Solid State Physics Yuanxu Wang School of Physics and Electronics Henan University 双语教学示范课程

Review of Section 2.1: Electronegativity What are ionization energy and affinity energy? What is the electronegativity? The relation between electronegativity and bonding? 2

§2 Bonding type in crystal 2.1 Covalent bonding When two types of atoms with large electronegativity form crystal, the electrons of outmost orbital will not be broken away from the atoms. Such crystal is called atomic crystal. When two atoms approach, only single electron with different spin can match each other (two atomic orbital is overlapped), concentrating electron cloud in the field between two nucleus. The system energy reduces, and a stable covalent bond is formed. A pair of electrons connecting two atoms are called paired electrons, such pattern is covalent bond. Electrons shared by the neighboring atoms 3

 The saturation of covalent bonding When one atom is combined with others ， there is a maximum of numbers forming a covalent bond ， which is determined by unpaired electron numbers. This property is called the saturation of covalent bonding. For the ns and np outer configuration, the electron number of full filled shell is eight. If the valence electron number N is smaller than half number of full filled shell, N ＜ 4 ， the atoms mostly form N covalent bonds. If the number of valence electron N≥4 ， mostly form （ 8 - N ） covalent bonds, which is called “8-N regulation”. 4

 The directivity of covalent bond When two atoms combine with covalent bonding, they must choose the orientation that electron cloud density is maximal. The more overlapping electron cloud, the more stable covalent bond. This is the physical nature of “the directivity of covalent bond”. Covalent bond makes an electron cloud region appear between two atoms, reducing the repulsion between two nucleus and forming stable structure. Positive electrostatic repulsion between two nucleus, decreasing potential system energy ， which forms a stable structure. Covalent crystal always has a high hardness ， high melting point, little thermal expansion, small electrical conductivity. 5

2.2 Ionic bonding As an element that easily losing electrons and one that easily obtaining combine together, one becomes cation, and the other is anion, and they form an ionic crystal. Ionic crystals always have high hardness, high melting point, little thermal expansion, weak electrical conductivity. Electrons transfers between two atoms to form two oppositely charged ions. Strong electrostatic forces dominate The structures of NaCl 、 CsCl are typical ionic crystal. Electrons transferred to adjacent atoms. The driving force of combining process is the coulomb force between cation and anion. The nearest neighbor of an anion must be cation. 6

2.3 Metal bonding For the first 、 second main groups, and transitional element, as they form crystals, there are metal atoms and metal ion that losing electrons in same lattice due to their little electronegativity. The metal atoms change their electron states at any time. Model for metal bonding: all atoms lose outer layer valence electrons and become atomic core. Atomic cores are immersed in electron cloud of shared electrons. The power of metal crystal combining process is the static coulomb force of atomic kernel and shared electrons. The more compact atoms are, the tighter electron cloud and atomic core are ， the lower coulomb energy. 7

2.3 Metal bond Metal crystal always have good electrical conductivity 、 thermal conductivity 、 malleability. Electrons free to move through sample. No directivity or saturation! 8

2.4 Molecule bond The force for binding molecules is called Van der Waals forces.  Binding of polar molecules Polar molecule has electric dipole moment, two atoms in two molecule approached must be heterosexual, this can reduce system energy.  Binding of polar molecules and non-polar molecule  Binding of non-polar molecules bond Polar molecule first produces electric dipole ， then the induced force electric dipole moment and induced dipole moment combines two molecules. Instantaneous electric dipole moment in low temperature is the reason that non-polar molecules form crystal. 9

2.5 Hydrogen bond The hydrogen combined with large electronegativity atom X (F, Cl, O, N) by covalent bond ， is close with large electronegativity Y, hydrogen can be as the medium between X and Y, producing bond X-H…Y, this is called “hydrogen bonding”. For most of crystals, the crystal binding may be complex, existing several types bonding simultaneously. Such as graphite is mixed type, owning covalent bond, molecule bond, and metal bond. Important Note 10

Summary: Bonding type What are ionization energy and affinity energy? What is electronegativity? The relation between electronegativity and bonding? 11