Chemical Structure: Structure of Matter. Elements, Ions & Isotopes

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Elements, Ions & Isotopes

University of Lincoln presentation

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

1. Elements

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

Dictionary Definition

A substance that cannot be decomposed

into simpler substances

Are all elements simply collections of atoms?

YES, normally Elemental

mercury (liquid), Hg

Elementalcopper, Cu

Elemental helium

(gas), HeElemental gold, Au

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

Symbols

A = MASS NUMBERZ = ATOMIC NUMBER =number of protonsN =number of neutrons

A = N + Z

For Example

1H NaCa

Classification of elements

• Metals• Non-metals• Semi-metals

Ti V Cr Mn Fe Co Ni Cu Zn

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

Sb Te I

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

B C N O F

Metals

Semi-metals

Non-Metals

Classification of elements

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

Allotropes of Carbon

C60 fullerene structure

Graphite layered structure

Diamond structure

Allotropes of other elements?

• Tin, Sn• Phosphorus, P• Arsenic, As• Oxygen, O• Sulphur, S• Selenium, Se

2. Ions

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

The Periodic Table

n=2n=3

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

f - block elements

Ti V Cr Mn Fe Co Ni Cu Zn

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

Sb Te I

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

B C N O F

LanthanoidsActinoids

d – block elements

Hydrogen and s – block elements

p – block elements

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

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

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

Group 1 & 2 elements (metals)

Group 1 Group 2

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

Group 16 & 17 elements (non-metals)

O F Ne

S Cl Ar

Se Br Kr

Groups

16 17 18

3. Isotopes

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

DefinitionIsotopes of an element have the same

number of protons, but different numbers of neutrons

Mass number (A)

Atomic number (Z)

Protons (Z) = 29

Neutrons (N) = 34

Protons (Z) = 29

Neutrons (N) = 36

Henri Becquerel

Marie & Pierre Curie

Radioactivity discovered in 1896

Stable v. Radioactive Isotopes

StableRadioactive

There are approximately 1,700 isotopes known to

Chart of the nuclides

Black squares denote STABLE isotopes

Chart of the nuclides

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

Relative Atomic Mass, Ar

Mg 24 78.7

Mg 2510.1

% 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

Acknowledgements

• JISC• HEA• Centre for Educational Research and

Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org

Chemical Structure: Structure of Matter. Elements, Ions & Isotopes

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