Chapter 2: Atomic Structure

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Cory Parsons
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1 Chapter 2: Atomic Structure Atom: Nucleus: protons and neutrons (neutral in charge) Electrons Electrons and protons are charged: e=1.6x10-19 Mass of protons and neutrons = 1.67x10-27 kg Mass of electron = 9.11x10-31kg We can examine and describe structure of materials at 5 different levels: 1. Macrostructure. 2. Microstructure. 3. Nanostructure. 4. Short- and long-range atomic arrangements. 5. Atomic structure. Microstructure: length-scale of ~10 to 1000 nm. It typically includes such features as average grain size, grain size distribution, grain orientation, and other features related to defects in materials. Macrostructure: length-scale is ~>1000nm. Features include porosity, surface coatings, and internal or external micro-cracks. Amorphous Materials: lack a long-range ordering of atoms or ions. Amorphous materials have only short-range atomic arrangements Crystalline: exhibit periodic geometrical arrangements of atoms or ions. Crystalline materials have short- and long-range arrangements. The atomic Structure of the Atom 2.2 fundamental concepts: Atomic number, Z: number of protons in each atom. Atomic mass A: sum of masses of protons and neutrons. In one mole of a substance there are 6.02 x atoms or molecules. Mass in grams of Avogadro number N A of atoms. The quantity N A = 6.02 x atoms/mol is the number of atoms or molecules in a mole.

2 1.0 amu/atom=1g/mol The number of neutrons for an element may vary. Atoms of elements that have two or more diff atomic masses are called isotopes. Atomic weight: Weighted average of the atomic masses of the atom s isotopes. An alternate unit for atomic mass is the amu, which is 1/12 the atomic mass of carbon electrons in atoms Atomic models Bohr atomic model: Electrons revolve around nucleus in discrete orbitals (F2-1) Energies of electrons are quantized An electron my change energy (quantum jump) See F2-2 Wave-mechanical model Electron exhibit both wave-like and particle-like char Position of electrons is described by a probabilty distribution or electron cloud See F2-3 Quantum numbers Every electron is char by 4 parameters called quantum numbers Size, shape, and spatial orientation of an electron s prob density are specified by 3 of these QNs Shells are specified by a principal QN, n, n=1,2,3,.. or n=k,l,m,n, see T2-1 n is related to the distance of an electron from nucleus or its position. 2 nd QN, l: signifies subshell l=s,p,d,f. l is related to the shape of the electron subshell 3rd QN, ml: number of energy states for each subshell In absense of an external magnetic field, states within each subshell are identical. When a magnetic field is applied these

3 subshell states split, each state assuming slightly diff energy. 4 th QN; associated with each electron is a spin moment. Two values (1/2 and -1/2) F2-4 Electron Configurations Pauli Exclusion principle: each electron state can hold no more than 2 electrons with opposite spins F2-5 : Na: 1s 2 2s 2 2p 6 3s 1 Valence of an atom: number of electrons that participate in bonding or chemical reactions. Usually, valence is the number of electrons in the outermost filled shell. Electro-negativity: tendency of an atom to gain an electron. Atoms with almost completely filled outer energy levels such as cl are strongly electronegative and readily accept electrons. Atoms with nearly empty outer levels such as Na readily gives up electrons and have low electro negativity. High atomic number elements also have low electro-negativity because the outer electrons are at a greater distance from the positive nucleus, so that they are not as strongly attracted to the atom. 2.4 The periodic table Electronegativety: Left to right, bottom to top Binding Energy and Inter-atomic Spacing: Inter-atomic spacing: equilibrium distance between atoms caused by a balance between repulsive and attractive forces. In metallic bond, for example, attraction between electrons and ion cores is balanced by repulsion between ion cores. Equilibrium separation occurs when total inter-atomic energy (IAE) of the pair of atoms is at a min, or when no net force is

4 acting to either attract or repel the atoms. The inter-atomic spacing in a solid metal is approximately equal to twice the atomic radius r. The minimum energy is the binding energy, or the energy required to create or break the bond. Materials having a high binding energy also have a high strength and a high melting temperature. Ionically bonded materials have a particularly large binding energy because of the large difference in electro-negativities between the ions. Metals have lower binding energies because the electronegativities of the atoms are similar. A steep slope, which correlates with a higher binding energy & a higher melting point, means that a greater force is required to stretch the bond; thus, the material has a high modulus of elasticity. Not all properties of engineered materials are microstructure sensitive. Modulus of elasticity is one such property. If we have two aluminum samples that have essentially the same chemical composition but different grain size, we can expect that the modulus of elasticity of these samples will be about the same. The modulus of elasticity can be linked directly to the strength of bonds between atoms. Thus, the modulus of elasticity depends primarily on the atoms that make up the material. However, the yield strength of these samples will be quite different. The yield strength, therefore, is a microstructure sensitive property. Materials that display a steep curve with a deep trough have low linear coefficients of thermal expansion. Another property that can be linked to the binding energy or inter-atomic force-distance curves is the coefficient of thermal expansion (CTE). In order for the atoms to move from their equilibrium separation, energy must be supplied to the material.

5 If a very deep inter-atomic energy (IAE) trough caused by strong atomic bonding is characteristic of the material, the atoms separate to a lesser degree and have a low, linear coefficient of thermal expansion. 3.1Chemical bonding In solids, atoms are held by bonds. They provide strength, electrical & thermal conductivities to solids i.e.: strong bonds results in high melt temp, high E, shorter interatomic distances, lower thermal expansion. Valence electrons affects inter-atomic attractions: o In noble gases, limited interactions with other atoms due to very stable arrangement of 8e in the outer orbital. Other elements achieve stable configuration of 8e in their outer orbital through: 1. Receiving extra electrons. 2. Releasing electrons. 3. Sharing electrons. Atomic Bonding 1. Ionic bond. 2. Covalent bond. 3. Metallic bond. Ionic bonds: Found in many ceramics is produced when an electron is donated from one electropositive atom to an electronegative atom, creating positively charged cations and negatively charged anions. Results from mutual attraction of +ve and ve charges.

6 Na Cl + Na - Cl + - Example: Na + surrounds themselves with as many ve Cl - as possible and Cl - surrounds themselves with max no. of +ve Na ions. Columbic attractions involve all neighbors; ionically bonded materials are very stable. i.e.: to form MgO, 570kJ/mol released, MgO must be raised to around 2700 o C before it overcomes this energy and melt. Mechanically strong and hard, but brittle. Relatively high Melting points. Electrical insulators. E N E E A R A r B n r A r Covalent bonds: A covalent bond is formed between two atoms when each atom donates an electron that is needed in bond formation. F + F F F or F-F B r n

7 In F 2, 2 atoms are held together by a covalent bond of 160kJ/mol. Neither of these 2 atoms develops strong attraction to other F atoms that may approach them. In ionic bonds, columbic attraction bring as many as unlike ions into neighboring positions as space will allow. However, covalent bonds are formed between specific atoms and no. of neighbors is limited by no. of bonds. Found in many polymeric and ceramic materials. These bonds are strong and most inorganic materials with covalent bonds exhibit high levels of strength, hardness, and limited ductility. Most plastic materials based on carbon-carbon (C-C) and carbonhydrogen (C-H) bonds show relatively lower strengths and good levels of ductility. Most covalently bonded materials tend to be relatively good electrical insulators. 0.25( X A X B ) % ionic character 1 e 100 Metallic bonds Typical metals have delocalized electrons that can move in 3D. So it s common to speak of an electron cloud because the outer, least strongly bonded electrons are able to move throughout the metal structure. Is formed as a result of atoms of low electro-negativity elements donating their valence electrons and leading to the formation of a sea of electrons. The positively charged atom cores are bonded by mutual attraction to the negatively charged electrons. The valence electrons in metal are delocalized into an energy band. These electrons are able to move throughout the metal. Valence electrons fill only the bottom half of band. Their average energy is lower than that of the 3s electron with individual atoms. This energy diff provides the metallic bond. 2

8 In brief, energy would have to be supplied to overcome the metallic bond and separate atoms from one another and to re-establish the individual atomic orbital. Metallic bonds are non-directional and relatively strong. As a result, most pure metals show a high Young s modulus and ductility. They are good conductors of heat and electricity and reflect visible light.

Materials Science. Atomic Structures and Bonding

Materials Science. Atomic Structures and Bonding Materials Science Atomic Structures and Bonding 1 Atomic Structure Fundamental concepts Each atom consists of a nucleus composed of protons and neutrons which are encircled by electrons. Protons and electrons