The Periodic Properties of Elements

How do the pumps and channels differentiate between sodium and potassium ions to selectively move one out of the cell and the other into the cell To answer this question, we must examine the ions more closely. Both ions are cations of group lA metals. All group 1A 

In order for a nerve cell to transmit a signal, sodium and potassium ions must flow in opposite directions through specific ion channels in the cell membrane. 

The relative size of sodium and potassium ions is an example of a periodic property one that is predictable based on an element s position within the periodic table. In this chapter, we examine several periodic properties of elements, including atomic radius, ionization energy, and electron affinity. We will see that these properties, as well as the overall arrangement of the periodic table, are explained by quantum-mechanical theory, which we examined in Chapter 7. The arrangement of elements in the periodic table— originally based on similarities in the properties of the elements— reflects how electrons fill quantum-mechanical orbitals. 

A Dmitri Mendeleev is credited with the arrangement of the periodic table. 

A FIGURE 8.1 Eka-aluminum and Eka-silicon Mendeleev s arrangement of elements in the periodic table allowed him to predict the existence of these elements, now known as gaUimn and germanium, and anticipate their properties. 

Why do some elements react more dramatically than others If we drop a piece of gold metal into water, nothing happens. Dropping lithium metal into water initiates a slow reaction in which bubbles gradually form on the surface of the metal. By contrast, potassium metal reacts suddenly and violently with water, as shown here. Why do lithium and potassium react so differently with water, even though they are from the same family of the periodic table To understand such differences we will examine how some key atomic properties change systematically as we traverse the periodic table. 

As we saw in Chapter 6, the periodic nature of the periodic table arises from repeating patterns in the electron configurations of the elements. Elements in the same column contain the same number of electrons in their valence orbitals—the occupied orbitals that hold the electrons involved in bonding. For example, O ([He]2s 2p ) and S ([NejSs Sp ) are both in group 6A. The similarity of the electron distribution in their valence s and p orbitals leads to similarities in the properties of these two elements. 

Electron configurations can be used to explain differences as well as similarities in the properties of elements. Despite similarities in their electron distributions, elemental oxygen and sulfur differ in fundamental ways. For example, at room temperature oxygen is a colorless gas but sulfur is a yellow solid. Might we explain these physical differences in noting that the outermost electrons of O are in the second shell, whereas those of S are in the third shell We will see that even though elements share some similarities for being in the same column of the periodic table, there can be differences because elements are in different rows of the table. 

We begin our discussion with a brief history of the discovery of the periodic tabie. 

2 EFFECTIVE NUCLEAR CHARGE We begin exploring many of the properties of atoms by examining the net attraction of 

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A detailed discussion of redox reactions must wait until Chapter 19, after we explore the nature of the atom, periodic properties of the elements, and thermodynamics. For now, we focus on only a few types of redox reactions that are common and relatively simple. 

One of the many periodic properties of the elements that can be explained by electron configurations is size, or atomic radius. You might wonder, though, how we can talk about a definite "size" for an atom, having said in Section 5.8 that the electron clouds around atoms have no specific boundaries. What s usually done is to define an atom s radius as being half the distance between the nuclei of two identical atoms when they are bonded together. In the Cl2 molecule, for example, the distance between the two chlorine nuclei is 198 pm in diamond (elemental carbon), the distance between two carbon nuclei is 154 pm. Thus, we say that the atomic radius of chlorine is half the Cl-Cl distance, or 99 pm, and the atomic radius of carbon is half the C-C distance, or 77 pm. 

What were the periodic properties of the elements upon which Mendeleev and Meyer established their periodic table of the elements ... 

Several other elements seemed out of order. For example, their atomic masses placed iodine (1) before tellurium (Te), but their chemical properties required the opposite order. Mendeleyev concluded that the atomic masses must have been determined incorrectly and put these two elements in positions reflecting their properties. We now know that the periodic properties of the elements are based on their atomic numbers, not their atomic masses, which explains Mendeleyev s difficulty with the placement of certain elements. 

Sargent-Welch Scientific Company 1980 Table of periodic properties of the elements Skokie, Illinois, pp.l. 

So far in this book we have covered the major principles and explored the most important models of chemistry. In particular, we have seen that the chemical properties of the elements can be explained very successfully by the quan- turn mechanical model of the atom. In fact, the most convincing evidence of that model s validity is its ability to relate the observed periodic properties of the elements to the number of valence electrons in their atoms. 

Electron affinity is another periodic property of the elements. Research and write a report on what electron affinity is and describe its group and period trends. 

In previous chapters, you learned about atomic structure, electron arrangement, and periodic properties of the elements. The elements within a group on the periodic table have similar properties. Many of these properties are due to the number of valence electrons. These same electrons are involved in the formation of chemical bonds between two atoms. 

Mendeleev s powerful insight. For many chemists, the periodic table was the last theoretical tool they needed, since the table made clear the framework of matter. There would be much more work done refining and adding data to the table over the next century, but the basic principles were set. The discovery of the missing elements and the addition of the noble gases confirmed the truth of the periodic law and the utility of the table. John Newlands, whose work had identified many of the periodic properties of the elements, was eventually awarded the Davy Medal by the Royal Society in 1887, and, in 1998, the Royal Society of Chemistry unveiled a plaque at his birthplace acknowledging his discovery of the periodic law. 

More About the Periodic Table Periodic Properties of the Elements... 

Table of Periodic Properties of the Elements" pub. by Sargent-Welch Scientific Company, company, 1978. 

Sodium and oxygen combine to form sodium oxide, which has the formula Na20. Use the periodic table to predict the formulas of the oxides of potassium, rubidium, and cesium. What periodic property of the elements are you using ... 

Across a period, the elements become less metallic and more nonmetallic with corresponding changes in chemical properties. The arrangement of the elements in the periodic table makes it easier to see trends in their properties within groups and across periods. Two important properties of elements are the size of their atoms and the ease (or lack of ease) with which they lose an electron. Both are functions of the periodic similarities of electronic configuration, causing both size and ease of electron loss to be periodic properties of the elements. 

Even in the twenty-first century, although historians recognize that others, especially Meyer, should be given considerable credit for the discovery of the periodic properties of the elements, most textbooks credit only Mendeleev. SEE ALSO Bunsen, Robert Mendeleev, Dimitri Periodic Table. 

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