10.4.2: Periodic Law
- Page ID
- 476545
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How are these items related to one another?
We have all enjoyed looking around a market for delicious foods to eat later at home. When you get to the market you know you need to get fruits, vegetables and grains to ensure you eat a balanced diet. In the market, these items are all grouped together to make it easier to find the type of fruit, vegetable or grain you are looking for; it would be inconvenient if the different fruits were all scattered in different places. The periodic table is organized in a similar way, ensuring similar elements are found in the same group or period.
The Periodic Law
When Mendeleev put his periodic table together, nobody knew about the existence of the atomic nucleus. Later experiments showed that the periodic relationship shown in the periodic table was not based on the atomic mass, but on the atomic number. (The atomic number is a property of the atomic nucleus which will be discussed in greater detail in the next chapter.) As it turns out, the atomic mass increases as the atomic number increases. For most of the periodic table the increases are on a similar scale, but there were some exceptions which were the cause of the discrepancies in Mendeleev's original periodic table.
When ordered by atomic number, the discrepancies within Mendeleev's table disappeared. Tellurium has an atomic number of 52, while iodine has an atomic number of 53. So even though tellurium does indeed have a greater atomic mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are credited with being most responsible for the modern periodic law: When elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties. The result is the periodic table as we know it today. Each new horizontal row of the periodic table corresponds to the beginning of a new period because a new principal energy level is being filled with electrons. Elements with similar chemical properties appear at regular intervals, within the vertical columns called groups.
When elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties
Section Summary
- Elements of the periodic table are arranged in order of increasing atomic number.
- The periodic law states: "When elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties."