1Åm = 1×10 −10 m. 1Åm = 1×10 −10 m.
The ångström is often used in natural sciences and technology to express the sizes of atoms, molecules, and microscopic biological structures, the lengths of chemical bonds, the arrangement of atoms in crystals, the wavelengths of electromagnetic radiation, and the dimensions of integrated circuit parts
The unit was named after the Svensk physicist Anders Jonas Ångström (1814–1874).
Ångström was one of the pioneers in the field of spectroscopy, and is known also for studies of astrophysics, heat transfer, terrestrial magnetism, and the aurora borealis.
In 1868, Ångström created a chart of the spectrum of solar radiation that expressed the wavelengths of electromagnetic radiation in the electromagnetic spectrum in multiples of 1×10−10 m.
Since the human eye is sensitive to wavelengths from about 4,000 to 7,000 angstroms, that choice of unit allowed sufficiently accurate measurements of visible wavelengths without resorting to fractional numbers.
The unit then spread to other sciences that deal with atomic-scale structures.
the enthalpy change for any chemical reaction is independent of the route, providing the starting conditions and final conditions, and reactants and products, are the same
the enthalpy change that occurs when 1 mole of a solid ionic compound is separated into gaseous ions under standard conditions.
(This endothermic definition is given in Table 13 of the IB booklet)
NaCl(s) → Na+ (g) + Cl- (g) ΔHlatticeO (NaCl)
the enthalpy change occurs when 1 mole of the substance is formed from its constituent elements in their standard states under standard conditions.
Na(s) + ½ Cl2 (g) → NaCl(s)ΔHfO (NaCl)
Na(s) + ½ Cl2 (g)Na+(g) + Cl- (g)
Na(s) + ½ Cl2 (g) → Na(g) + ½ Cl2 (g) ΔHatomO (Na)
Na(g) + ½ Cl2 (g) → Na(g) + Cl(g)ΔHatomO (Cl)
( ΔHatomO (Cl) = ½ E(Cl-Cl),
bond enthalpies are provided in Table 10 of the IB booklet )
the enthalpy change that occurs when 1 mole of gaseous atoms are formed from the element in its standard state under standard conditions.
Electrons being removed and added
Na(g) + Cl(g) → Na+ (g) + e- + Cl(g)ΔHiO (Na)
Na+ (g) + e- + Cl(g) → Na+ (g) + Cl-(g) ΔHeO (Cl)
the energy required to remove 1 mole of electrons from 1 mole of isolated gaseous atom.
the energy released when 1 mole of electron is added to 1 mole of isolated gaseous atom.
(Data provided in Table 7 of the IB booklet)
H Na+(g) + e- + Cl(g)
Na(g) + Cl(g) Na+(g) + Cl- (g)
Na(g) + ½ Cl2 (g)
Na(s) + ½ Cl2 (g)
H Mg2+ (g)+ O2-(g)
Mg2+(g) + 2e- + O(g)
Mg(g) + O(g)
Mg(g) + ½ O2 (g)
Mg(s) + ½ O2 (g)
ΔHlatO (ab)= ΔHatomO (a) + ΔHatomO (b) + ΔHiO (a) +
ΔHeO (b) - ΔHfO (ab)
Theoretical Lattice enthalpies can be calculated by assuming the crystal is made up from perfect spherical ions and that the only interaction is due to electrostatic forces between the ions (ionic model); it can be calculated from the size, charge and packing of the constituent ions
ΔHlatO = Knm/ ( RMn+ +RXm- )
K is a constant which depends on the geometry of lattice.
ΔHlatO= Knm/ ( RMn+ +RXm- )