- Explain early attempts at organizing chemical elements
- Understand how Mendeleev organized his periodic table
- Know the improvements that Moseley made on Mendeleev’s table
- Know the periodic law
- Describe components of the modern periodic table: periods, groups, metals, nonmetals, and metalloids
- periodic law
- periodic table
Early Attempts to Organize Elements
By the year 1700, only a handful of elements had been identified and isolated. Several of these, such as copper and lead, had been known since ancient times. As scientific methods improved, the rate of discovery dramatically increased (Figure below).
Dates of discovery of the chemical elements.
With the ever-increasing number of elements, chemists recognized that there may be some kind of systematic way to organize the elements. The question was: how?
A logical way to begin to group elements together was by their chemical properties. In other words, putting elements in separate groups based on how they reacted with other elements. In 1829, a German chemist, Johann Dobereiner (1780-1849), placed various groups of three elements into groups called triads. One such triad was lithium, sodium, and potassium. Triads were based on both physical as well as chemical properties. Dobereiner found that the atomic masses of these three elements, as well as other triads, formed a pattern. When the atomic masses of lithium and potassium were averaged together (6.94 + 39.10)/2 = 23.02, it was approximately equal to the atomic mass of sodium (22.99). These three elements also displayed similar chemical reactions, such as vigorously reacting with the members of another triad: chlorine, bromine, and iodine (Figure below).
The elements chlorine, bromine, and iodine: Chlorine is a greenish-yellow gas. Bromine is a dark orange liquid. Iodine is a shiny blue-black solid. Though different in appearance, they have very similar chemical properties.
While Dobereiner’s system would pave the way for future ideas, a limitation of the triad system was that not all of the known elements could be classified in this way.
English chemist John Newlands (1838-1898) ordered the elements in increasing order of atomic mass and noticed that every eighth element exhibited similar properties. He called this relationship the Law of Octaves. Unfortunately, there were some elements that were missing and the law did not seem to hold for elements that were heavier than calcium. Newlands’s work was largely ignored and even ridiculed by the scientific community in his day. It was not until years later that another more extensive periodic table effort would gain much greater acceptance and the pioneering work of John Newlands would be appreciated.
Mendeleev’s Periodic Table
In 1869, Russian chemist and teacher Dmitri Mendeleev (1836-1907) published a periodic table of the elements. The following year, German chemist Lothar Meyer independently published a very similar table. Mendeleev is generally given more credit than Meyer because his table was published first and because of several key insights that he made regarding the table.
Mendeleev was writing a chemistry textbook for his students and wanted to organize all of the known elements at that time according to their chemical properties. He famously organized the information for each element on to separate note cards that were then easy to rearrange as needed. He discovered that when he placed them in order of increasing atomic mass, certain similarities in chemical behavior repeated at regular intervals. This type of a repeating pattern is called periodic. A pendulum that swings back and forth in a given time interval is periodic, as is the movement of the moon around the Earth. Figure below shows an early version of Mendeleev’s table.
Mendeleev’s first published periodic table shows elements arranged in vertical columns according to increasing atomic mass. The atomic mass is the number which follows each symbol. Elements with question marks were unknown at the time, but were discovered at a later date.
In this table, atomic mass increases from top to bottom of vertical columns, with successive columns going left to right. As a result, elements that are in the same horizontal row are groups of elements that were known to exhibit similar chemical properties. One of Mendeleev’s insights is illustrated by the elements tellurium (Te) and iodine (I). Notice that tellurium is listed before iodine even though its atomic mass is higher. Mendeleev reversed the order because he knew that the properties of iodine were much more similar to those of fluorine (F), chlorine (Cl), and bromine (Br) than they were to oxygen (O), sulfur (S), and selenium (Se). He simply assumed that there was an error in the determination of one or both of the atomic masses. As we will see shortly, this turned out not to be the case, but Mendeleev was indeed correct to group these two elements as he did.
Notice that there are several places in the table that have no chemical symbol, but are instead labeled with a question mark. Between zinc (Zn) and arsenic (As) are two such missing elements. Mendeleev believed that elements with atomic masses of 68 and 70 would eventually be discovered and that they would fit chemically into each of those spaces. Listed in Table below are other properties that Mendeleev predicted for the first of these two missing elements, which he called “eka-aluminum,” compared with the element gallium.
Mendeleev's Predictions for Eka-Aluminum
Formula of oxide
The element gallium was discovered four years after the publication of Mendeleev’s table, and its properties matched up remarkably well with eka-aluminum, fitting into the table exactly where he had predicted. This was also the case with the element that followed gallium, which was named eventually named germanium.
Mendeleev’s periodic table gained wide acceptance with the scientific community and earned him credit as the discoverer of the periodic law. Element number 101, synthesized in 1955, is named mendelevium after the founder of the periodic table. It would, however, be several years after Mendeleev died before the several discrepancies with the atomic masses could be explained and before the reasons behind the repetition of chemical properties could be fully explained.
A wax sculpture of Mendeleev.
Interactive periodic table websites:
The Periodic Law
Recall that Rutherford’s experiments which resulted in the discovery of the nucleus occurred in 1911, long after Mendeleev’s periodic table was developed. Just two years later, in 1913, English physicist Henry Moseley (1887-1915) examined x-ray spectra of a number of chemical elements. His results led to the definition of atomic number as the number of protons contained in the nucleus of each atom. He then realized that the elements of the periodic table should be arranged in order of increasing atomic number rather than increasing atomic mass.
When ordered by atomic number, the discrepancies within Mendeleev’s table disappeared. Tellurium has an atomic number of 52, while iodine has an atomic number of 53. So even though tellurium does indeed have a greater atomic mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are credited with being most responsible for the modern periodic law: When elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties. The result is the periodic table as we know it today (Figure below). Each new horizontal row of the periodic table corresponds to the beginning of a new period because a new principal energy level is being filled with electrons. Elements with similar chemical properties appear at regular intervals, within the vertical columns called groups.
The periodic table of the elements.
The Modern Periodic Table
The periodic table has undergone extensive changes in the time since it was originally developed by Mendeleev and Moseley. Many new elements have been discovered, while others have been artificially synthesized. Each fits properly into a group of elements with similar properties. The periodic table is an arrangement of the elements in order of their atomic numbers so that elements with similar properties appear in the same vertical column or group.
Figure above shows the most commonly used form of the periodic table. Each square shows the chemical symbol of the element along with its name. Notice that several of the symbols seem to be unrelated to the name of the element: Fe for iron, Pb for lead, etc. Most of these are the elements that have been known since ancient times and have symbols based on their Latin names. The atomic number of each element is written above the symbol. Each square on this version of the periodic table also shows the average atomic mass of the element.
A period is a horizontal row of the periodic table. There are seven periods in the periodic table, with each one beginning at the far left. A new period begins when a new principal energy level begins filling with electrons. Period 1 has only two elements (hydrogen and helium), while periods 2 and 3 have 8 elements. Periods 4 and 5 have 18 elements. Periods 6 and 7 have 32 elements because the two bottom rows that are separated from the rest of the table belong to those periods. They are pulled out in order to make the table itself fit more easily onto a single page.
A group is a vertical column of the periodic table. There are a total of 18 groups. There are two different numbering systems that are commonly used to designate groups and you should be familiar with both. The traditional system used in the United States involves the use of the letters A and B. The first two groups are 1A and 2A, while the last six groups are 3A through 8A. The middle groups use B in their titles. Unfortunately, there was a slightly different system in place in Europe. To eliminate confusion the International Union of Pure and Applied Chemistry (IUPAC) decided that the official system for numbering groups would be a simple 1 through 18 from left to right. Many periodic tables show both systems simultaneously
Metals, Nonmetals, and Metalloids
Elements can be classified in a number of different ways. Classifying by period and/or group is important because it is based on their electron configuration. Another way is to classify elements based on physical properties. Three broad classes of elements are metals, nonmetals, and metalloids.
A metal is an element that is a good conductor of heat and electricity. Metals are also malleable, which means that they can be hammered into very thin sheets without breaking. They are ductile, which means that they can be drawn into wires. When a fresh surface of any metal is exposed, it will be very shiny because it reflects light well. This is called luster. All metals are solid at room temperature with the exception of mercury (Hg), which is a liquid. Melting points of metals display a very wide variance. The melting point of mercury is −39°C, while the highest melting metal is tungsten (W), with a melting point of 3422°C. On the periodic table of Figure above, the metals are blue and are located to the left of the bold stair-step line. About 80 percent of the elements are metals (see examples in Figure below).
The elements mercury, gold, and copper show the properties of metals. Mercury (left) is the only liquid metal, but has high luster. Gold (middle) is malleable and can be formed into very thin sheets called gold leaf. Copper is very ductile and a good conductor. Copper (right) is used extensively in electrical wiring.
A nonmetal is an element that is generally a poor conductor of heat and electricity. Most properties of nonmetals are the opposite of metals. There is a wider variation in properties among the nonmetals than among the metals, as seen in Figure below. Nonmetals exist in all three states of matter. The majority are gases, such as nitrogen and oxygen. Bromine is a liquid. A few are solids, such as carbon and sulfur. In the solid state, nonmetals are brittle, meaning that they will shatter if struck with a hammer. The solids are not lustrous. Melting points are generally much lower than those of metals. On the periodic table, the nonmetals are the green squares and are located to the right of the stair-step line.
Nonmetals have properties that are unlike those of metals. Sulfur (left) is brittle and its distinctive yellow color lacks luster. Bromine (center) is the only liquid nonmetal and must be carefully handled due to its toxicity. Helium (right), a colorless and unreactive gas, is lighter than air and thus is used in blimps.
A metalloid is an element that has properties that are intermediate between those of metals and nonmetals. Silicon is a typical metalloid (Figure below). It has luster like a metal, but is brittle like a nonmetal. Silicon is used extensively in computer chips and other electronics because its electrical conductivity is in between that of a metal and a nonmetal. Metalloids can also be called semimetals. On the periodic table, the elements colored red, which generally border the stair-step line, are considered to be metalloids. Notice that aluminum borders the line, but it is considered to be a metal since all of its properties are like those of metals.
Silicon is an example of a metalloid, having some properties of a metal and some of a nonmetal.
Explore some interactive periodic tables at:
Watch this Khan Academy video on Groups of the Periodic Table (11:51):
- Mendeleev arranged chemical elements into a periodic table by arranging them in order of increasing atomic mass.
- According to the periodic law, physical and chemical properties of elements are periodic functions of their atomic numbers.
- In the modern periodic table, elements are arranged into periods and groups. Elements in the same group have similar properties.
- Elements can be classified into the categories of metals, nonmetals, or metalloids.
Lesson Review Questions
- Compare Dobereiner’s triads to the modern periodic table. What aspects of his idea agreed with the modern table and what aspects did not?
- Why did Mendeleev leave empty spaces in his periodic table?
- Explain the significance of the discovery of gallium to Mendeleev’s periodic table.
- What discovery occurred after Mendeleev’s periodic table was published that allowed Moseley to improve it?
- Identify each physical property as more characteristic of a metal or of a nonmetal.
- lower melting points
- good conductor of electric current
- Use Figure above to answer the following.
- How many elements were known by 1700?
- How many elements were discovered between 1735 and 1843?
- The atomic mass of beryllium (Be) is 9.01 amu. The atomic mass of calcium (Ca), which is in the same group as beryllium, is 40.08 amu. What would you predict to be the atomic mass of magnesium (Mg), which is also in the same group but is between Be and Ca?
- Use the periodic table to put the following elements into pairs that would be expected to have similar chemical properties: Ba, S, Br, Ca, K, F, Se, Na
- Identify the elements below as a metal, nonmetal, or metalloid.
- Write the name and symbol of the element located in each position on the periodic table.
- period 2 and group 16
- period 5 and group 9
- period 4 and group 5A
- period 6 and group 12
Further Reading / Supplemental Links
Interactive web sites:
Tutorial and video: