Data of periodic Table.docx

7/28/2019 Data of periodic Table.docx

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Periodic TableI INTRODUCTION

Periodic Table, table of the chemical elements arranged to illustrate patterns of recurring chemical and

physical properties. Elements, such as oxygen, iron, and gold, are the most basic chemical substances

and cannot be broken down by chemical reactions. All other substances are formed from combinations

of elements. The periodic table provides a means of arranging all the known elements and even those

yet to be discovered.

Although all elements differ from one another, some show similarities that allow scientists to

categorize them. Around 1870, Russian chemist Dmitry Mendeleyev used these similarities to

construct the original periodic table. Since that time, newly discovered elements have been added and

the tables layout has changed; however, the modern periodic table conveys essentially the same

information as the one that Mendeleyev created. In the modern table, elements with similar properties

fall into columns called groups or families. Group 1 of the periodic table, for example, contains a

number of soft metals, all of which react vigorously with water to form hydrogen gas.

II ARRANGEMENT OF THE TABLE


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The elements within the modern periodic table are arranged from left to right, top to bottom, in order

of increasing atomic number. An elements atomic number is the number of protons in its nucleus.

There are 92 naturally occurring elements, ranging from hydrogen, which has atomic number 1, to

uranium, whose atomic number is 92. The periodic table also includes artificially created elements,

whose atomic numbers are higher than 92. These additions must be prepared through nuclear

reactions. The most recent element to be synthesized was an element with 114 protons in the nucleus

of each of its atoms. None of the artificially created elements with atomic numbers higher than 109

have yet been officially named.

Whereas the ordering of the elements is completely determined by their atomic numbers, the

arrangement into vertical columns, called groups, is determined by a number of factors. These factors

include chemical properties, physical properties, and the number of electrons thought to exist in the

outer shells of the elements atoms. (The electrons that surround the nucleus of an atom are arranged

in concentric shells.)

The placement of elements into groups within the periodic table is not completely clear-cut. Some

scientists disagree about minor differences in the placement of elements such as hydrogen and

helium. Helium, which does not react with other elements, is usually placed in group 18, which houses

the noble gases. This group also includes neon, argon, and krypton, all of which are also very

unreactive. Scientists who group the elements based primarily on the number of outer-shell electrons

place helium with elements such as magnesium, calcium, and barium among the alkaline earth metals

of group 2. Elements in group 2 have two electrons in their outermost shell.

The periodic table has been published in various shapes and sizes, but the most commonly used

modern form begins with a column of group 1 metals on the left-hand side, followed by a column of

group 2 alkaline earth metals. These columns are followed by a block of 40 elements divided into ten


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columns of four elements each. The groups in this block, collectively called the transition metals, are

numbered 3 through 12. Groups 13 through 18 make up the right-hand side of the table. A diagonal

dividing line separates the nonmetals in the upper-right portion of this block, such as oxygen, carbon,

and nitrogen, from the metals such as tin and lead in the lower left portion.

There is an additional block of 28 elements, divided into two rows of 14 elements each, that is usually

placed beneath the main table. These are the rare earth elements, whose properties are all

remarkably similar. They are so similar to one another that chemists have difficulty separating them

when they occur together as mixtures. This additional block really belongs between the first block,

consisting of groups 1 and 2, and the transition metal block. For convenience it is placed at the bottom

of the table rather than in its proper place. Otherwise the periodic table would be very wide and would

not lend itself to being represented on wall charts.

Scientists refer to the horizontal rows in the periodic table as periods. Periods vary in length. Moving

through the table from top to bottom, the successive periods contain 2, 8, 8, 18, 18, 32, and 32

elements. These numbers correspond to the maximum number of electrons that can beaccommodated in the largest electron shell in an atom of any element belonging to that period.

III USES OF THE PERIODIC TABLE

The periodic table is an important tool for scientists and students studying the chemical elements. If a

person knows the main properties of each of the groups in the periodic table and how chemical

properties vary within a group, he or she can predict the properties of any particular element with a

reasonable degree of confidence. For example, if a student needs to know the properties of francium,

she can predict that it, like other elements of group 1, will be a soft metal and will react even more

vigorously with water than the elements above it. If another student wants to predict what compounds

tellurium will form in combination with hydrogen, he can guess that the two elements will form H2Te

because the other elements in telluriums group form similar hydrogen compounds: H2O, H2S, and

H2Se.

IV HISTORY


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A number of chemists contributed to the development of the

periodic table during the 19th century; however, Mendeleyev is regarded as the primary discoverer.

His version of the table was the first to accommodate all the known elements and also successfully

predicted the existence of several elements that had not yet been discovered. These undiscovered

elements included gallium, scandium, and germanium.

At first, the ordering of the elements was based on atomic weightthe weight of a single atom of the

element. This method of ordering broke down in a few cases, such as tellurium and iodine. The atomic

weight of tellurium is actually higher than that of iodine, suggesting that tellurium belongs in group 17

while iodine belongs in group 16. Judging by their chemical properties, however, tellurium belongs in

group 16 and iodine in 17. This anomaly was resolved in 1914, when it was discovered that atomic

number provides a better basis than atomic weight for ordering the elements. Tellurium has an atomic

number of 52 and iodines is 53.

Elements with similar properties are placed in the same group of the periodic table, but for many

years it was a mystery why these elements behaved similarly. At the beginning of the 20th century,when theories of physics changed rapidly, an approximate explanation for the repeating patterns

within the periodic table was found. Scientists discovered that the elements within a single group of

the periodic table possess the same number of outer-shell electrons, particles that had not even been

discovered when scientists produced the first periodic tables.


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