Notes Ch. 6.1 Organizing the Elements Chemistry

Searching For an Organizing Principle By 1700 only 13 elements had been identified. Chemists suspected that other elements existed. But some were only found in compounds and could not be isolated. Between 1765-1775 chemists identified 5 new elements, including hydrogen, nitrogen, and oxygen. To figure out how many elements existed they needed to find a logical way to organize the elements.

Chemists used the properties of elements to sort them into groups. In 1829 J. W. Dobereiner (a German chemist) published a classification system. He grouped elements into triads (a set of 3 elements with similar properties). For example - chlorine, bromine, and iodine form a triad. They look different but have similar chemical properties. For example, they react easily with metals. This triad didn’t work because not all known elements could be grouped into triads.

Mendeleev’s Periodic Table In 1869 Dmitri Mendeleev published a table of the elements. To form his table he wrote the properties of each element on a separate note card. He could then move the cards around until he found an organization that worked. The organization he chose was a periodic table. He arranged the elements into groups based on a set of repeating properties. Mendeleev arranged the elements in his periodic table in order of increasing atomic mass.

Mendeleev also had questions marks at some places because he knew an element should be there but it hadn’t been discovered yet. Some of the elements were discovered because they could figure out the properties of the missing elements and then find the elements.

The Periodic Law Mendeleev found that some of his elements didn’t work out in the right groups based on using the atomic mass. Mendeleev developed his table before scientists knew about the structure of atoms. In 1913 Henry Moseley determined the atomic number for each element. So he reorganized the periodic table based on increasing atomic number.

There are 7 rows, or periods, in the periodic table There are 7 rows, or periods, in the periodic table. Period 1 has 2 elements, period 2 has 8 elements, period 4 has 18 elements, and period 6 has 32 elements. Each period corresponds to a principal energy level. There are more elements in higher numbered periods because there are more orbitals in higher energy levels.

The elements in a column, or group, in the periodic table have similar properties. The properties of the elements within a period change as you move across a period from left to right. The pattern of the properties within a period repeats as you move from one period to the next. This is periodic law - when elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties.

Metals, Nonmetals, and Metalloids The groups on the periodic table are labeled 1 to 18 from left to right. The elements are also grouped into 3 broad classes based on general properties. These three classes are metals, nonmetals, and metalloids.

Metals Most elements are metals (about 80%). Metals are good conductors of heat and electric current. Metals will have high luster (or sheen). This is because the metal can reflect light. All metals are solids at room temp (except for mercury). Many metals are ductile, they can be drawn into wires. Most are also malleable, they can be hammered into thin sheets.

Nonmetals These elements are in the upper-right corner of the periodic table. Most nonmetals are gases at room temperature. A few are solids. One nonmetal, bromine, is a dark-red liquid. There is a lot of variation among nonmetals. They usually have properties that are opposite of metals. In general they are poor conductors of heat and electric current. Solid nonmetals are usually brittle.

Metalloids The heavy stair-step line that separates metals and nonmetals. Elements that border this line are metalloids. Metalloids have properties of metals and nonmetals. Sometimes they may act like a metal, other times they may be more like a nonmetal.