ELEMENTS
On the basis of their chemical composition, substances can be classified either as elements or compounds.
Robert Boyle was the first scientist to use the term element in 1661. Antoine Laurent Lavoisier (1743-94), a French chemist, was the first to establish an experimentally useful definition of an element. He defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
* The number of elements known at present are more than 100. Ninety-two elements are naturally occurring and the rest are man- made.
* Majority of the elements are solid.
* Eleven elements are in gaseous state at room temperature.
* Two elements are liquid at room temperature–mercury and bromine.
* Elements, gallium and cesium become liquid at a temperature slightly above room temperature (303 K).
Elements can be normally divided into metals, non-metals and metalloids.
Metals usually show some or all of the following properties:
* They have a lustre (shine).
* They have silvery-grey or golden-yellow colour.
* They conduct heat and electricity.
* They are ductile (can be drawn into wires).
* They are malleable (can be hammered into thin sheets).
* They are sonorous (make a ringing sound when hit).
Examples of metals are gold, silver, copper, iron, sodium, potassium etc. Mercury is the only metal that is liquid at room temperature.
Non-metals usually show some or all of the following properties:
* They display a variety of colours.
* They are poor conductors of heat and electricity.
* They are not lustrous, sonorous or malleable.
Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc.
Some elements have intermediate properties between those of metals and non-metals, they are called metalloids; examples are boron, silicon, germanium etc.
Source: This topic is taken from NCERT TEXTBOOK
COMPOUNDS
A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.
What do we get when two or more elements are combined?
Activity 2.10:
* Divide the class into two groups. Give 5 g of iron filings and 3 g of sulphur powder in a china dish to both the groups.
Group I
* Mix and crush iron filings and sulphur powder.
Group II
* Mix and crush iron filings and sulphur powder. Heat this mixture strongly till red hot. Remove from flame and let the mixture cool.
Groups I and II
* Check for magnetism in the material obtained. Bring a magnet near the material and check if the material is attracted towards the magnet.
* Compare the texture and colour of the material obtained by the groups.
* Add carbon disulphide to one part of the material obtained. Stir well and filter.
* Add dilute sulphuric acid or dilute hydrochloric acid to the other part of the material obtained.(Note: teacher supervision is necessary for this activity).
* Perform all the above steps with both the elements (iron and sulphur) separately.
The gas obtained by Group I is hydrogen, it is colourless, odourless and combustible– it is not advised to do the combustion test for hydrogen in the class. The gas obtained by Group II is hydrogen sulphide. It is a colourless gas with the smell of rotten eggs.
You must have observed that the products obtained by both the groups show different properties, though the starting materials were the same. Group I has carried out the activity involving a physical change whereas in case of Group II, a chemical change (a chemical reaction) has taken place.
* The material obtained by group I is a mixture of the two substances. The substances given are the elements– iron and sulphur.
* The properties of the mixture are the same as that of its constituents.
* The material obtained by group II is a compound.
* On heating the two elements strongly we get a compound, which has totally different properties compared to the combining elements.
* The composition of a compound is the same throughout. We can also observe that the texture and the colour of the compound are the same throughout.
Questions
1. Did the material obtained by the two groups look the same?
2. Which group has obtained a material with magnetic properties?
3. Can we separate the components of the material obtained?
4. On adding dilute sulphuric acid or dilute hydrochloric acid, did both the groups obtain a gas? Did the gas in both the cases smell the same or different?
Source: This topic is taken from NCERT TEXTBOOK
DIFFERENCE BETWEEN COMPOUNDS AND MIXTURES
Thus, we can summarise the physical and chemical nature of matter in the following graphical organiser :
Source: This topic is taken from NCERT TEXTBOOK