1. The periodic table
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2. THE SEARCH FOR ORDER
Until 1750, scientists had identified only 17 elements, mainly metals, such as copper and iron.
As the number of known elements grew, so did the need to organize them into groups based on their properties.
In 1789, Antoine Lavoisier grouped the know elements into categories he called metals, nonmetals, gases, and earths.

3. Mendeleev’s proposal
For the next 80 years scientists looked for different ways to classify elements, but no system worked for all the known elements.
In the 1860s Dmitri Mendeleev developed an approach for organizing the elements while playing the card game solitaire.
Mendeleev made a “deck of cards” of the elements, listing an element’s name, mass, and properties on each card. When he arranged the cards in order of increasing mass a pattern emerged.

4. Mendeleev’s periodic table

5. Modern periodic table
In the modern periodic table, elements are arranged by increasing atomic number (number of protons)
Properties of elements repeat in a predictable way when atomic numbers are used to arrange elements into groups
Each row in the table represents a period
Each column in the table represents a group

6. Modern periodic table
Elements in a group have similar electron configurations and thus share similar chemical properties.
This pattern of repeating properties is the periodic law.
Elements in the table are classified as metals, nonmetals, and metalloids.

8. Elements and atomic mass
There are four pieces if information for each element.
Atomic number
Element symbol
Element name
Atomic mass

9. Elements and atomic mass
The atomic mass is a value that depends on the distribution of an element’s isotopes in nature and the masses of those isotopes.
In nature, most elements exist as a mixture of two or more isotopes.
Atomic mass is calculated roughly by adding the number of protons and neutrons found in an element.