Chemistry

Organization of the Periodic Table and its Applications



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The periodic table is an organization of all the known elements (including those only synthesized in a lab). Each box on the periodic table represents an element and includes the chemical symbol, atomic number, and average atomic mass. The chemical symbol is basically an abbreviation of the name of the element. However, some of the chemical symbols refer to the Latin name and are therefore not intuitively obvious. For example, the chemical symbol for iron is Fe, from the Latin word ferrum. The atomic number is simply the number of protons in one atom of the element. It is important because it is the number of protons that identifies an atom as a certain element. This is because the number of electrons and neutrons can change. The average atomic mass is an average of the masses of all of the isotopes of an element, taking into account the amount of each isotope that is typically present in nature. In other words, the atomic mass of an element is the sum of the atomic masses of each isotope multiplied by its relative abundance.

 GROUPS

The groups on the periodic table are the vertical columns. The elements in each group, with the exception of the transition metals (groups 3-12) have the same valence electron configuration (for help with electron configurations, see the next section), and thus similar properties. Group one contains the alkali metals which have ns1 valence configurations. This means that their oxidation state is typically 1+. (note: Hydrogen is not an alkali metal, although it does have the same electron configuration and oxidation state) Group two contains the alkaline earth metals which have valence configurations of ns2. They typically have a 2+ oxidation state. Groups 13 through 16 are named for the first element in the column and therefore are the Boron, Carbon, Nitrogen, and Oxygen groups respectively. The oxidation states for these groups are usually 3+, 4+, 3-, and 2- however, there is much more variation of oxidation state in these groups, so those numbers are certainly not the rule. The elements of group 17 are called the Halogens and their oxidation states are 1-. The last group, group 18, contains the noble gases, sometimes called the inert gases for their low reactivity. These elements tend not to form compounds because they have completed valence shells (ns2np6).

 PERIODS AND ELECTRON CONFIGURATIONS

The horizontal rows on a periodic table are called the periods. When determining the electron configuration of an element, each period represents an electron shell (principle quantum number n). The periodic table is then divided into blocks, which designate the subshells (angular momentum quantum number l). The subshells are named by letters: s, p, d, and f. The s-block on the periodic table includes groups 1 and 2, as well has Helium. The p-block includes groups 13 through 16. The d-block includes groups 3 through 12. Lastly, the f-block includes the lanthanide and actinide series.

 To determine the electron configuration for an element, all one has to do is look at the periodic table. For example, to determine the electron configuration of Aluminum, element 13, begin in the upper left of the periodic table (Hydrogen) and list each shell and subshell with a superscript for the number of electrons it includes (due to formatting issues, the superscript is not present in this article- the number following each letter should be superscript). Each subshell holds an electron for each group on the periodic table; so the s-block, which is two groups wide, holds two electrons. Getting back to the aluminum example, the configuration obtained by tracing the periodic table would be 1s2 2s2 2p6 3s2 3p1. Notice that since Aluminum is the first element in the p block of the third shell, the superscript number is a one. To check your work, add up all of the superscript numbers; they should add up to the number of electrons in an aluminum atom in its ground state (13).

There are, however, a few tricky areas. The principle quantum number (shell) for the d-block is offset by one. For example, although the first row of transition metals is in the 4th period, they actually hold the 3d designation. Similarly, the f-block is offset by two. Thus the actinide series holds the designation 4f, and the lanthanide series holds the designation 5f. In addition, there are several elements whose observed configurations do not match the one suggested by the periodic table (Copper is an example).

 PERIODIC TRENDS

The arrangement of the elements in the periodic table allows for the identification of trends such as atomic radius, ionization energy, and electronegativity.

 Atomic radius decreases as one traces from left to right across the periodic table. This is because no new electron shells are added, yet protons are added to the nucleus. The additional positive charge in the nucleus pulls the electron shells closer. As one moves down the columns of the periodic table, atomic radius increases because electrons shells are added.

 Ionization energy (the energy required to remove one electron) increases as one traces from left to right across the table because no new electron shells are added, yet protons are added to the nucleus, increasing its pull. Thus it takes more energy to remove an electron. As one moves down the columns of the periodic table, the ionization energy decreases because more electron shells are added. This means that the electron to be removed is not only farther away from the positive nucleus, but also shielded from the pull of the nucleus by the lower shells of electrons, and is therefore easier to remove. *notice that ionization energy follows the opposite trend of atomic radius*

 Electronegativity (the amount an atom attracts electrons) increases as one moves from left to right across the periodic table for the same reason that ionization energy increases: that is, the pull of the nucleus increases with the addition of protons, yet no new electrons shells are added. Moving down the columns, electronegativity decreases, also for the same reason ionization energy decreases. The addition of electron shells causes shielding and increases distance from the positive nucleus.

 METALS, NONMETALS, AND METALLOIDS

The location of metals, nonmetals, and metalloids on the periodic table can be observed in relation to the stair step line. The stair step line is a line that zigzags through the elements of Boron (B), Silicon (Si), Germanium (Ge), Arsenic (Ar), Antimony (Sb), Tellurium (Te), and Polonium (Po). The elements located on the stair step line (those just listed) are considered metalloids. All elements to the left of the line, including the lanthanide and actinide series, are considered metals. The elements to the right of the line are nonmetals. Hydrogen is the exception to this rule because it is located to the left of the line but is a nonmetal.

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