chemical structure: structure of matter. elements, ions & isotopes
DESCRIPTION
Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.TRANSCRIPT
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Elements, Ions & Isotopes
University of Lincoln presentation
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What you should know…• Elements and their classification• Atoms/molecules• Symbols of the elements• Allotropy• The Octet rule• Ions – cations/anions• Oxidation/reduction• Ionisation energy/electron affinity• Isotopes• Atomic mass• Relative atomic mass
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1. Elements
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IUPAC Definition
An element is matter, all of whose atoms are alike
in having the same positive charge on the nucleus
International Union of Pure and Applied Chemistry
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Dictionary Definition
A substance that cannot be decomposed
into simpler substances
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Are all elements simply collections of atoms?
YES, normally Elemental
mercury (liquid), Hg
Elementalcopper, Cu
Elemental helium
(gas), HeElemental gold, Au
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Some elements only exist as molecules
These elements exist as diatomic molecules*
* A molecule is two or more atoms bonded together
H2 N2 O2 F2Cl2
Br2
I2
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Symbols
A = MASS NUMBERZ = ATOMIC NUMBER =number of protonsN =number of neutrons
A = N + Z
XZ
A
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For Example
20
40 11
22
1
1H NaCa
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Classification of elements
• Metals• Non-metals• Semi-metals
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H
BeLi
Na
K
Rb
Cs
Fr
Mg
Ca
Sr
Ba
Ra
Sc
Y
La
Ac
Ti V Cr Mn Fe Co Ni Cu Zn
Zr
Hf Ta W Re Os Ir Pt Au Hg Tl
Nb Mo Tc Ru Rh Pd Ag Cd In Sn
Pb Bi Po At Rn
Xe
Kr
Ar
Ne
Sb Te I
Ga
Al
Ge
Si P S Cl
As Se Br
Ce Pr Nd Pm Sm
Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
He
B C N O F
Metals
Semi-metals
Non-Metals
Classification of elements
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Allotropes
Some elements exist in more than one structural form. This property is called ALLOTROPY
Consider carbon – 2 common allotropes are graphite and diamond.
Both consist only of atoms of carbon, C, but their structures are very different, and hence their properties differ
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Allotropes of Carbon
C60 fullerene structure
Graphite layered structure
Diamond structure
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Allotropes of other elements?
• Tin, Sn• Phosphorus, P• Arsenic, As• Oxygen, O• Sulphur, S• Selenium, Se
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2. Ions
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The Octet RuleBecause filled orbitals give the best STABILITY, all
elements try to attain a noble gas configuration (i.e. 8 electrons in their valence shell)
2 ways of doing this:(i) losing electrons; or(ii) gaining electrons(which ever uses the least energy)
This is the driving force behind the chemistry of the elements and is called the OCTET RULE
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The Periodic Table
n=1
n=2n=3
n=4
n=5n=6n=7
The Periodic Table consists of rows of 8 elements (s + p block only)
Each row corresponds to a different quantum number (n=1–7)
Each column has the same VALENCE CONFIGURATION
ns
1 ns
2
ns
2n
p1 n
s2n
p2
ns
2n
p3
ns
2n
p4
ns
2n
p5
ns
2n
p6
f - block elements
H
BeLi
Na
K
Rb
Cs
Fr
Mg
Ca
Sr
Ba
Ra
Sc
Y
La
Ac
Ti V Cr Mn Fe Co Ni Cu Zn
Zr
Hf Ta W Re Os Ir Pt Au Hg Tl
Nb Mo Tc Ru Rh Pd Ag Cd In Sn
Pb Bi Po At Rn
Xe
Kr
Ar
Ne
Sb Te I
Ga
Al
Ge
Si P S Cl
As Se Br
Ce Pr Nd Pm Sm
Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
He
B C N O F
LanthanoidsActinoids
d – block elements
Hydrogen and s – block elements
p – block elements
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Definition
An ION is a charged atom or molecule.
There are 2 types of ion:A CATION is positively charged
An ANION is negatively charged
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Cations
• Cations are formed when an atom loses 1 or more valence electron:
Na Na+ + e-
Mg Mg2+ + 2e-
• The loss of electrons is known as OXIDATION and is a typical reaction of metals
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Valence ElectronsAtomic number
Element Symbol
Electronic configuration
Atomic number
Element Symbol
Electronic configuration
1 H 1s1 11 Na [Ne]3s1
2 He 1s2 12 Mg [Ne]3s2
3 Li [He]2s1 13 Al [Ne]3s23p1
4 Be [He]2s2 14 Si [Ne]3s23p2
5 B [He]2s22p1 15 P [Ne]3s23p3
6 C [He]2s22p2 16 S [Ne]3s23p4
7 N [He]2s22p3 17 Cl [Ne]3s23p5
8 O [He]2s22p4 18 Ar 1s22s22p63s23p6
9 F [He]2s22p5 19 K [Ar]4s1
10 Ne 1s22s22p6 20 Ca [Ar]4s2
Group 1 = [NG]ns1
Group 2 = [NG]ns2
Elements in these groups want to LOSE their outer (valence) electrons to gain the noble gas configuration [NG]:
Na Na+ + e-
Mg Mg2+ + 2e-
Electronic configuration of both cations = [Ne]
The energy required to remove a valence electron is called the IONISATION ENERGY
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Group 1 & 2 elements (metals)
Li Be
Na Mg
K Ca
Rb Sr
Cs Ba
Fr Ra
Group 1 Group 2
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Anions
• Anions are formed when an atom gains 1 or more valence electron:
F + e- F-
O + 2e- O2-
• The gain of electrons is known as REDUCTION and is a typical reaction of non-metals
Group 16 = [NG]ns2np4
Group 17 = [NG]ns2np5
Elements in these groups want to GAIN valence electrons to attain the noble gas configuration [NG] ns2np6, which is the noble gas sitting on their RHS in the Periodic Table
F + e- F-
O + 2e- O2-
Electronic configuration of both anions = [Ne]
Electron affinity is a measure of how easy it is to gain a valence electron
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Group 16 & 17 elements (non-metals)
O F Ne
S Cl Ar
Se Br Kr
I Xe
At Rn
Groups
16 17 18
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3. Isotopes
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Definition
In 1913 Soddy proposed the existence of ISOTOPES
Definition: Atoms of the same elements with different atomic masses
Frederick Soddy
Nobel Prize (Chemistry) 1921
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DefinitionIsotopes of an element have the same
number of protons, but different numbers of neutrons
Eg.
29
29
Mass number (A)
Atomic number (Z)
Protons (Z) = 29
Neutrons (N) = 34
Protons (Z) = 29
Neutrons (N) = 36
Cu
Cu65
63
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Henri Becquerel
Marie & Pierre Curie
Radioactivity discovered in 1896
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Stable v. Radioactive Isotopes
0
200
400
600
800
1000
1200
1400
1600
StableRadioactive
There are approximately 1,700 isotopes known to
exist
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Chart of the nuclides
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Black squares denote STABLE isotopes
Z
N
Chart of the nuclides
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Atomic Mass, A
• For simplicity, atomic masses are given relative to the mass of 12C
• 12C = 12.0000 amu
• amu = atomic mass unit = 1.660x 10-27kg similar to the mass of a proton or neutron (see Lecture 1)
Mass number (A) is used as the atomic mass
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Relative Atomic Mass, Ar
Mg 24 78.7
Mg 2510.1
Mg 26
11.2
% abundance
The relative atomic mass of an element is the weighted mean of the atomic masses of all the stable isotopes for that element.
For example:
Ar (Mg) = 24.3
Atomic mass
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Acknowledgements
• JISC• HEA• Centre for Educational Research and
Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org