Placeholder Image

字幕表 動画を再生する

  • The periodic table is the most powerful tool chemists have for organizing chemical information.

  • Without it, chemistry would be a chaotic, confusing jumble of seemingly random observations.

  • What makes the periodic table really invaluable is its use as a predictive tool. You can predict

  • a lot about the chemical behavior of an element if you know where it is on the periodic table.

  • Each element is represented by one square on the periodic table, with a one or two-letter

  • chemical symbol. Many of the chemical symbols are derived from the English name for the

  • element, but some come from other languages. For example, the symbol for silver is Ag,

  • from the Latin word argentum. The symbol for lead is Pb, from the Latin word plumbum.

  • Above the chemical symbol is the atomic number of the element, and below the symbol are the

  • full name of the element and its atomic mass.

  • Most elements are metals, and you can find them on the left and in the middle of the

  • periodic table. Metallic elements are typically shiny and are good conductors of heat and

  • electricity. Nonmetals are found on the upper right of the periodic table (except for Hydrogen

  • there on the left, it’s also a nonmetal). Nonmetals generally are NOT shiny and are

  • NOT good conductors of heat or electricity. The dividing line between metals and nonmetals

  • on the periodic table is drawn as a thick staircase. The elements that are found on

  • either side of that staircase are often called METALLOIDS, and they have properties that

  • fall between metals and nonmetals.

  • Notice that the atoms are listed in order of increasing atomic number, as you read the

  • periodic table from left to right, top to bottom. Each element has a unique atomic number

  • - that’s the number of protons in the nucleus of the atom. So why are the elements organized

  • into rows and columns? Why don’t we just put the elements in a long list?

  • It turns out, if you arrange elements by atomic number, a pattern emerges. There is a periodicity,

  • or a repeating, of certain characteristics. For example, every so often, an inert gas

  • appears. Right next to it will be an element that reacts violently with water. This periodic

  • repetition is known as the PERIODIC LAW. This is the basis for organizing the elements in

  • the Periodic Table into columns.

  • A vertical column of elements is called a GROUP or a FAMILY. The elements in a group

  • have similar chemical properties. We now know that’s because they have similar valence

  • electron configurations.

  • There are 7 horizontal rows in the periodic table. These rows are called PERIODS. Each

  • row corresponds to a different energy level occupied by electrons.

  • The two groups on the left are the alkali metals and the alkali earth metals. The s

  • orbitals in the outermost shell of the atom are being filled in these groups.

  • On the right is a block of 6 columns. These elements have the outermost p orbitals being

  • filled. Notice on the far right are the Noble gases, which all have a filled valence shell

  • of electrons.

  • In the middle is a block of 10 columns, the transition metals. In these elements, the

  • outermost d orbitals are being filled.

  • The asterisks take you to the bottom of the Periodic Table, where there are two rows of

  • 14 columns, the inner transition metals - also known as the Lanthanides and Actinides. These

  • elements have the outermost f orbitals being filled.

  • The groups in the periodic table have been numbered in a variety of ways over the

  • years. Depending on which periodic table you look at, it may have 1, 2, or even 3 different

  • systems for numbering the groups. You may see the groups labeled with Roman

  • numerals and As and Bs - this system was popular in North America and Europe. Unfortunately,

  • the designations were somewhat arbitrary - in North America, the A groups were the s and

  • p blocks, known as theRepresentative Elements,” and the B groups were the d block, theTransition

  • Metals.” Meanwhile, in Europe, the A groups were on the left, and the B groups were on

  • the right. In both systems, there was one triple-sized group called Roman numeral VIII.

  • To eliminate all this confusion, the International Union of Pure and Applied Chemistry (IUPAC)

  • proposed a system that numbers the groups 1-18, with no As or Bs.

  • This is an example of the sorts of refinements that have changed the Periodic Table gradually

  • over the years, as new discoveries were made and chemists came to agreements about how

  • to present the new information. You may not even recognize the first periodic table - fewer

  • than 70 elements had been discovered in the mid 1800s - they didn’t know about noble

  • gases yet. At that point, there was no agreed upon way to list the elements that was of

  • any help to chemists. For example, listing them in the order of discovery didn’t tell

  • you anything about their chemical behavior, so that kind of list would be useless as any

  • kind of predictive tool. This was the state of affairs when Russian chemist Dmitri Mendeleev

  • developed the Periodic Table.

  • In 1869, Mendeleev came up with the idea of listing the elements in order of increasing

  • ATOMIC MASS. Almost simultaneously, Lothar Meyer in Germany published a nearly identical

  • system for classifying elements. We generally give credit for the discovery to Mendeleev,

  • because he devoted so much time and effort championing this new system and he helped

  • it become widely accepted.

  • Mendeleev insisted that elements with similar properties be listed together, and because

  • of this, there were gaps in his table. Mendeleev boldly proposed the existence of a number

  • of elements that had not yet been found, that would one day fill in these gaps. He named

  • them for their positions in his table. For example, the proposed element eka-aluminum

  • would reside under aluminum, and eka-silica would go under silicon. Some years later,

  • these elements were indeed found, and their characteristics closely matched Mendeleev’s

  • predictions. This was a powerful example of the utility of Mendeleev’s periodic table

  • as a PREDICTIVE tool, something that chemists didn’t have before.

  • In 1913, English physicist Henry Moseley made an important modification to the Periodic

  • Table. Moseley, a member of Ernest Rutherford’s research group, was probing metallic elements

  • with X-rays and measuring the wavelength of the X-ray emissions. He found that each element

  • gave different results. Moseley developed a mathematical relationship between the X

  • ray wavelengths produced by different elements and their atomic number, which increased by

  • 1 for each element. Moseley suggested that the atomic number was more significant for

  • predicting chemical behavior than the atomic mass as had been previously thought.

  • Moseley reorganized the elements in the periodic table, listing them in increasing order of

  • atomic number instead of atomic mass. This resolved some inconsistencies with Mendeleev’s

  • table. For instance, Argon has a greater atomic mass than Potassium, but a lower atomic number.

  • Like Mendeleev, Moseley left gaps in the Periodic Table where he proposed several yet-undiscovered

  • elements should fit. These included atomic numbers 43, 61, 72, and 75.

  • Moseley’s proposed elements and many more have since been discovered. There are 92 naturally

  • occurring elements, and many elements not found in nature have been synthesized. Were

  • running out of room to put the all the new elements! We just might have to enlarge the

  • Periodic Table in the near future.

The periodic table is the most powerful tool chemists have for organizing chemical information.

字幕と単語

ワンタップで英和辞典検索 単語をクリックすると、意味が表示されます

B1 中級

化学のこと。周期表入門 (Chemistry: Introduction to the Periodic Table)

  • 7 0
    林宜悉 に公開 2021 年 01 月 14 日
動画の中の単語