PERIODIC TRENDS AND CHEMICAL REACTIONS

Recall that we can classify elements as metals, metalloids, and nonmetals. oQo (Section 7.6) Except for hydrogen, which is a special case, the nonmetals occupy the upper right portion of the periodic table. This division of elements relates nicely 

2 HYDROGEN The first nonmetal we consider, hydrogen, forms compounds with most other nonmetals and with many metals. 

3 GROUP 8A THE NOBLE GASES Next, we consider the noble gases, the elements of group 8A. which exhibit very limited chemical reactivity. 

4 GROUP 7A THE HALOGENS We then explore the most electronegative elements the halogens, group 7A. 

5 OXYGEN We next consider oxygen, the most abundant element by mass in both Earth s crust and the human body, and the oxide and peroxide compounds it forms. 

Increasing ionization energy Decreasing atomic radius Increasing electronegativity Decreasing metallic character 

The chemistry exhibited by the first member of a nonmetal group can differ from that of subsequent members. For example, nonmetals in period 3 and below can accommodate a larger number of bonded neighbors. (Section 8.7) Another important difference is that the first element in any group can more readily form tt bonds. This trend is due, in part, to size because small atoms are able to approach each other more closely. As a result, the overlap of p orbitals, which results in the formation of tt bonds, is more effective for the first element in each group (T FIGURE 22.2). More effective overlap means stronger tt bonds, reflected in bond enthalpies. (Section 8.8) For example, the difference between the enthalpies of the C—C bond and the C=C bond is about 270 kj/mol  

As we shall see, tt bonds are particularly important in the chemistry of carbon, nitrogen, and oxygen, each the first member in its group. The heavier elements in these groups have a tendency to form only single bonds. 

Of the elements Li, K, N, P, and Ne, which (a) is the most electronegative, (b) has the greatest metallic character, (c) can bond to more than four atoms in a molecule, (d) forms tt bonds most readily  

Analyze We are given a bst of elements and asked to predict several properties that can be related to periodic trends. 

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PERIODIC TRENDS AND CHEMICAL REACTIONS We begin with a review of periodic trends and types of chemicai reactions, which will help us focus on general patterns of behavior as we examine each family in the periodic table. 

General Concepts Periodic Trends and Chemical Reactions... 

Understanding the wealth of information found in the organization of the periodic table is a central skill for general chemistry. You will always have a periodic table available for ACS exams, and likely for most classroom tests as well. Therefore, knowing the trends within the periodic table will allow prediction of properties, even for unfamiliar elements. Relative sizes of atoms and ions, trends in ionization energy, and trends in electronegativity are all important to understanding the behavior of elements. The differences between metals and nonmetals and their reactions are also based on periodic trends. Trends within families and trends within periods can both reveal much about the physical properties and chemical reactions expected for the elements. 

Figure 1.5.1. A periodic tabie showing the trends in eiectronegativity. Elements tend to become more electronegative toward the right and to the top of the periodic table. Fluorine is the most electronegative element. The noble gases are not assigned an electronegativity because they do not normally enter into chemical reactions.

Oxidation numbers are assigned to elements to name inorganic compounds, to keep track of electrons in electron transfer (oxidation-reduction) reactions, and to explore trends in chemical reactivity across the periodic table. 

The periodic table has been described as the chemist s best friend. Chemical reactions involve loss, gain, or sharing of electrons. In this chapter, we have seen that the fundamental basis of the periodic table is that it reflects similarities and trends in electron configurations. It is easy to use the periodic table to determine many important aspects of electron configurations of atoms. Practice until you can use the periodic table with confidence to answer many questions about electron configurations. As we continue our study, we will learn many other useful ways to interpret the periodic table. We should always keep in mind that the many trends in chemical and physical properties that we correlate with the periodic table are ultimately based on the trends in electron configurations. 

We first discuss the overall chemical process predicted, followed by a discussion of reaction mechanisms. Under the simulation conditions, the HMX was in a highly reactive dense fluid phase. There are important differences between the dense fluid (supercritical) phase and the solid phase, which is stable at standard conditions. Namely, the dense fluid phase cannot accommodate long-lived voids, bubbles, or other static defects, since it has no surface tension. Instead numerous fluctuations in the local environment occur within a timescale of 10s of femtoseconds. The fast reactivity of the dense fluid phase and the short spatial coherence length make it well suited for molecular dynamics study with a finite system for a limited period of time. Under the simulation conditions chemical reactions occurred within 50 fs. Stable molecular species were formed in less than a picosecond. We report the results of the simulation for up to 55 picoseconds. Figs. 11 (a-d) display the product formation of H2O, N2, CO2 and CO, respectively. The concentration, C(t), is represented by the actual number of product molecules formed at the corresponding time (. Each point on the graphs (open circles) represents a 250 fs averaged interval. The number of the molecules in the simulation was sufficient to capture clear trends in the chemical composition of the species studied. These concentrations were in turn fit to an expression of the form C(/) = C(l- e ), where C is the equilibrium concentration and b is the effective rate constant. From this fit to the data, we estimate effective reaction rates for the formation of H2O, N2, CO2, and CO to be 0.48, 0.08,0.05, and 0.11 ps, respectively. 

During the Stone Age, the material research was limited to the mechanical treatment of natural products. When Dalton discovered atomicity and Mendeleev revealed the periodic table, the research trends drastically changed in the intervening period and research was focussed on fundamental principles of basic molecular structure and simple chemical reactions. During the late twentieth century and the early twenty-first century, an exciting revolution in chemistry has taken place, with multidisciplinary approaches in nanoscience and nanotechnology to the creation of molecules with pre-specified complex structures to perform novel functions, hi the present century, research is focussed on control of crystal structures, nanostractures and microstructures with distinct mechanical, electrical, optical and magnetic properties [1-5]. 

The reaction of H2O and Li left) to produce LiOH and H2(g) is much slower than the analogous reaction between H2O and Na right). The higher reactivity of sodium reiative to lithium is one of the many predictions and trends from chemical periodicity discussed in this chapter. 

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