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ENG2000: R.I. Hornsey Atom: 1
ENG2000 Chapter 2Atoms and Bonding
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
ENG2000: R.I. Hornsey Atom: 3
Atoms• For our purposes, atoms are made from three
fundamental particles proton (charge = +q, m = 1.67 x 10-27kg) neutron (charge - 0, m = 1.67 x 10-27kg) electron (charge = -q, m = 9.1 X 10-31kg) q = 1.6 x 10-19C
• 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
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 1023 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
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.jpghttp://csep10.phys.utk.edu/astr162/lect/light/bohrframe/bohr2.gif
nucleus
orbitingelectrons
ENG2000: R.I. Hornsey Atom: 6
Energy levels• These are the first three energy
levels for an isolated H atom• 1eV = 1.6 x 10-19J
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
potentialenergy
n=3
n=2
n=1
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
potentialenergy
n=3
n=2
n=11s
2s2p
3s3p3d
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
€
λ=hmv
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
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, ml (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, ms
ms = ± 1/2
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 ml corresponds to the tilt (or inclination) of the orbit
ms represents the two directions the ‘planet’ can spin
ml
msn
l
ENG2000: R.I. Hornsey Atom: 12
Maximum number of statesn sub-shell # states max # electrons
sub-shell* shell
1 K s 1 2 2
2 Ls 1 2
8p 3 6
3 M
s 1 2
18p 3 6
d 5 10
4 N
s 1 2
32p 3 6
d 5 10
f 7 14
* # states x 2, because two electrons (with ± spin) can exist in each state
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 = 1s1
He = 1s2
Li = 1s22s1
Be = 1s22s2
B = 1s22s22p1
Ne = 1s22s22p6
Na = 1s22s22p63s1
Al = 1s22s22p63s23p1
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.gifhttp://www.chemtutor.com/scheme.gif
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
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
ENG2000: R.I. Hornsey Atom: 17
http://helios.augustana.edu/physics/301/periodic-table-fix.jpg
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
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 (FA)
and a repulsive force (FB), which increases when the atoms are sufficiently close for the outer shells to overlap
equilibrium is reached when FA + FB = 0
this is at r0 on the following page
• The potential energy at r0 is the bonding energy, E0
and represents the energy required to separate the atoms to an infinite distance
e.g. thermal energy to melt the material
ENG2000: R.I. Hornsey Atom: 20
Callister
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.gifhttp://www.astro.lsa.umich.edu/users/cowley/lecture11/images/NaCl.jpg
ENG2000: R.I. Hornsey Atom: 22
• Here, the attractive forces are coulombic, arising from the attraction of oppositely charged ions E0 ≈ 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
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, CH4
http://www.mse.cornell.edu/courses/engri111/images/covalent.gif
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
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
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
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
+ – + –
ENG2000: R.I. Hornsey Atom: 28
Melting temperaturesType Substance Energy (eV/atom) Melt. Temp (°C)
IonicNaCl 3.3 801
MgO 5.2 2800
CovalentSi 4.7 1410
C 7.4 >3550
Metallic
Hg 0.7 -39
Al 3.4 660
Fe 4.2 1538
W 8.8 3410
van der WaalsAr 0.08 -189
Cl2 0.32 -101
Hydrogen NH3 0.36 -78
H20 0.52 0
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