Atomic Structure & Interatomic Bonding
Introduction:
Some of the important properties of solid materials depend on geometrical atomic arrangements, and also the interactions that exist among atoms or molecules . In some instances, the type of bond allows us to explain a material’s properties.
Ex :
Carbon, which may exist as both Graphite(soft & greasy) and
Diamond(the hardest known material). This dramatic disparity in properties is directly due to a type of interatomic bonding found in graphite that does not exist in diamond .
Atomic Structure:
? Atoms = nucleus (protons and neutrons) + electrons
Mass Charge
Proton 1.67 ? 10-27 kg + 1.60 ? 10 -19 C
Neutron 1.67 ? 10-27 kg Neutral
Electron 9.11 ? 10-31 kg - 1.60 ? 10 -19 C
. Electron mass = 1/1836 that of a proton ?
Radius of an atom= 0.1 nm = 0.1 x10 -9 m (1Angstrom) ?
50,000,000 atoms lined up measure 10mm !!! ?
? Nucleus takes up 10 -14 of the total volume of atom and has diameter of 4 -15 fm (Femtometer = 10 -15 m)
?# of protons gives chemical identification of the element = atomic number (Z)
?# of neutrons (N) defines isotope number
?Atomic mass (A) = mass of protons + mass of neutrons
? The atomic mass unit (amu) is often used to express atomic weight.
1 amu is defined as 1/12 of the atomic mass of the most common isotope of carbon atom that has 6 protons (Z=6) and six neutrons (N=6). m proton ? m neutron = 1.67 ? 10-27 kg = 1 amu.
The atomic mass of the 12C atom is 12 amu.
Atomic mass (A) ? atomic number (Z) + # of neutrons (N)?
? The atomic weight of an element or the molecular weight of a compound may be specified on the basis of amu per atom (molecule) or mass per mole of material.
? In one mole of a substance there are 6.023 X 1023 (Avogadro’s number) atoms or molecules. These two atomic weight schemes are related through the following equation:
1 amu/atom (or molecule) = 1 g/mol
Ex:
The atomic weight of iron is 55.85 amu/atom, or 55.85 g/mol.
.. …………………………………..
? The number of atoms per cm3, n, for material of density ? (g/cm3) and atomic mass A (g/mol):
n = Nav × ? / A
Ex:
Graphite (carbon): ? = 2.3 g/cm3, A = 12 g/mol
n = 6.023 × 1023 atoms/mol × 2.3 g/cm3 / 12 g/mol = 11.5 × 10 22 atoms/cm3
Atomic Models:
? Quantum mechanics
- Bohr atomic model: An early atomic model, in which electrons are assumed to revolve around the nucleus in discrete orbital.
Bohr suggested 3 ideas :
i) An electron in an atom can revolve in specific orbits without emitting energy.
ii) An e- may jump from one ‘orbit’ to another – when it does, a single photon (quantum) of light is emitted with E =difference between Ei and Ef
E1 – E2 = hv
(where v = frequency, h = planks const 6.62 x 10 -34 Js)
iii) Permitted orbits are those for which the angular momentum (mvr) is a integer multiple of h/2?
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