# Modern Periodic Table

## Elements are arranged by increasing atomic number instead of atomic mass.

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Modern Periodic Table

Look at substances A–C in the photos above. They look very different from one another, but they have something important in common. All three are elements, or pure substances. Can you identify which elements they are? For ideas, listen to the amazing elements song at the URL below. The singer rapidly names all of the known elements while pictures of the elements flash by. Even if the video doesn’t help you name the elements pictured above, it will certainly impress you with the need to organize the large number of elements that have been discovered.

### The Modern Periodic Table

A periodic table is still used today to organize the elements. You can see a simple version of the modern periodic table in the Figure below. The modern table is based on Mendeleev’s table, except the modern table arranges the elements by increasing atomic number instead of atomic mass. Atomic number is the number of protons in an atom, and this number is unique for each element. The modern table has more elements than Mendeleev’s table because many elements have been discovered since Mendeleev’s time. You can explore an interactive version of the modern periodic table at this URL: http://www.ptable.com/.

### Reading the Table

In the table above, each element is represented by its chemical symbol, which consists of one or two letters. The first letter of the symbol is always written in upper case, and the second letter—if there is one—is always written in lower case. For example, the symbol for copper is Cu. It stands for cuprum, which is the Latin word for copper. The number above each symbol in the table is its unique atomic number. Notice how the atomic numbers increase from left to right and from top to bottom in the table.

Q: Find the symbol for copper in the periodic table above. What is its atomic number? What does this number represent?

A: The atomic number of copper is 29. This number represents the number of protons in each atom of copper. (Copper is the element that makes up the coil of wire in photo A of the opening sequence of photos.)

The alkali metal sodium (Na) reacting with water.

### Summary

• The modern periodic table is used to organize all the known elements. Elements are arranged in the table by increasing atomic number.
• In the modern periodic table, each element is represented by its chemical symbol. The number above each symbol is its atomic number. Atomic numbers increase from left to right and from top to bottom in the table.
• Rows of the periodic table are called periods. From left to right across a period, each element has one more proton than the element before it.
• Columns of the periodic table are called groups. Elements in the same group have similar properties.
• All elements can be classified in one of three classes: metals, metalloids, or nonmetals. Elements in each class share certain basic properties. From left to right across each period of the periodic table, elements change from metals to metalloids to nonmetals.

### Vocabulary

• group: Column of the periodic table, which contains elements with similar properties.
• period: Row of the periodic table that contains elements ranging from metals on the left to metalloids and to nonmetals on the right.
• periodic table: Table of elements arranged by increasing atomic number (modern periodic table) or by increasing atomic mass (Mendeleev’s periodic table).

### Practice

Practice using the modern periodic table by playing the element math game at the URL below. Be sure to check your answers. For any questions you answer incorrectly, click on the “Tell me more about...” tab to see where you went wrong.

### Review

1. What is the modern periodic table?
2. Compare and contrast the periods and groups of the modern periodic table.
3. In the modern periodic table above, find the element named lead (Pb). How many protons do atoms of lead have? To which class of elements does lead belong?
4. Which groups of the modern periodic table contain elements that are classified as metalloids?

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