Ceramics chapter 4.docx - 1 4 Atomic Bonding and Crystal...

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1 4: Atomic Bonding and Crystal Structure 4.1 ELECTRONIC CONFIGURATION OF ATOMS Each Quantum shell is referred to by a principal quantum number n , where n = 1, 2, 3, … The total number of electrons in a shell is 2 n 2 . Thus, the lowest-energy quantum shell ( n = 1) has only 2 electrons, and successively higher energy shells have 8 ( n = 2), 18 ( n = 3), 32 ( n = 4), and so on, electrons, respectively. Although electrons within a quantum shell have similar energy, no two are identical. To distinguish among these electrons, shells are divided into subshells called orbitals , which describe the probability of where pairs of electrons will be within the shell with respect to the nucleus. The first quantum shell has only two electrons, both in the s orbital, with a spherical probability distribution around the nucleus at a radius of approximately 0.5 Å. These two electrons have identical energy, but opposite magnetic behavior or spin. The second shell has eight electrons, two in s orbitals and six in p orbitals. All have higher energy than the two electrons in the first shell and are in orbitals farther from the nucleus. (For instance, the s orbitals of the second shell of lithium have a spherical probability distribution at a radius of about 3 Å.) The p orbitals are not spherical but have slight dumbbell-shaped probability distributions along the orthogonal axes. The length of the extension along the orthogonal axes is exaggerated to emphasize that there is a directional influence that will occur during bonding in contrast to the nondirectional bonding for s orbitals. The third quantum shell has d orbitals and so on. 4.2 BONDING When two or more atoms come together, the probability distribution of electrons can change to yield molecular orbitals (MOs) that are different than the atomic orbitals (AOs). This interaction determines if and how atoms will bond together to result in the thermodynamically stable lowest energy state. This in turn determines the behavior of the material such as chemical stability and electrical, thermal, optical, and mechanical properties. O EXAMPLE 4.4-- The helium atom has two s electrons and thus a full s orbital and a thermodynamically stable low energy state. When two helium atoms approach each other, their AO electron configuration is lower energy than a new combined MO configuration, so no bond results between the atoms. O EXAMPLE 4.5-- Each hydrogen atom has only one s electron. When two hydrogen atoms come together, there are a couple of possibilities, one that is a bonding MO and one that is an antibonding MO . Note that the energy required for bonding is much lower than for antibonding. Therefore, the bonding MO electron distribution results when two hydrogen atoms come together. Each s electron is shared between the two atoms, but neither electron is available for bonding with other hydrogen atoms. Thus, the thermodynamically stable configuration at room temperature and atmospheric pressure is a diatomic molecule H 2 . In a similar fashion, two oxygen atoms bond to form a stable O 2 molecule.
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