Download 1.3.4 Atoms and molecules Name Symbol Definition SI unit Notes

1.3.4 Atoms and molecules
Name
Symbol
Definition
nucleon number,
mass number
proton number,
atomic number
neutron number
electron rest mass
mass of atom,
atomic mass
atomic mass constant
mass excess
elementary charge,
proton charage
Planck constant
Planck constant/2π
Bohr radius
Rydberg constant
fine structure constant
ionization energy
electron affinity
A
1
Z
1
N
me
ma, m
N=A-Z
mu
∆
e
mu = ma( C)/12
∆ = ma - Amu
h
h
a0
R∞
α
Ei
Eea
12
h = h/2π
2
2
a0 = 4πε0 h /mee
R∞ = Eh/2hc
2
α = e /4πε0 h c 1
SI unit
1
kg
kg
kg
kg
C
Notes
(1)
(1), (2)
Js
Js
m
-1
m
J
J
(1)
Analogous symbols are used for other particles with subscripts: p for proton, n for
neutron, a for atom, N for nucleus, etc.
(2)
mu is equal to the unified atomic mass unit, with symbol u, i.e. mu = 1 u. In biochemistry
the name dalton, with symbol Da, is used for the unified atomic mass unit, although the
name and symbols have not been accepted by CGPM.
Chapter 1 - 1
Name
Symbol
Definition
electronegativity
dissociation energy
from the ground state
from the potential
minimum
principal quantum
number (H atom)
angular momentum
quantum numbers
magnetic dipole
moment of a molecule
magnetizability
of a molecule
Bohr magneton
χ
Ed, D
D0
De
χ = ½(Ei +Eea) J
n
E = -hcR/n
(3)
SI unit
(3)
J
J
J
2
Notes
(4)
(4)
1
see under Spectroscopy, section 3.5.
-1
m, µ
Ep = -m⋅B
ξ
m = ξB
JT
µB
µB = eh/2me
JT
JT
(5)
-2
-1
The concept of electronegativity was intoduced by L. Pauling as the power of an atom in
a molecule to attract electrons to itself. There are several ways of defining this quantity.
The one given in the table has a clear physical meaning of energy and is due to R.S.
Mulliken. The most frequently used scale, due to Pauling, is based on bond dissociation
energies in eV and it is relative in the sense that the values are dimensionless and that
only electronegativity differences are defined. For atoms A and B
χ r, A − χ r, B = (eV ) −1 / 2 E d (AB) − [E d (AA) + E d (BB)]
where χr denotes the Pauling relative electronegativity. The scale is chosen so as to make
the relative electronegativity of hydrogen χr,H = 2.1. There is a difficulty in choosing the
sign of the square root, which determines the sign of χr,A - χr,B. Pauling made this choice
intuitively.
(4)
The symbols D0 and De are mainly used for diatomic dissociation energies.
(5)
Magnetic moments of specific particles may be denoted by subscripts, e.g. µe, µp, µn for
an electron, a proton and a neutron.
Chapter 1 - 2
Name
Symbol
Definition
SI unit
nuclear magneton
magnetogyric ratio
(gyromagnetic ratio)
g-factor
nuclear g-factor
Larmor angular frequency
Larmor frequency
relaxation time,
longitudinal
transverse
electric field gradient tensor
activity (of a radioactive
substance)
decay (rate) constant,
disintegration (rate)
constant
half life
mean life
level width
disintegration energy
cross section
(of a nuclear reaction)
µN
γ
µN = (me/mp)µB
γ = µ/L
JT
-1 -1
s T
g
gN
ωL
vL
g = 2µ/µB
gN = µ/IµN
ωL = (e/2m)B
vL = ωL/2π
1
1
-1
s
Hz
Notes
-1
(6)
(7)
T1
T2
q
A
qαβ = -∂ V/∂α∂β
A = -dNB/dt
s
s
-2
Vm
Bq
λ, k
A = λNB
s
(9)
t½,T½
τ
Γ
Q
σ
NB(t½)= NB(0)/2
τ = 1/λ
Γ = h/τ
s
s
J
J
2
m
(9), (10)
(10)
2
-1
(8)
(8)
(9)
(6)
µ is the magnetic moment, L the angular momentum.
(7)
This quantity is commonly called Larmor circular frequency.
(8)
These quantities are used in the context of saturation effects in spectroscopy, particularly
spin-resonance spectroscopy (see section 3.5).
(9)
NB is the number of radioactive atoms B.
(10)
Half lives and mean lives are often given in years (a), see conversion tables.
t½ = τ ln2 for exponential decays.
Chapter 1 - 3