By 1860, scientists had already discovered 60 elements and determined their atomic masses. The Search for a Periodic Table Periodic Table: Basic Concepts

• By 1860, scientists had already discovered 60 elements and determined their atomic masses.

The Search for a Periodic Table

Periodic Table: Basic ConceptsPeriodic Table: Basic Concepts Periodic Table: Basic ConceptsPeriodic Table: Basic Concepts

• They noticed that some elements had similar properties.

• They gave each group of similar elements a name. Copper, silver, and gold were called the coinage metals; lithium, sodium, and potassium were known as the alkali metals; chlorine, bromine, and iodine were called the halogens.

Topic 5Topic 5

• Chemists also saw differences among the groups of elements and between individual elements.

The Search for a Periodic Table

• They wanted to organize the elements into a system that would show similarities while acknowledging differences.

• It was logical to use atomic mass as the basis for these early attempts.

Periodic Table: Basic ConceptsPeriodic Table: Basic Concepts Periodic Table: Basic ConceptsPeriodic Table: Basic Concepts Topic 5Topic 5

Döbereiner’s Triads

• The elements in a triad had similar chemical properties, and their physical properties varied in an orderly way according to their atomic masses.


• In 1829, the German chemist J.W. Döbereiner classified some elements

into groups of three, which he called triads.

• Döbereiner’s triads were useful because they grouped elements with similar properties and revealed an orderly pattern in some of their physical and chemical properties.

Döbereiner’s Triads

• The concept of triads suggested that the properties of an element are related to its atomic mass.


• Triads show a relationship among the densities that is true for many triads. Density increases with increasing atomic mass.

• The Russian chemist, Dmitri Mendeleev, was a professor of chemistry at the University of St. Petersburg when he developed a periodic table of elements.

Mendeleev’s Periodic Table

• Mendeleev was studying the properties of the elements and realized that the chemical and physical properties of the elements repeated in an orderly way when he organized the elements according to increasing atomic mass.


• Mendeleev later developed an improved version of his table with the elements arranged in horizontal rows.


• This arrangement was the forerunner of today’s periodic table.

• Patterns of changing properties repeated for the elements across the horizontal rows.

• Elements in vertical columns showed similar properties.


• Mendeleev’s insight was a significant contribution to the development of chemistry.


• He showed that the properties of the elements repeat in an orderly way from row to row of the table.

• This repeated pattern is an example of periodicity in the properties of elements.

• Periodicity is the tendency to recur at regular intervals.


• One of the tests of a scientific theory is the ability to use it to make successful predictions.


• Mendeleev correctly predicted the properties of several undiscovered elements.

• In order to group elements with similar properties in the same columns, Mendeleev had to leave some blank spaces in his table.

• He suggested that these spaces represented undiscovered elements.


• Mendeleev was so confident of the periodicity of the elements that he placed some elements in groups with others of similar properties even though arranging them strictly by atomic mass would have resulted in a different arrangement.





Periodic Table of the Elements


The Modern Periodic Table

• There are several places in the modern table where an element of higher atomic mass comes before one of lower atomic mass.

• This is because the basis for ordering the elements in the table is the atomic number, not atomic mass.


The Modern Periodic Table• The atomic number of an element is equal to

the number of protons in the nucleus. • Atomic number

increases by one as you move from element to element across a row.

• Each row (except the first) begins with a metal and ends with a noble gas.



• In between, the properties of the elements change in an orderly progression from left to right.

• The pattern in properties repeats after column 18.

• This regular cycle illustrates periodicity in the properties of the elements.



• The statement that the physical and chemical properties of the elements repeat in a regular pattern when they are arranged in order of increasing atomic number is known as the periodic law.


Relationship of the Periodic Table to Atomic Structure

• In the modern periodic table, elements are arranged according to atomic number.

• The atomic number tells the number of electrons it has.



• The lineup starts with hydrogen, which has one electron.

• Helium comes next in the first horizontal row because helium has two electrons. Lithium has three.


• If elements are ordered in the periodic table by atomic number, then they are also ordered according to the number of electrons they have.


• Notice on the periodic table that lithium starts a new period, or horizontal row, in the table.

• Why does this happen? Why does the first period have only two elements?

• Only two electrons can occupy the first energy level in an atom. The third electron in lithium must be at a higher energy level.


Nonmetals• Most nonmetals don’t conduct electricity,

are much poorer conductors of heat than metals, and are brittle when solid.

• Many are gases at room temperature; those that are solids lack the luster of metals.

• Their melting points tend to be lower than those of metals.

• With the exception of carbon, nonmetals have five, six, seven, or eight valence electrons.


Properties of Metals and Nonmetals


Metalloids• Metalloids have some chemical and physical

properties of metals and other properties of nonmetals.

• In the periodic table, the metalloids lie along the border between metals and nonmetals.


MetalloidsTopic 5Topic 5

• Silicon (Si) is probably the most well-known metalloid.

• Some metalloids such as silicon, germanium (Ge), and arsenic (As) are semiconductors.

Metalloids

• The ability of a semiconductor to conduct an electrical current can be increased by adding a small amount of certain other elements.

• Silicon’s semiconducting properties made the computer revolution possible.


• A semiconductor is an element that does not conduct electricity as well as a metal, but does conduct slightly better than a nonmetal.

Semiconductors and Their Uses

• Your television, computer, handheld electronic games, and calculator are electrical devices that depend on silicon semiconductors.

• All have miniature electrical circuits that use silicon’s properties as a semiconductor.


Semiconductors and Their Uses

• You learned that metals generally are good conductors of electricity, nonmetals are poor conductors, and semiconductors fall in between the two extremes.


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By 1860, scientists had already discovered 60 elements and determined their atomic masses. The Search for a Periodic Table Periodic Table: Basic Concepts