Download Lecture # 4

CHEM 1711 & 1811 Lecture # 4
1
ATOMIC STRUCTURE
•
Dalton (1808) gave modern def'n. of atoms by hypothesizing that
-
each element is made of tiny particles called atoms
-
compounds form when atoms combine in definite proportions
-
reactions reorganize how atoms are connected together
-
reactions do not create or destroy atoms
•
law of conservation of mass
•
microscopes invented since 1980's allows us to "see" atoms
SUB-ATOMIC PARTICLES
nucleus (~ 0.01 pm) has protons and neutrons
electrons in a delocalized "cloud" (~ 100 pm)
proton : positively charged nuclear particle; number is atom's identity ;
charge = + 1.602 x 10-19 C
(C is coulomb, SI charge unit)
e.g.
neutron:
lead, Pb
proton number Z = 82
nuclear particle having no electrical charge; some elements
have atoms with different numbers of neutrons
CHEM 1711 & 1811 Lecture # 4
2
mass number:
# of (p + n)
isotopes:
varieties of an element having different mass numbers
carbon-12
carbon-13
carbon-14
electron:
6p
6p
6p
6n
7n
8n
fast moving, negatively charged particle; much less massive
than p or n ; much more easily transferred between atoms ;
charge = - 1.602 x 10-19 C
DEMO
•
always an integer
Tesla coil excites a fluorescent light tube
atoms are net electrically neutral
check:
# of e- = # of p
How many electrons are in one silicon atom?
cation:
a positive ion ; an atom or molecule that has lost one or
more e-
anion:
a negative ion ; gained one or more e-
ATOMIC MASS MEASUREMENTS
•
electrons are the least massive sub-atomic particle
mp = mn = 1.67 x 10-27 kg
-
me = 9.11 x 10-31 kg
ignore net charge (to 3. s.f.) when considering mass
i.e. atom mass = ion mass
CHEM 1711 & 1811 Lecture # 4
atomic mass unit (amu or u) :
3
exactly 1/12 the mass of a carbon-12 atom
1 amu or 1 u = 1.6605 x 10-27 kg
conversion to SI
atomic mass:
measured mass of a given isotope ; many sig. figs. ; value
close to mass number
e.g.
•
oxygen-16
mass = 15.995 u
the relative abundance of isotopes is nearly constant for samples
taken at different locations on Earth
e.g.
chlorine has 2 isotopes
35
Cl
75.77 %
37
Cl
24.23 %
mass spectrum
atom count
35
•
37
mass/u
-
samples may be isotopically enriched by special purification
-
unusual isotopes may serve as labels
the average atomic mass for an element is found using the
experimental masses and abundances of all its isotopes
CHEM 1711 & 1811 Lecture # 4
e.g.
4
chlorine
average mass = (34.97 u) (75.77 %) + (36.97 u) (24.23 %)
= 35.45 u
(check P.T.)
SYMBOLS
mass #
net charge (default = 0)
16
8
proton #
(not often used)
-
O
2
how many atoms in
chemical formula
How many p, n and e- are in this ion? (its name is superoxide ion)
EXAMPLE
How many p and e- are in one phosphate ion (PO43-) ?