A substance made up of one kind of atom is an element. Gold is an element because it only contains gold atoms. Water is not an element because it contains hydrogen and oxygen atoms. Scientists list the elements by atomic number in the PERIODIC TABLE. The atomic number is the number of protons in the atom’s nucleus.
Scientists have identified 92 elements that occur naturally. Over three-fourths of the natural elements are METALS. The heaviest natural element is the metal uranium. Scientists have created heavier elements in nuclear reactors and particle accelerators.
Hydrogen and helium are the most common elements in the Universe. Stars are made almost entirely of these elements. The most common elements in the Earth’s crust are oxygen and silicon.
More than sixty elements are metals. They are elements with only one or two electrons in their outer shell. They all share similar properties (characteristics)—they are shiny and strong. Metals also conduct heat and electricity well—they are useful for cooking pots and electrical wiring.
The periodic table is a simple list of all the elements. The elements are arranged by their atomic number. The information given for each element includes atomic number, symbol, name, and mass number. The vertical columns of the table are called groups, and the horizontal rows are called periods.
The periodic table sets out the elements in a way that highlights similarities and trends in their properties. Elements in the same group (column) have similar properties. The properties change gradually along periods (rows) in the table—elements on the left are metals, elements on the right are nonmetals. As you move across periods, the atomic number increases. Also, at the start of a period, elements have one electron in their outer shell—by the end, they have eight.
The chemical properties of an element depend on the number of electrons in its outer shell. Elements in the same group have the same number of outer electrons. For example, all the elements in Group 1 (alkali metals) have one electron in their outer shells. They are all silvery white, highly reactive metals.
All elements have several forms called isotopes. Each isotope of an element has the same atomic number, but a different atomic mass. In carbon, 99 percent of the atoms are isotope carbon 12, and 1 percent are carbon 13. Both isotopes have six protons, but carbon 12 has six neutrons, while carbon 13 has seven.
Scientists can tell a great deal about the properties of an element, even before they do experiments—by looking at its position in the periodic table. They can decide whether it will be a metal or a nonmetal, judge how well it will conduct electricity, and predict how it will react with other elements.