eng2000: r.i. hornsey atom: 1 eng2000 chapter 2 atoms and bonding

ENG2000: R.I. Hornsey Atom: 1

ENG2000 Chapter 2Atoms and Bonding


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


http://helios.augustana.edu/physics/301/periodic-table-fix.jpg


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


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


Callister


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


• 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


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


• 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


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


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


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

+ – + –


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


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

eng2000: r.i. hornsey atom: 1 eng2000 chapter 2 atoms and bonding

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