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# Periodic Law

## Properties of elements are periodic functions of their atomic numbers.

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Finding Patterns in Elemental Behavior

### Finding Patterns in Elemental Behavior

Credit: Liza

#### Why It Matters

• Early attempts to organize the elements either listed the known elements by atomic mass or focused on “triads” that had similar chemical properties, such as lithium, sodium, and potassium.
• Dmitri Mendeleev, often credited with the invention of the modern periodic table, combined both of these approaches, but left gaps to ensure that elements with similar properties were lined up together.
• Credit: Shehal Joseph
Source: http://www.flickr.com/photos/shehal/2453867520/

An early version of Mendeleev's periodic table. Notice that it looks different from the more modern periodic table [Figure2]

• The gaps in Mendeleev’s table corresponded to undiscovered elements, and many of their properties could be predicted based on their position in the table.

#### Can You Apply It?

1. Mendeleev placed tellurium before iodine on his periodic table, even though tellurium has a higher atomic mass. Why?
2. The modern periodic table is not arranged by atomic mass. What atomic property is used instead to reliably group together elements with similar chemical characteristics?
3. Carbon can be burned in oxygen to produce CO2 gas. Sand and quartz are two common forms of SiO2, which makes up over half of the Earth’s crust. Name two other elements (X) that are likely to form a compound with the formula XO2.
4. At room temperature, chlorine is a yellowish gas, bromine is a brown liquid, and iodine is a shiny blue-black solid. Why are these elements considered similar enough to group together on the periodic table?

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