Download Nucleon number

Chapter 1
1.1 Atoms and Molecules
At the end of this topic, students should be
able to:
Describe proton, electron and neutron.
Define proton no.,Z, nucleon no., A and isotope.
Write isotope notation.
Define relative atomic mass and relative molecular
mass based on the C-12 scale.
Sketch and explain the function of the following
main components of a simple mass spectrum:
Analyze mass spectrum of an element.
Name cations, anions and salt according to the
IUPAC.
1.1 Atoms and Molecules
What is matter?
Matter is anything that has mass and
occupies space.
All matters consist of tiny particles called
atoms.
3 states of matter are solid, liquid and
gas.
For examples are metal, plastics, gas, etc.
1.1.1 Atoms
Protons and neutrons are found in
the nucleus of the atom, while
electrons surround the nucleus to
form ‘electron cloud’.
The properties of these particles are
summarized in the table.
Nucleon
the particles that are found in the
nucleus.
consist proton and neutron.
By proton we can identified the
element.
PROTON NUMBER AND NUCLEON
NUMBER
Proton number (atomic number), Z.
 The proton number = the number of
protons in the nucleus of an atom.
Nucleon number (mass number), A.
 The nucleon number = the total number of
protons and neutrons in the nucleus of an atom.
1.1.2 Isotopic notation
Nucleon number
Atom or
ion
Proton number
For neutral atom, number of protons =
number of electrons.
For +ve ion, number of electron is less than
number of protons.
For –ve ion, number of electrons is more
than number of protons.
Example:
Number of protons = proton number
= 13
Number of neutrons = nucleon number – proton number
= 27 - 13
= 14
Number of electron = proton number – charge carried by
species
= 13 – ( +3)
= 10
Exercise:
Determine the number of proton,
neutron and electron in the following
species.
a)
b)
Exercise:
The atomic number of lead (Pb) is 82 and
the mass number is 207.
1. Write out the atomic notation for lead.
2. Give the number of protons, neutrons
and electrons.
1.1.3 ISOTOPES
Two or more atoms of the same
element having same proton
number but different nucleon
number.
Eg : 1) hydrogen isotope
1
2
H
1
1
3
H
1
2) carbon isotope
12
6
C
14
;
6
C
H
Isotopes of an element have the same:
1. number of protons (proton number)
2. charge of nucleus of the atoms (ionization
energy; electron affinity; size of the atom;
electronegativity are the same)
3. number of electrons in a neutral atom
4. electronic configuration (the number of
valence electrons)
5. chemical properties
Isotopes of an element have different:
1. number of neutrons (nucleon number) in
the nucleus of the atoms
2. relative isotopic mass
3. physical properties (e.g boiling point /
melting point, density, effusion rate,…)
1.1.4 Molecule
A molecule consists of a small number of
atoms joined together by covalent bond.
Diatomic molecule: contains two atoms
(example: H2, Cl2, HCl, CO)
Polyatomic molecule: contains more than
two atoms (example: H2O, NH3)
1.1.5 Ion
An ion is a charged species formed from a
neutral atom or molecule when electrons
are gained or lost as the result of a
chemical reaction.
Cation: a positively charged ion (number e
< number p) (example: Mg2+, Al3+)
Anion: a negatively charged ion (number e
> number p) (example: Cl, OH)
Monatomic ion: ion contains only
one nucleus (example: Fe3+, S2)
Polyatomic ion: ion contains more
than one nucleus (example: H3O+,
CN)
1.1.6 RELATIVE MASS
Isotopes carbon -12 as a reference or
standard for comparing the masses of
other atoms.
RELATIVE ATOMIC MASS, Ar
Average mass of one atom of the element
relative to 1/12 times the mass of one atom
of carbon-12.
Ar = average mass of one atom of the element
1/12 x mass of one atom of C-12
The mass of C-12 is defined exactly
12.00 a.m.u.
1 atomic mass unit : a mass unit equal
to 1/12 the mass of a C-12 atom
Example:
Oxygen consists of three isotopes 16O
;17O and 18O in the ratio of 99.76 : 0.04 :
0.20. Calculate the relative atomic mass
(to 4 decimal point) of oxygen.
ANSWER :
16.0044
RELATIVE MOLECULAR MASS, Mr
The mass of one molecule of the
substance relative to 1/12 times the mass
of one atom of carbon-12.
Mr =
average mass of one molecule of the substance
1/12 x mass of one atom of C-12
= sum of the relative atomic masses of all
the atoms shown in the molecular formula.
Example:
The relative molecular mass of
carbon dioxide, CO2.
Answer:
Mr CO2 = Ar C + 2 Ar O
= 12 + 2(16)
= 44
Mass spectrum
the horizontal axis –
i. the m/e ratio @
ii. nucleon number @
iii. isotopic mass @
iv. relative atomic mass of the ions
entering the detector.
The vertical axis - the abundance or
detector current or relative abundance or
ion intensity or percentage abundance of
the ions.
1.
Information from a mass spectrum of
an element
the isotopes which are present in the
element
the relative isotopic mass of each
isotope
the abundance of each isotope
2.
Thus, the relative atomic mass of
the element can be determined
Relative abundance
63
9.1
8.1
0
24
25
26
m/e
The mass spectrum of magnesium shows that
naturally occurring magnesium consists of
three isotopes:
24Mg, 25Mg
and 26Mg.
The height of each line is
proportional to the abundance of
each isotope.
In this example, 24Mg is the most
abundance of the three isotopes.
Ar of Mg:
= 24.33
Average
atomic
mass
=
Q m
Q
i
i
i
i
i
where
Q = the abundance of an isotope of the element
= the percentage of the isotope found in the
naturally occurring element
m = the relative isotopic mass of the element
Notes :
1) The height of each peak measures the relative
abundance of the ion which gives rise to that peak.
2) The total number of peaks in the mass spectrum of
an element shows the types of naturally occurring
isotopes.
3) The ratio of mass/charge for each species is found
from the value of the accelerating voltage
associated with a particular peak. Many ions have a
charge of +1 elementary charge unit, and the ratio
m/e is numerically equal to m, the mass of the ion.
(1 elementary charge unit = 1.60x10 19 C)
4) The ion with the highest value of m/e is the
molecular ion, and its mass gives the molecular
mass of the compound