Representing Free Elements in Chemical Equations

Having classified the elements according to their ground-state electron configurations, we can now learn how chemists represent elements in chemical equations. 

Metals Because metals do not exist in discrete molecular units but rather in complex, three-dimensional networks of atoms, we always use their empirical formulas in chemical equations. The empirical formulas are the same as the symbols that represent the elements. For example, the empirical formula for iron is Fe, the same as the symbol for the element. 

Nonmetals There is no single mle regarding the representation of nonmetals in chemical equa- tibns. Carbon, for example, exists in several allotropic forms. Regardless oi the allotrope, we use its empirical formula C to represent elemental carbon in chemical equations. Often the symbol C will be followed by the specific allotrope in parentheses as in the equation representing the conversion of graphite to diamond, two of carbon s allotropic forms  

For nonmetals that exist as polyatomic molecules, we generally use the molecular formula in equations H2, N2, O2, F2, CI2, Br2,12, and P4, for example. In the case of sulfur, however, we usually use the empirical formula S rather than the molecular formula 85. Thus, instead of writing the equation for the combustion of sulfur as 

Noble Gases All the noble gases exist as isolated atoms, so we use their symbols He, Ne, Ar, Kr, Xe, and Rn. 

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Figure 17.7 Hydrogen exchange kinetics of free ACTR and CBP and their complex. Representative spectra (a) and uptake curves (b) from peptides in CBP and ACTR that become a-helical in the complex. Secondary structure elements shown In diagram indicate a-helical (boxes), loop (lines), and unstructured (dotted lines) that ACTR adopts in complex with CBP [29]. The vertical dashed lines in the spectra denote the centroids of the undeuterated and fully deuterated states. The dashed lines in the uptake curves denote data fitting using Equation 17.20. (c) The kinetic analysis provides estimates of peptide-averaged protection, depicted as different colored bars mapped onto the sequence and secondary structure found in the complex. Adapted from Ref [81] with permission, 2011 American Chemical Society. (See insert for color representation of the figure.)...

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