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The Periodic Table and Periodic Law. Chapter 6. The Search for a Periodic Table . By 1860, scientists had already discovered 60 elements and determined their atomic masses. . They noticed that some elements had similar properties.
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The Periodic Table and Periodic Law Chapter 6
The Search for a Periodic Table By 1860, scientists had already discovered 60 elements and determined their atomic masses. 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.
The Search for a Periodic Table Chemists also saw differences among the groups of elements and between individual elements. 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.
In 1829, the German chemist J.W. Döbereiner classified some elements into groups of three, which he called triads. 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.
Döbereiner’s Triads Triads show a relationship among the densities that is true for many triads. Density increases with increasing atomic mass. 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. The concept of triads suggested that the properties of an element are related to its atomic mass.
Mendeleev’s Periodic Table 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 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’s Periodic Table 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 Periodic Table 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.
Mendeleev’s Periodic Table 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’s Periodic Table 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.
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.
The Modern Periodic Table 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 Modern Periodic Table 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 protons (hence electrons) it has.
Relationship of the Periodic Table to Atomic Structure If elements are ordered in the periodic table by atomic number, then they are also ordered according to the number of electrons they have. 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.
Relationship of the Periodic Table to Atomic Structure Notice on the periodic table that lithium starts a new period (or series), 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.
Relationship of the Periodic Table to Atomic Structure Lithium starts a new period at the far left in the table and becomes the first element in a group. A group, sometimes also called a family, consists of the elements in a vertical column.
Relationship of the Periodic Table to Atomic Structure Groups are numbered from left to right. Lithium is the first element in Group 1 and in Period 2. Check this location on the periodic table.
Relationship of the Periodic Table to Atomic Structure Elements with atomic numbers 4 through 10 follow lithium and fill the second period. Each has one more electron than the element that preceded it. Neon, with atomic number 10, is at the end of the period.
Relationship of the Periodic Table to Atomic Structure Eight electrons are added to Period 2 from lithium to neon, so eight electrons must be the number that can occupy the second energy level.
Relationship of the Periodic Table to Atomic Structure The next element, sodium, atomic number 11, begins Period 3. Sodium’s 11th electron is in the third energy level. The third period repeats the pattern of the second period. Each element has one more electron than its neighbor to the left, and those electrons are in the third energy level.
Atomic Structure of Elements Within a Period The first period is complete with two elements, hydrogen and helium. Hydrogen has one electron in its outermost energy level, so it has one valence electron.
Atomic Structure of Elements Within a Period Every period after the first starts with a Group 1 element. These elements have one electron at a higher energy level than the noble gas of the preceding period. Therefore, Group 1 elements have one valence electron.
Atomic Structure of Elements Within a Period As you move from one element to the next across Periods 2 and 3, the number of valence electrons increases by one. Group 18 elements have the maximum number of eight valence electrons in their outermost energy level.
Atomic Structure of Elements Within a Period Group 18 elements are called noble gases. The noble gases, with a full complement of valence electrons, are generally unreactive.
Atomic Structure of Elements Within a Period The period number of an element is the same as the number of its outermost energy level, so the valence electrons of an element in the second period, for example, are in the second energy level. A Period 3 element such as aluminum has its valence electrons in the third energy level.
Atomic Structure of Elements Within a Group The number of valence electrons changes from one to eight as you move from left to right across a period; when you reach Group 18, the pattern repeats. For the main group elements, the group number is related to the number of valence electrons. The main group elements are those in Groups 1, 2, 13, 14, 15, 16, 17, and 18.
Atomic Structure of Elements Within a Group For elements in Groups 1 and 2, the group number equals the number of valence electrons. For elements in Groups 13, 14, 15, 16, 17, and 18, the second digit in the group number is equal to the number of valence electrons.
Atomic Structure of Elements Within a Group Because elements in the same group have the same number of valence electrons, they have similar properties. Sodium is in Group 1 because it has one valence electron. Because other elements in Group 1 also have one valence electron, they have similar chemical properties.
Atomic Structure of Elements Within a Group Chlorine is in Group 17 and has seven valence electrons. All the other elements in Group 17 also have seven valence electrons and, as a result, they have similar chemical properties. Throughout the periodic table, elements in the same group have similar chemical properties because the have the same number of valence electrons.
Atomic Structure of Elements Within a Group Four groups have commonly used names: the alkali metals in Group 1, the alkaline earth metals in Group 2, the halogens in Group 17, and the noble gases in Group 18.
Atomic Structure of Elements Within a Group The word halogen is from the Greek words for “salt former” so named because the compounds that halogens form with metals are saltlike. The elements in Group 18 are called noble gases because they are much less reactive than most of the other elements.
Atomic Structure of Elements Within a Group Because the periodic table relates group and period numbers to valence electrons, it’s useful in predicting atomic structure and, therefore, chemical properties.
Atomic Structure of Elements Within a Group For example, oxygen, in Group 16 and Period 2, has six valence electrons (the same as the second digit in the group number), and these electrons are in the second energy level (because oxygen is in the second period). Oxygen has the same number of valence electrons as all the other elements in Group 16 and, therefore, similar chemical properties.
Physical States and Classes of the Elements The color coding in the periodic table on pages 92 and 93 identifies which elements are metals (blue), nonmetals (yellow), and metalloids (green).
Physical States and Classes of the Elements The majority of the elements are metals. They occupy the entire left side and center of the periodic table. Nonmetals occupy the upper-right-hand corner. Metalloids are located along the boundary between metals and nonmetals.
Physical States and Classes of the Elements Each of these classes has characteristic chemical and physical properties, so by knowing whether an element is a metal, nonmetal, or metalloid, you can make predictions about its behavior. Elements are classified as metals, metalloids, or nonmetals on the basis of their physical and chemical properties.
Metals Metals are elements that have luster, conduct heat and electricity, and usually bend without breaking. With the exception of tin, lead, and bismuth, metals have one, two, or three valence electrons. Click box to view movie clip.
Metals All metals except mercury are solids at room temperature; in fact, most have extremely high melting points. The periodic table shows that most of the metals (coded blue) are not main group elements.
Metals The elements in Groups 3 through 12 of the periodic table are called the transition elements. All transition elements are metals. Click box to view movie clip.
Metals Many are commonplace, including chromium (Cr), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), silver (Ag), and gold (Au). Some are less common but still important, such as titanium (Ti), manganese (Mn), and platinum (Pt). Some period 7 transition elements are synthetic and radioactive.
