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.
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 TableIn 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.)
Periods of the Modern Periodic Table
Rows of the modern periodic table are called periods , as they are in Mendeleev’s table. From left to right across a period, each element has one more proton than the element before it. Some periods in the modern periodic table are longer than others. For example, period 1 contains only two elements: hydrogen (H) and helium (He). In contrast, periods 6 and 7 are so long that many of their elements are placed below the main part of the table. They are the elements starting with lanthanum (La) in period 6 and actinium (Ac) in period 7. Some elements in period 7 have not yet been named. They are represented by temporary three-letter symbols, such as Uub. The number of each period represents the number of energy levels that have electrons in them for atoms of each element in that period.
Q: Find calcium (Ca) in the periodic table above. How many energy levels have electrons in them for atoms of calcium?
A: Calcium is in period 4, so its atoms have electrons in them for the first four energy levels.
Groups of the Modern Periodic TableColumns of the modern table are called groups , as they are in Mendeleev’s table. However, the modern table has many more groups—18 compared with just 8 in Mendeleev’s table. Elements in the same group have similar properties. For example, all elements in group 18 are colorless, odorless gases, such as neon (Ne). (Neon is the element inside the light in opening photo C.) In contrast, all elements in group 1 are very reactive solids. They react explosively with water.
Classes of Elements
All elements can be classified in one of three classes: metals, metalloids, or nonmetals. Elements in each class share certain basic properties. For example, elements in the metals class can conduct electricity, whereas elements in the nonmetals class generally cannot. Elements in the metalloids class fall in between the metals and nonmetals in their properties. An example of a metalloid is arsenic (As). (Arsenic is the element in opening photo B.) In the periodic table above, elements are color coded to show their class. As you move from left to right across each period of the table, the elements change from metals to metalloids to nonmetals.
Q: To which class of elements does copper (Cu) belong: metal, metalloid, or nonmetal? Identify three other elements in this class.
A: In the periodic table above, the cell for copper is colored yellow. This means that copper belongs to the metals class. Other elements in the metals class include iron (Fe), sodium (Na), and gold (Au). It is apparent from the table that the majority of elements are metals.