ICSE Class 7 Chemistry Chapter 1 Elements and Compounds Atoms and Molecules

Elements And Compounds - Atoms And Molecules

In This Chapter You Will Learn

Elements, symbols and their significance

Atomicity and molecular formulae of elements

Molecular formulae of compounds

Atoms and molecules

Compounds

Introduction

There are millions of substances in this world. These substances differ from each other in their composition, properties and uses because they all are made up of different kinds of matter. They can be pure or impure. To study these substances accurately and conveniently, they need to be classified under three categories: elements, compounds and mixtures on the basis of some similarities and dissimilarities.

All substances are made up of very tiny particles called molecules. Molecules are formed from even smaller particles called atoms. Atoms and molecules are so small that they can only be seen through a powerful microscope.

Substances

Pure substances: Pure substances are made up of same kind of atoms or molecules and have a definite set of properties. They are all homogeneous, i.e., their composition is uniform throughout the bulk. Both elements and compounds are pure substances.

Elements are made up of same kind of atoms and compounds are made up of the same kind of molecules.

Impure substances: Impure substances are made up of two or more pure substances mixed together in any proportions. They do not have any definite set of properties but they retain the properties of constituent substances. They may be homogeneous or heterogeneous, i.e., their composition is not uniform throughout the bulk, e.g.: air, sugar solution, sand and stone, etc.

At present 118 elements are known. Of these, 90 are natural elements (most of them are in combined state in the earth's crust) while rest 28 have been artificially created. Some elements are solids, some are liquids and some are gases.

Do You Know?

Of the 118 elements, known to us, some are radioactive in nature, because they emit radiations which may be harmful.

1.2 Classification Of Elements

Based on their properties, elements are classified into:

(i) metals (ii) non-metals

(iii) metalloids (iv) noble (or inert) gases.

Metals: Most of the elements known to us are metals.

1. They are usually lustrous and hard solids. [Exceptions: Mercury and Gallium are liquids. Sodium and potassium are soft.]

2. Metals are malleable and ductile [Zinc is brittle].

Note: Malleable (can be beaten into thin sheets).

Ductile (can be drawn into wire without breaking).

3. They have high melting and boiling points.

4. They are good conductors of heat and electricity.

5. They are sonorous substances because they produce a specific sound when struck.

Examples: Gold, silver, copper, aluminium, zinc, iron, tin, etc.

Do You Know?

Chalk, milk and our bones contain a common metal calcium.

Chlorophyll contains magnesium which helps to capture the energy from sunlight for photosynthesis by plants.

In mammals iron is found in red blood cells in haemoglobin which carries oxygen in the body.

Non-metals:

1. Non-metals are elements with a dull surface, i.e., they do not shine.

2. They are fewer in number as compared to metals. They are either soft solids or gases, except bromine, which is a liquid.

3. Non-metallic solids are brittle in nature.

4. They are all bad conductors of heat and electricity. [Except carbon in the form of graphite and gas carbon].

5. They have low melting and boiling points [Exception: Carbon in the form of Graphite has a high melting point].

6. They are neither malleable nor ductile.

7. They are not sonorous.

8. They display a variety of colours.

Examples: Sulphur, phosphorus, carbon, oxygen, hydrogen, chlorine, nitrogen, etc.

Do You Know?

Sulphur, phosphorus and iodine are soft solids.

Carbon exists as soft solid in the form of charcoal, coal and soot but in the form of diamond it is the hardest naturally occurring solid.

Graphite is another form of carbon which is soft, lustrous and a good conductor of heat and electricity.

Diamond, due to its brilliant shine is used as a gem for making jewelleries.

Oxygen, hydrogen, chlorine and nitrogen are all gases.

Metalloids: These elements show some properties of metals and some properties of non-metals. They are hard solids.

Examples: boron, silicon, germanium, arsenic, antimony, etc.

Inert or noble gases: These elements do not react chemically with other elements or compounds, so they are known as noble (or inert) gases. They are found in air, in traces. There are only six noble gases - helium, neon, argon, krypton, xenon and radon.

1.3 Symbols Of Elements

1. Each element is denoted by a symbol which is usually the first letter of its name in English or Latin [written in capital].

Example: Oxygen is an element. It is denoted by the symbol 'O'. Similarly, hydrogen is denoted by a symbol 'H'. Now-a-days, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.

2. However, when the first letter of more than one element is same, the symbol is denoted by two letters, first letter is written in capital while the second is written in small letter.

Example: Carbon, cobalt and copper are the elements whose first letter is 'C'. Carbon is denoted by the symbol 'C'. Cobalt is denoted by two letters 'Co'. Copper is denoted by the symbol 'Cu' [taken from its Latin name cuprum]. Chromium is denoted by Cr while chlorine by Cl.

3. These symbols also represent an atom of that element.

Example:

(i) 'H' represents the element hydrogen as well as one atom of hydrogen.

(ii) 'C' represents the element carbon as well as one atom of carbon.

4. Other symbols have been taken from the names of elements in Latin, German or Greek.

Example: The symbol of iron is Fe from its latin name Ferrum, sodium is Na from Natrium, potassium is K from Kalium. Therefore, each element has a name and a unique chemical symbol.

Table 1.2 - Names and symbols

Compounds: "A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass."

A compound can be broken down into simpler substances. For example: table salt is a compound which can be broken down into two elements (more simple substances) sodium and chlorine.

Some common compounds are water, carbon dioxide, sugar, common salt, chalk, washing soda, alcohol, etc.

The smallest unit of a compound is molecule

Teacher's Note

Learning element symbols helps us read labels on household products and understand what materials are used in everyday items like batteries, cookware, and cosmetics.

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