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Noble Gases and Valence e - Ionization Energy and Bonding.

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Presentation on theme: "Noble Gases and Valence e - Ionization Energy and Bonding."— Presentation transcript:

1 Noble Gases and Valence e - Ionization Energy and Bonding

2 A look back at the Periodic Table Halogens, group 7A or 17: F, Cl, Br, I – Nonmetals – Element form is diatomic F 2 Cl 2 Br 2 I 2 – At room temperature, F 2 and Cl 2 are gases, Br 2 is liquid, I 2 is solid – Reactivity decreases down group F 2 is the most reactive of all elements I 2 is the least reactive halogen 2

3 Trends in Element Families Alkali metals, group 1A or 1: Li, Na, K, Rb, Cs – Metals stored under oil to keep them from reacting with O 2 – Impart bright, characteristic colors to flames Li = red Na = orange K, Rb, Cs = lavender – All react with water to produce H 2 – Reactivity increases down group Cs is most reactive alkali metal Li is least reactive alkali metal 3

4 Trends in Element Families Noble gases, group 8A or 18: He, Ne, Ar, Kr, Xe – Nonmetals – Element form is monatomic – At room temperature, all are gases – Most significant property is that they are almost completely unreactive Compounds of Xe, Kr, and recently Ar have been made with F and O, but they decompose very easily No compounds of He have ever been prepared 4

5 How do Atoms Join? The behavior of the noble gas, alkali metal, and halogen families is key to understanding bonding – Noble gases generally do not form compounds – Alkali metals form compounds by losing one electron to form a + 1 ion (cation) – Halogens form compounds by gaining one electron to form a – 1 ion (anion) 5

6 Ion sizes A cation is always smaller than its parent atom An anion is always larger than its parent atom 6

7 Lets Practice… Which ion is larger? – Seor Se -2 Which ion is smaller? – Al or Al +3

8 Ionization energy Ionization energy (IE) is the energy needed to remove the outermost electron from an atom – Low IE electron is easier to remove – High IE electron is harder to remove 8

9 Ionization energy IE decreases down a group (easier to remove e – as you go down a group) IE generally increases across a period from left to right (harder to remove e – as you go across a period) 9

10 Ionization Energy

11 Lets Practice… Which has a larger ionization energy? – Cs or F Which has a smaller ionization energy? – Be orMg

12 12 2 He 10 Ne 18 Ar 17 Cl 9F9F 19 K 11 Na 3 Li Group 8A Noble gases happy to be me Group 7A Halogens just one more electron... Group 1A Alkali metals just one less electron...

13 Valence electrons This behavior suggests that – The number of electrons in a noble gas is especially stable – That number of electrons forms a noble gas core of stable electrons unavailable for bonding – Only electrons outside the noble gas core participate in the formation of chemical bonds between atoms – Those electrons are called VALENCE ELECTRONS 13

14 Valence electrons 14 1H1H 2 He 3 Li 11 Na 19 K 4 Be 12 Mg 20 Ca 5B5B 6C6C 7N7N 8O8O 9F9F 10 Ne 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 1A 2A 3A 4A 5A 6A 7A 8A FOR MAIN-GROUP ELEMENTS: number of valence electrons = group number

15 Depicting Valence Electrons Show the valence electrons for a main-group element using a LEWIS DOT STRUCTURE 15 Li Lithium Group 1A 1 valence electron C C Carbon Group 4A 4 valence electrons O O Oxygen Group 6A 6 valence electrons

16 Lewis Dot Structures Dot number is important, positions are not 16 N Nitrogen Group 5A 5 valence electrons N N N

17 Lewis Dot Structures All the atoms in the same family have the same dot structure 17 Halogens Group 7A 7 valence electrons F Cl Br I

18 Lets Practice… Draw the Lewis Dot Structure for: – Si – Rb – As – Br

19 Chemical Bonding The stability of the noble gases suggests that for main group elements, 19 Atoms form chemical bonds by losing, gaining, or sharing valence electrons in order to achieve the same number of valence electrons as the nearest noble gas

20 Octet Rule Noble gases have 8 valence shell electrons (except He) – Atoms lose, gain, or share valence electrons in order to achieve 8 valence shell electrons – This is the OCTET RULE Atoms near He (H and Li) will try to achieve just 2 valence shell electrons 20

21 Lets Practice… Octet rule predicts that an atom of Flourine will try to _____(gain/lose) _____(1,2,3,4) electron(s) when forming a compound. Octet rule predicts that an atom of Calcium will try to _____(gain/lose) _____ (1,2,3,4) electron(s)when forming a compound.

22 Three Types of Chemical Bonding Chemical Bonds can be classified into 3 broad categories, based on how valence electrons are arranged: AtomsBondCharacterProperties Metal + nonmetal ionice - transferredForms a salt= crystal of cations and anions Brittle Solid does not conduct electricity, but conducts electricity when melted or dissolved in water Nonmetal + nonmetal covalente - sharedComposed of neutral molecules Brittle Does not conduct electricity in any state (solid, liquid, or dissovled in water) Metal + metal metallice - pooledLattice of cations embedded in sea of electrons Malleable Solid and liquid both conduct electricity

23 23 Ionic bonds Ionic bonds form between metals and nonmetals – Metal gives up electrons – Nonmetal accepts electrons – Ions form – Because of opposite charges, ions stick together to form a crystal – Resulting compound is a SALT NaCl, salt

24 24 Ionic bonds: electron transfer Metals transfer electrons to nonmetals to form ions: – Na has 1 valence e –, wants to lose it to attain a Ne core – Cl has 7 valence e –, wants to gain 1 to attain an octet, like Ar Solution is to transfer 1 electron from Na to Cl – Electron transfer forms ions – Ions stick together because of opposite charges Cl Na Cl 1– Na 1+

25 25 Dot structures for ionic compounds Draw dot structures for these ionic compounds – Show the atoms before and ions after e – transfer – Use an arrow to show e – transfer – Show charges on ions after e – transfer – Write ions near each other but not together (they are not sharing the electrons) MgONa 2 OCaCl 2

26 26 Electronegativity Electronegativity is the ability to attract bond e – The higher the EN, the greedier the atom

27 27 Electronegativity Electronegativity increases across a period and decreases down a group

28 28 Covalent bonds: electron sharing Nonmetals share electrons to form covalent bonds: – Each Cl has 7 valence e –, wants to gain 1 e – to get an octet like Ar – Neither atom is willing to give up an electron – Solution is to share their unpaired electrons – The shared pair of e – is a single covalent bond Cl Cl Cl Cl

29 29 Covalent bonds Covalent bonds form between nonmetals – Nonmetals share unpairedvalence electrons – Each atom owns all of its bond electrons – Each atom achieves an octet (or 2, for hydrogen) – One shared pair of electrons is shown with a single line Cl Cl or Cl Cl ––

30 30 Bonding in O 2 Oxygen is a diatomic element – Each unpaired electron must get into a bond – The atoms cannot achieve a filled valence level with a single bond, so a double bond forms: they share two pairs of electrons O O O O O O or =

31 31 How to draw a dot structure Count valence e – Put atom with lowest EN in center Arrange other atoms around it symmetrically Form single bonds between atoms (1 line = 2 e – ) Put lone pairs around terminal atoms to give each an octet (2 for H), then finish central atom octet If central atom does not get octet, move in lone pairs to make double or triple bonds

32 32 Lewis dot structures Draw a Lewis dot structure for each species – NH 3 HBrCO 2 – OH 1– NH 4 1+ HCN – H 2 CONO 3 1– PF 3 – SO 2 C 2 H 4 Cl 2 O

33 33 How equally do atoms share e – ? Hydrogen and fluorine share one pair of e – in a single covalent bond In the Lewis dot structure, they appear to share the electrons equally, but do they?

34 34 How equally do atoms share e – ? When you put HF in an electric field, the molecules line up, as if the F end were negative and the H end positive. The electrons are shared unequally.

35 35 How equally do atoms share e – ? When bond e – are shared unequally, the bond is said to be a polar covalent bond. A polar covalent bond has a dipole: one end is more negative than the other end

36 36 Electronegativity and bond polarity Bond polarity depends on the difference in EN

37 37 Evaluating bond type H–Br – H = 2.1, Br = 2.8 – EN = 0.7 – Bond is polar covalent with Br end more negative H–Br –

38 38 Evaluating bond type C = O – C = 2.5, O = 3.5 – EN = 1.0 – Bond is polar covalent with O end more negative – Doesnt matter whether bond is single or double C=O –

39 39 Evaluating bond type KCl – K = 0.8, Cl = 3.0 – EN = 2.2 – Bond is ionic; e – transferred from K to Cl K 1+ Cl 1–

40 40 Metallic bonding: a sea of electrons Lattice of positive metal cations (nucleus + core e – ) Lattice surrounded by sea of mobile valence e – – Valence e – can move => metals conduct electricty – Cations can slide past each other without breaking bonds => malleable

41 41 Metal alloys Alloys are mixtures that contain at least one metal – Examples: brass, bronze, stainless steel – Alloys usually stronger and harder than the pure metal – Alloys are not compounds because they do not have a definite composition Table salt is a compound of 1 Na + 1 Cl = NaCl Water is a compound of 2 H + 1 O = H 2 O Brass is a mixture of Cu and Zn in any proportions

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