3,N2 etc.

Atomicity

It is the number of atoms found in the molecule of a substance. On the basis of atomicity, they are further of four types -

(a) Monoatomic - These molecules consist of only one atom. For example - Na,Ca, He etc.

(b)Diatomic - These molecules consist of two atoms. For example - Elementary gases such as O2, H2, N2 etc.

(c)Triatomic - These molecules consist of three atoms. For example - O3 (Ozone)

(d)Polyatomic - These molecules consist of more than three atoms. For example P4 (consists of four atoms) , S8 (consists of 8 atoms)etc.

Molecules of Compounds

They are formed by the combination of atoms of two or more elements. For example - HCl(consists of 1 atom of hydrogen and 1atom of chlorine),CO2 (consists of 1 atom of carbon and two atoms of oxygen)etc.

Formula of Compounds

The formula of compounds tell us about the number of atoms of the different elements found in each molecule of that compound. For example - HCl (consists of 1 atom of hydrogen element and one atom of chlorine element).

How to write a chemical formula

We should know the symbols and valencies (capacity to combine) f the elements properly to write their chemical formula.

Elements Symbols Valencies

Hydrogen H 1

Oxygen O 2

Carbon C 4

Chlorine Cl 1

Nitrogen N 3

Magnesium Mg 2

Sodium Na 1

Iron Fe 2

Potassium K 1

Calcium Ca 2

Lead Pb 2

Sulphur S 2

Aluminum Al 3

Rules to be followed while writing the formula for a compound

Step 1 - First of all the symbols of elements that form the compound are written.

For example - For writing the formula of sodium chloride write

1 1

Na Cl

Step 2 - Now interchange their valencies

Step 3 - Write the interchanged numbers below the symbols in the formula.

(No need to write the interchanged number if it is one)

NaCl

Importance of writing the molecular formula for a compound

1.It tells about the name of the compound.

For example - H2O → Water

2.It tells about the element present in the compound.

For example - H2O (In the molecule of water, two elements hydrogen and oxygen are present)

3.It tells about the number of atoms present in different elements of that compound.

For example - H2O (Water consists of two atoms of hydrogen and one atom of oxygen)

Mixtures - A mixture consists of the combination of two or more substances.

1.Homogenous Mixtures - These are the mixtures in which different components of the mixture don't settle independently as they are very well mixed. It seems to be uniform. For example - air, salty water etc.

2.Heterogeneous Mixtures - These are the mixtures in which the components of the mixture are unevenly distributed. For example - sand and water, oil etc.

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