# Elements & the Periodic Table

An element is a substance that cannot be broken down into simpler substances by chemical means.

• Pure substances can be also compounds. But compounds can be chemically broken down to the elements of which they are made.
• Elements cannot be broken down into simpler substances.
• Each element has a symbol and an unique atomic number(Know the symbols of elements with atomic number from 1 to 36 in the periodic table).

# History of Elements

1. Greeks – They thought that everything was a mixture of four elements. Fire, air, water and earth.
2. Robert Boyle – More than 1000 years later defined an element as a substance that cannot be broken down into simpler materials.
3. Humphry Davy – Developed powerful new electrochemical techniques for breaking down compounds into elements.
4. Henry Moseley – Using X-rays he found that the atomic nucleus of each element had a characteristic positive charge. He called this charge the atomic number.

# The Periodic Table

In the periodic table the elements are grouped together with similar chemical properties.

• Atomic number: The number of protons in the nucleus of that atom.
• Mass number (Atomic weight/relative atomic mass): The sum of the number of protons and neutrons in the nucleus of an atom of that element.
• A horizontal row of elements in the periodic table is called a period.

#### Modern Periodic Table facts:

• The elements are arranged in order of increasing atomic number.
• When they are arranges like this, the properties of the elements vary periodically.

# History of the periodic table

## Law of Triads

Johann Dobereiner arranged the elements in groups of 3, where the properties of the middle element were midway between those above and below it. This only worked for a few of the elements. Unfortunately, this couldn’t seem to be applied to the remaining 40 elements.

triad is a group of three elements with similar chemical properties in which the atomic weight of the middle element is approximately equal to the average of the other two.

## Law of Octaves

John Newlands arranged the elements in the order of increasing atomic weight and he noticed that the properties seemed to repeat themselves every eighth element. However, this worked for only the first 17 elements then known. He attempted to force the other elements to fit this pattern but was unsuccessful. His Law if Octaves was not accepted by other chemists.

Newland’s Octaves are groups of elements arranged in order of increasing atomic weight, in which the first and the eighth element of each group have similar properties.

## Mendeleev’s periodic table

Some years later, Dmitri Mendeleev arranged the elements in order of their atomic weights, He placed similar elements in groups. He also left gaps in the table in order to fit them into their proper groups and predicted the properties of the missing elements.

Mendeleev’s Periodic Law: When elements are arranged in order of increasing atomic weight (relative atomic mass), the properties of the elements vary periodically.

### Differences in the modern periodic table from Mendeleev’s periodic table

• Gaps have been filled which were left for undiscovered elements.
• More elements discovered since 1869 have been included.
• d-block (transition) elements are arranged as subgroups in a separate block instead of being beside each main group.
• Elements are arranged in order of increasing atomic number instead of increasing atomic weight.
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