Lecture 5

Electronic Configuration of Periodic Table

Aufbau Principle: Add one proton to nucleus and one electron to the lowest-energy orbital available in the electron shell. s block: 2 boxes  1 orbital  2 electrons. For s: = 0; m= 0 i.e. 1 orbital on s subshell. p block: 6 boxes  3 orbitals  with 2 electrons each. For p: =1; m= -1, m= 0, m= 1 i.e. 3 orbitals on p subshell d block: 10 boxes  5 orbitals  with 2 electrons each. For d:  = 2; m = -2, m = -1, m= 0, m =1, m =2 i.e. 5 orbitals on d subshell f block: 14 boxes  7 orbitals  with 2 electrons each. For f:  =3; m =-3, m =-2, m =-1, m =0, m =1, m =2, m =3 i.e. 7 orbitals on f subshell

Electron Energy Diagrams A visual method of showing the energy of electrons within an element: 3 steps to build an energy diagram: e.g. Iron [Fe] Z=26  26 protons  26 electrons Step 1: Write down shorthand electronic configuration. Step 2: Build energy diagram with subshells Step 3: Fill in electrons with arrows counting up to Z. Step 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 (short hand)

Electron Energy Diagrams A visual method of showing the energy of electrons within an element: 3 steps to build an energy diagram: e.g. Iron [Fe] Z=26  26 protons  26 electrons Step 1: Write down shorthand electronic configuration. Step 2: Build energy diagram with subshells Step 3: Fill in electrons with arrows counting up to Z. Step 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 (short hand)

Electron Energy Diagrams Step 3 Energy 3d 4s 3p 3s 2p 2s 1s Step 2

Valence Electrons Every element has both core electrons and valence electrons, e.g. Magnesium: Mg Z=12  12 electrons: 1s 2 2s 2 2p 6 3s 2 core electronsvalence electrons Core electrons are electrons in fully filled shells Valence electrons are electrons in the outermost shell that is not fully filled with the exception of the noble gases that all have fully filled shells He: 1s 2, Ne: {He} 2s 2 2p 6, Ar: {Ne} 3s 2 3p 6, Kr: {Ar} 4s 2 3d 10 4p 6, Xe: {Kr} 5s 2 4d 10 5p 6.Rn: {Xe} 6s 2 4f 14 5d 10 6p 6.

Construction of the Periodic Table Electron shells fill in a systematic fashion so that patterns can be recognised in the electronic configuration. Elements listed in horizontal Rows are called Periods. A new period is started each time the value of the principle quantum number, n, increases, i.e. each time the valence electrons enter a new shell. Arrange rows so that elements with similar electronic configuration lie above one another to form vertical columns called Groups, similar electronic configuration meaning similar chemistry.

Group Properties Group 1 - elements with only one valence electron: These are called the Alkali-Metal Group Electronic configuration 3 Lithium Li{He}2s 1 11 SodiumNa{Ne}3s 1 19 PotassiumK{Ar}4s 1 37 RubidiumRb{Kr}5s 1 55 CesiumCs{Xe}6s 1 87 FranciumFr{Rn}7s 1 Physical Properties metals i.e good conductors, soft, low melting point and boiling point

Group I Chemical Properties Na. + :Cl. →NaCl (Na + and :Cl : - ) Alkali +Halogen →Ionic Compound Na. + H 2 O→NaOH + H + Alkali +Water →Ionic Compound NaOH → Na + + OH - cations: positive ions (e.g. Na +, K +, H + ) anions: negative ions (Cl , OH  ) Me→ Me + + e  (Me = Alkali metal) ; ; ; ;

Group 17 (or 7A) Halogens Elements with one electron less than their nearest nobel gas: The Halogens: (Greek: halogen=salt former) Electronic Configuration: 9 FluorineF{He}2s 2 2p 5 17 ChlorineCl{Ne}3s 2 3p 5 35 BromineBr{Ar}4s 2 3d 10 4p 5 53 IodineI{Kr}5s 2 4d 10 5p 5 Physical Properties: Highly coloured - volatile - non-metals - bad conductors - occur in nature as diatomic molecules X 2 (X=Halogen), e.g. Cl 2. F 2 and Cl 2 are gases, Br 2 is a liquid and I 2 is a solid.

Chemical Properties of the Halogens X + H  HX or better: X 2 + H 2  2 HX X + Me  MeX (Me + + X  ) (see group 1) Aqueous solutions of HX contain high concentrations of H+, i.e. are acids: HX +H 2 O  H + + X - + H 2 O

Group 18 (or 8A) - The Noble Gases Special group of elements within the periodic table. They all have full electron shells and are highly non-reactive. Physical properties: Colourless gases (at normal temperature and pressure) – lowest boiling and melting points of all elements. Chemical properties: The most chemically unreactive of all elements, up to recently thought to be totally unreactive and were called Inert Gas Elements. Summary Groups (columns) have similar electronic configuration and similar chemistry, e.g., Noble gases have full shells, Alkali metals have one electron more than Noble gases. Halogens have one electron less than Noble gas configuration.

Summary Continued. Properties of Periods (Rows): Periods are characterised by the gradual filling of valence shells – all atoms in a period have different electronic configuration. Similar chemical behaviour is not expected, however a gradual change of the metallic properties of Group 1 to the non- metallic properties of Group 17 is expected.