TOPIC 4: THE PERIODIC TABLE AND STRUCTURE OF ATOM By: Chemistry Lecturer School of Allied Health Sciences City University College of Science and Technology

HISTORY There are about 115 elements on earth’s crust. Many of this elements found in the form of compounds or mineral. GROUP, PERIOD & PROTON NUMBERS: Elements arranged in PT based on their proton numbers. Have 18 vertical groups call GROUPS which named Group 1, 2, 13, 14….18.

Elements have similar chemical properties are placed in same group. Have 7 horizontal rows called PERIODS which named Period 1, 2,3…7

Special names for some Groups: Group 1,2,13 – group metals Group 15-18 – non metals (silicon & carbon) GROUP NAME 1 ALKALI METALS 2 ALKALINE EARTH METALS 17 HALOGENS 18 INERT NOBEL GASES 3-12 TRANSITION METALS

By knowing proton number of element- we can determine Group & Period it is placed in. Element W has proton num 15. what Group & Period occupy in PT? Num. proton = Num. electron = 15 Electron configuration = 2.8.5 = W has 3 electron shells & 5 valence electron = valence electron 3 to 8 = +10 = 5 + 10 = 15 = W placed in Period 3 and Group 15 *Period= look at number of shells Group= look at valence number

Valence electrons = Group number Valence electron = 1, Group =1 Group 1 and 2 Valence electrons = Group number Valence electron = 1, Group =1 Valence electron = 2, Group =2 Group 13-18 Group number = Valence electron + 10 Valence electron = 3, Group =13 Valence electron = 7, Group =17 Group Period 1 2 13 14 15 16 17 18 H He Li 2.1 Be 2.2 B 2.3 C 2.4 N 2.5 O 2.6 F 2.7 Ne 2.8 3 Na 2.8.1 Mg 2.8.2 Al 2.8.3 Si 2.8.4 P 2.8.5 S 2.8.6 Cl 2.8.7 Ar 2.8.8 4 K 2.8.8.1 Ca 2.8.8.2 Period number = number of filled electron shells Na ; 2.8.1 = Period 3 Li ; 2.1 = Period 2 K ; 2.8.8.1 = Period 4

EXERCISE 1: Elements P, Q, R, S, T have proton number 5, 9, 11, 14, 20. Write down electron configuration for each & give Group & Period: ELEMENT ELECTRON CONFIGURATION GROUP PERIOD P Q R S T

ANSWER 1: Elements P, Q, R, S, T have proton number 5, 9, 11, 14, 20. Write down electron configuration for each & give Group & Period: ELEMENT ELECTRON CONFIGURATION GROUP PERIOD P 2.3 13 2 Q 2.7 17 R 2.8.1 1 3 S 2.8.4 14 T 2.8.8.2 12 4

As we go across a period from left to right PT: The electronegativity increase The atomic radius decrease The oxides of elements show a decrease in basicity but increase in acidity The atomic size decreases from sodium to argon because: Increase number of electrons in same shells results in stronger electrostatic force exerted by protons on electron Electrostatic force pulls the electron shells closer to nucleus  decrease in atomic size

GROUP 18 ELEMENTS (NOBLE GASES) Helium (He) Neon (Ne) Argon (Ar) Krypton (Kr) Xenon (Xe) Radon (Rn) All noble gases can’t dissolve in water & can’t conduct electricity & heat. Colourless gases with low meting & boiling point

Not chemically reactive because they exist as monoatomic gases – can’t combine ionically or covalently with other element. It has attained duplet or octet electron arrangement Have eight valence electron As their valence shells complete, this atoms can’t accept, donate or share electrons with other elements.

USES OF GROUP 18 GROUP USES HELIUM Fill air ships & weather balloons NEON Burns with orange red colour – used to light up billboards on hotels, buildings & airport runways ARGON Used in filament bulbs to prevent burning of tungsten filament KRYPTON High speed flash bulbs in photography Used in electric bulbs

GROUP 1 ELEMENTS ( Alkali metals) Consist: Lithium (Li) Sodium (Na) Potassium (K) Rubidium (Rb) Caesium (Cs) Francium (Fr) Soft & cut easily Conduct heat & electricity Hv high melting & boiling points

Chemical properties of alkalis metal Reactive elements & must kept in paraffin Cannot exist in free form – need combine with other elements. Have 1 valence electron During chemical reaction – they donate electron to obtain nose gas stability

GROUP 17 ELEMENTS (Halogens) Consists: Fluorine (F) Chlorine (Cl) Bromine (Br) Iodine (I) Astatine (At) Reactive non-metals Found in minerals on earth’s crust All halogen made up of diatomic molecules

Cannot conduct electricity Weak conductors of heat because consist of covalent molecules Chemical properties: All halogen have 7 valence electrons. During chemical reaction-halogen atoms receive electron to form anion-to attain the noble gas electron arrangement

The Transition Elements Is metals which placed between Group 2 & G.13 Physical & chemical properties of elements are similar. All period 4 transition metals are solids. Atomic radius increase very slightly towards right. Density, melting & boiling point increase across the period from Sc to Cu Zn decrease in density, melting & boiling points. Not reactive compared to alkalis metals because they have stronger metallic bonds among their atoms.

Have shining surfaces & high tensile strength. Good conductors of heat & electricity Special features: Coloured ions – they form ions, compound & solutions which are coloured. Complex ions Varying oxidation numbers Catalytic properties

Uses of Transition Elements in Industry Used as catalyst in specific industrial process. Mercury used in thermometers because its high melting & boiling point & liquid state Tungsten – make filament because not melt easily Iron- make vehicles & electrical appliances Chromium – used to coat iron to prevent rusting of iron substance

SUMMARY Horizontal rows in periodic table of Element call periods. The number given to each period indicates the number of filled electron shells. Eg: period 3 shows all elements of that period have 3 shells filled with electron. Number of proton increase across the period from left to right. They increase by 1 proton.

Physical & chemical properties of the elements change gradually across the Period. smaller the atom & closer the electron to nucleus  more difficult it release electrons to form cations or +ve ion – electropositivity decrease Smaller the atom & more valence electron in last shell  easier to accept or share electrons with other atoms  electronegativity increase across period from sodium to argon.

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