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Discovering a Pattern In 1869, Russian chemist Dmitri Mendeleev arranged the elements in order of increasing atomic mass. Periodic Properties of the Elements When the elements are arranged in order of increasing atomic mass, those that had similar properties occurred in a repeating pattern. These repeating patterns are periodic, meaning that they happen at regular intervals.

Discovering a Pattern, continued Predicting Properties of Missing Elements Mendeleev’s arrangement had gaps in its pattern. Mendeleev predicted that elements yet to be found would fill these gaps. He also predicted the properties of the missing elements. By 1886, all of the gaps had been filled and Mendeleev’s predictions were right.

Changing the Arrangement A few elements’ properties did not fit in the pattern of Mendeleev’s table. In 1914, British scientist Henry Moseley found the number of protons—the atomic number—in an atom. When the elements were arranged by atomic number, they fit the pattern in Mendeleev’s table.

The Periodic Table and Classes of Elements Elements are classified as metals, nonmetals, and metalloids, according to their properties. The number of electrons in the outer energy level of an atom is one characteristic that helps determine which category an element belongs in. The zigzag line on the periodic table can help you recognize which elements belong in which category.

The Periodic Table and Classes of Elements, continued Metals are found to the left of the zigzag line. Atoms of most metals have few electrons in their outer energy level. Most metals are shiny, ductile, malleable, and are good conductors of electric current and thermal energy.

The Periodic Table and Classes of Elements, continued Nonmetals are found to the right of the zigzag line. Atoms of most nonmetals have an almost complete set of electrons in their outer energy level. Nonmetals are not shiny, ductile, or malleable, and are poor conductors of electric current and thermal energy.

The Periodic Table and Classes of Elements, continued Metalloids are the elements that border the zigzag line. Atoms of metalloids have about half of a complete set of electrons in their outer energy level. Metalloids have some properties of metals and some properties of nonmetals. Metalloids are also called semiconductors.

Decoding the Periodic Table Each Element is Identified by a Chemical Symbol Each square on the periodic table includes an element’s name, chemical symbol, atomic number, and atomic mass. For most elements, the chemical symbol has one or two letters. The newest elements have temporary three-letter symbols.

Decoding the Periodic Table, continued Rows Are Called Periods Each horizontal row of elements is called a period. The chemical and physical properties of elements in a row follow a repeating pattern as you move across the period. Columns Are Called Groups Each vertical column of elements is called a group. Elements in the same group often have similar chemical and physical properties.

Group 1: Alkali Metals Alkali metals are elements in Group 1 of the periodic table. Alkali metal properties: group contains metals 1 electron in the outer level very reactive softness, color of silver, shininess, low density

Group 2: Alkaline-Earth Metals Alkaline-earth metals are elements in Group 2. Alkaline-earth metal properties: group contains metals 2 electrons in the outer level very reactive, but less reactive than alkali metals color of silver, higher densities than alkali metals

Group 3–12: Transition Metals Transition metals are in Groups 3–12. Some of the transition metals are shown below.

Group 3–12: Transition Metals, continued Properties of Transition Metals vary widely but include: groups contains metals 1 or 2 electrons in the outer level less reactive than alkaline-earth metals shininess, good conductors of electric current and thermal energy

Group 3–12: Transition Metals, continued Lanthanides and Actinides Some transition metals from Periods 6 and 7 appear in two rows at the bottom of the periodic table. Elements in the first row are called lanthanides and elements in the second row are called actinides.

Group 13: Boron Group Aluminum is the most common element from Group 13. Group 13 properties: group contains one metalloid and five metals 3 electrons in the outer level reactive solids at room temperature

Group 14: Carbon Group Group 14 properties: group contains one nonmetal, two metalloids, and two metals 4 electrons in the outer level reactivity varies among the elements solids at room temperature

Group 15: Nitrogen Group Group 15 properties: group contains two nonmetals, two metalloids, and two metals 5 electrons in the outer level reactivity varies among the elements solids at room temperature (except for nitrogen, which is a gas)

Group 16: Oxygen Group Group 16 properties: group contains three nonmetals, one metalloids, and one metal 6 electrons in the outer level reactive solids at room temperature (except for oxygen, which is a gas)

Group 17: Halogens Halogens are the elements in Group 17. Group 17 properties: group contains nonmetals 7 electrons in the outer level very reactive poor conductors of electric current, never in uncombined form in nature

Group 18: Noble Gases Noble gases are the elements in Group 18. Group 18 properties: group contains nonmetals 8 electrons in the outer level (except helium, which has 2) unreactive colorless, odorless gases at room temperature

Hydrogen The properties of hydrogen do not match the properties of any single group, so hydrogen is set apart. a nonmetal 1 electron in the outer level reactive colorless, odorless gas at room temperature, low density