1. ENG2000: R.I. Hornsey Atom: 1 ENG2000 Chapter 2 Atoms and Bonding

2. ENG2000: R.I. Hornsey Atom: 2 Overview Atomic structure  fundamentals  electrons and atoms Atomic bonding  bonding mechanisms and forces  bonding types  molecule Atomic bonding is determined by the electronic configurations of the atoms Atomic bonding determines all the fundamental physical and electronic, magnetic, optical etc properties

3. ENG2000: R.I. Hornsey Atom: 3 Atoms For our purposes, atoms are made from three fundamental particles  proton (charge = +q, m = 1.67 x 10 -27 kg)  neutron (charge - 0, m = 1.67 x 10 -27 kg)  electron (charge = -q, m = 9.1 X 10 -31 kg)  q = 1.6 x 10 -19 C An element is defined by its atomic number, Z  Z = number of protons in the atomic nucleus  1 (H) ≤ Z ≤ 92 (U) for naturally occurring elements The atomic mass (A) is the sum of proton and neutron masses in the nucleus  # neutrons (N) can vary to give different isotopes of the same element

4. ENG2000: R.I. Hornsey Atom: 4 Masses The atomic weight (really a mass) is typically given in units of grams per mole (g/mol.)  1 mole of a substance contains 6.023 x 10 23 particles – Avogadro’s Number  e.g. iron has an atomic weight of A = 55.85 g/mol. Where several isotopes of a substance are present, the atomic weight is calculated from the appropriate fractions of the weights of the individual isotopes

5. ENG2000: R.I. Hornsey Atom: 5 Bohr Atom In the early years of the 20th century, atomic spectroscopy indicated that electron energies are quantised  the Bohr planetary model of the atom is an early attempt to visualise a system that would yield quantised energies  it is incomplete because it does not explain why the orbiting electrons do not emit electromagnetic radiation http://www.marxists.org/reference/subject/philosophy/images/bohr.jpg http://csep10.phys.utk.edu/astr162/lect/light/bohrframe/bohr2.gif nucleus orbiting electrons

6. ENG2000: R.I. Hornsey Atom: 6 Energy levels These are the first three energy levels for an isolated H atom 1eV = 1.6 x 10 -19 J  the energy gained by an electron accelerated through a potential difference of 1V To move between energy levels requires a ‘quantum jump’ More refined measurements showed that each ‘n’ level was in fact composed of several discrete energies Better models needed 0 -1.5eV -3.4eV -13.6eV potential energy n=3 n=2 n=1

7. ENG2000: R.I. Hornsey Atom: 7 Other energy levels Due to electrostatic (and other) interactions between electrons, each primary energy is in fact several closely spaced levels These are named s, p, d, f  after the shapes of the spectroscopic lines in the early experiments  sharp, principal, diffuse, fine Energy levels are identified by four quantum numbers 0 -1.5eV -3.4eV -13.6eV potential energy n=3 n=2 n=1 1s 2s 2p 3s 3p 3d

8. ENG2000: R.I. Hornsey Atom: 8 Wave mechanics Numerous pieces of evidence suggest that all particles can be thought of as both particles and waves  interference effects – quintessentially wave-like phenomena – can be seen with electrons  quantum-mechanical tunnelling (see later)  called wave-particle duality A particle’s wavelength is calculated from the de Broglie formula (1924)  where h is Planck’s constant (1901); h = 6.62 x 10 -34 Js  m is the mass, v is the velocity

9. ENG2000: R.I. Hornsey Atom: 9 Callister The spatial properties of the wave (x, y, t, intensity) are closely related to the probability of finding the particle at a particular location  the important part here is that the wave mechanical nature of an electron implies that we do not know the precise position  only a probability function giving the likelihood of an electron’s position

10. ENG2000: R.I. Hornsey Atom: 10 Quantum numbers Principal quantum numbers are n = 1, 2, 3, 4 …  they correspond to energy shells K, L, M, N, … Second quantum number, l, is [s, p, d, f]  related to the spatial shape of the energy level  the number of sub-shells is limited to the ‘n’ for the level A third number, m l (the magnetic quantum number), describes the number of available energy states per sub-shell  1 for s, 3 for p, 5 for d, 7 for f  the energies of these states are identical in the absence of a magnetic field, but split when a field is applied The last quantum number is the spin, m s  m s = ± 1/2

11. ENG2000: R.I. Hornsey Atom: 11 Planetary picture Very, very roughly these for quantum numbers can be visualised in terms of a planetary orbit  n corresponds to the radius of the orbit  l corresponds to the shape of the orbit  m l corresponds to the tilt (or inclination) of the orbit  m s represents the two directions the ‘planet’ can spin mlml msms n l

12. ENG2000: R.I. Hornsey Atom: 12 Maximum number of states nsub-shell# statesmax # electrons sub-shell*shell 1K s12 2 2L s12 8 p36 3M s12 18 p36 d510 4N s12 32 p36 d510 f714 * # states x 2, because two electrons (with ± spin) can exist in each state

13. ENG2000: R.I. Hornsey Atom: 13 Notation The conventional notation is: n [s,p,d,f] #  where # is the number of available states that actually contain electrons For example:  H = 1s 1  He = 1s 2  Li = 1s 2 2s 1  Be = 1s 2 2s 2  B = 1s 2 2s 2 2p 1  Ne = 1s 2 2s 2 2p 6  Na = 1s 2 2s 2 2p 6 3s 1  Al = 1s 2 2s 2 2p 6 3s 2 3p 1

14. ENG2000: R.I. Hornsey Atom: 14 Filling the energy levels Electrons occupy the lowest energy state available  note that e.g. 4s < 3d, so fills first http://www.webelements.com/webelements/elements/media/e-config/H.gif http://www.chemtutor.com/scheme.gif

15. ENG2000: R.I. Hornsey Atom: 15 Valence electrons The number of electrons occupying the outermost shell of an atom – the valence electrons – is important for determining the chemical properties of the atom  because these electrons will be involved with the bonding of atoms Atoms with one electron too many (e.g. Na) or one too few (e.g. F) are highly reactive Atoms with full shells (e.g. Ne, Ar) tend to be inert

16. ENG2000: R.I. Hornsey Atom: 16 Periodic table The periodic table of the elements was originally drawn up according to the chemical properties of the elements  as we have seen these properties are closely related to the atomic electron configurations  the seven horizontal rows are called ‘periods’  chemical properties vary from one end of the period to the other  each column – a ‘group’ – displays similar chemical properties and similar valence structures

17. ENG2000: R.I. Hornsey Atom: 17 http://helios.augustana.edu/physics/301/periodic-table-fix.jpg

18. ENG2000: R.I. Hornsey Atom: 18 Groups Group 0 contains the inert (noble) gases Group IV includes Si  important materials in Si chip manufacture are B (III) and P (V) – as we will see later  together, these materials are between metals and non- metals Group VII are the ‘halogens’ and are one electron deficient in the valence shell Groups IA and IIA are the alkali and alkaline earth metals Groups IIIB – IIB are the transition metals, which have partially filled lower (d) energy states  includes ‘real’ metals and magnetic materials

19. ENG2000: R.I. Hornsey Atom: 19 Bonding Atomic bonding determines many of the physical properties of a material If two isolated atoms are brought closer together the net force varies with distance  there is a mechanism-specific attractive force (F A )  and a repulsive force (F B ), which increases when the atoms are sufficiently close for the outer shells to overlap  equilibrium is reached when F A + F B = 0  this is at r 0 on the following page The potential energy at r 0 is the bonding energy, E 0  and represents the energy required to separate the atoms to an infinite distance  e.g. thermal energy to melt the material

20. ENG2000: R.I. Hornsey Atom: 20 Callister

21. ENG2000: R.I. Hornsey Atom: 21 Ionic bonding Ionic bonding occurs in materials composed of a metallic and a non-metallic element  the metallic element easily donates its electron to the non- metallic element  the metal becomes a positive ion, while the non-metal is negatively ionised http://www.agen.ufl.edu/~chyn/age4660/lect/lect_02/2_11a.gif http://www.astro.lsa.umich.edu/users/cowley/lecture11/images/NaCl.jpg

22. ENG2000: R.I. Hornsey Atom: 22 Here, the attractive forces are coulombic, arising from the attraction of oppositely charged ions  E 0 ≈ 600 – 1500 kJ/mol., or 3 – 8 eV/atom  this relatively large bonding energy is reflected in typically high melting temperatures for ionically bonded materials  including ceramics

23. ENG2000: R.I. Hornsey Atom: 23 Covalent bonding As the name suggests, covalent bonds are formed by sharing valence electrons between the constituent atoms  thereby causing all atoms to achieve a full – and stable – outer shell  the classic example is methane, CH 4 http://www.mse.cornell.edu/courses/engri111/images/covalent.gif

24. ENG2000: R.I. Hornsey Atom: 24 Covalent bonds are also common in elements from the right-hand side of the periodic table  notably the semiconductors silicon and germanium, as well as carbon  also compound semiconductors, e.g. GaAs and InP The number of atoms participating in the bond is determined by the number of valence electrons  Si is in group IV, so has 4 valence electrons, and therefore bonds with 4 neighbouring atoms

25. ENG2000: R.I. Hornsey Atom: 25 Metallic bonding Metallic elements have one or two (possibly three) ‘loose’ valence electrons  which are relatively freely donated by all atoms The result is a structure in which ionised atoms (because they have donated their electron) are ‘suspended’ in a ‘sea’ of electrons  the ions are fixed in place because the negatively charged electron sea exerts an equal attraction in all directions +ve ion cores -ve electron sea

26. ENG2000: R.I. Hornsey Atom: 26 Metallic bonding Because the donated electrons are freely mobile, the electrical conductivity of metals is high  heat can also be transmitted by electrons, so metals are good thermal conductors Ionically and covalently materials are typically good electrical insulators  there is another mechanism for thermal transport which means that e.g. ceramics can be good thermal conductors http://207.10.97.102/chemzone/lessons/03bonding/mleebonding/metallicblue.gif

27. ENG2000: R.I. Hornsey Atom: 27 Other bonding types Ionic, covalent and metallic are the primary bonding types Secondary bonds are those that exist between all atoms, but are relatively weak and may be obscured by the primary bonds van der Waals bonds are typically only 0.1eV/atom (c.f. 8eV/atom for ionic)  and results from atomic or molecular dipoles Dipoles can result from molecular bonds – especially those involving H atoms – atomic vibrations, or external electric fields +–+–

28. ENG2000: R.I. Hornsey Atom: 28 Melting temperatures TypeSubstanceEnergy (eV/atom)Melt. Temp (°C) Ionic NaCl3.3801 MgO5.22800 Covalent Si4.71410 C7.4>3550 Metallic Hg0.7-39 Al3.4660 Fe4.21538 W8.83410 van der Waals Ar0.08-189 Cl 2 0.32-101 HydrogenNH 3 0.36-78 H20H200.520

29. ENG2000: R.I. Hornsey Atom: 29 Summary Atomic structure is determined by quantum mechanics  four quantum numbers determine energy states  states may or may not be occupied by electrons Atomic structure determines chemical and physical properties of the elements  periodic table Structure also determines how atoms bond  primary – ionic, covalent, metallic  secondary – van der Waals, hydrogen