Valency

Learning Objectives:
1. Understand what is valency
2. Learn how to define valences of elements
3. Learn normal and excited states of atoms
4. Learn how to determine chemical formula of molecules
5. Understand why atoms are grouped in molecules

1. Valency D.M K.M S.H Z.M I.S

2. Contents Introduction: Learning Objectives How can  we  know the valency of an element? Quiz Definition of valency Why do atoms bond? Normal and excited states of atoms How to define the chemical formula of a molecule?

3. Introduction: Learning Objectives 2. Learn how to define valences of elements 3. Learn normal and excited states of atoms 4. Learn how to determine chemical formula of molecules 1. Understand what is valency 5. Understand why atoms are grouped in molecules

4. Valency Property of an atom of a chemical element to form certain number of chemical bonds with atoms of other chemical elements. H О Н Na Cl O O Al F F F

5. Why do atoms bond? Stable octe t – the arrangement of electrons of a chemical element in such a way that the atom has 8 electrons in their valence shell (s and p shell of outer energy level)! 2 8 1 2 8 + 2 8 7 + 1 e – 2 8 8 – lost electrons obtains electrons Electrostatic force 2 6 6 2 2 8 8 2 share electrons bonds – 1 e – All atoms desire to have stable octet! There are 2 ways to reach this: Some elements (metals) loose their valence electrons and some (non-metals) gain. = atoms are binded Some elements (non-metals) share their electrons. O O Na Cl

6. How can we know the valency of an element? Paired electrons       Empty orbital …2 s 2 2 p 2 Unpaired electron = valency      and Group number = max valency ( except : N,O,F) Unpaied electrons All electrons on the outermost energy level are valence electrons! The last shell is al ways consists of s or s & p electrons! One of the ways to determine valency of a particular element is via its electrons configuration: 1 H : 1s 1   → 1 unpaired electron → valency is 1 → 1 bond 8 O : …2s 2 2p 4         → 2 unpaired electrons → valency is 2 → 2 bonds H О Н Valences of metals of s and p block is defined by the number of electrons at the last shell! Valences I II III Li, Na, K, Rb , Cs Be, Mg, Ca, Ba, Sr Al What about non-metals?

7. Normal and excited states of atoms A state of an atom where all electrons fill orbitals according the principle of Aufbau and Hund’s rule is normal state. Excitement can happen only if empty orbitals and paired electrons are present . ↑↓ ↑ ↓ ↑ ↓ ↑ ↑ ↑ ↑ ↑ ↑ ↑ The state when the excitement cannot happen anymore is called maximum excited state.           A state of an atom where one of paired electrons moved to empty orbitals of the same shell is excited state. Elements with only normal states of atoms are considered to be constant valency elements Valences I II H and F O Due to several states of some elements they may have several valences Elements Valences Group 15: P III and V Group 16: S II, IV, VI Group 17: Cl, Br, I I, III, V, VII

8. How to define the valency if chemical formula is known? Fe 2 O 3 x II x∙2 = II∙3 In compound X x Y y valency X intiger x = valency Y intiger y   Check-up Find unkno wn valenc es : CrO 3 CrO Mn 2 O 7 x∙1 = II ∙3 x II x = 6 x = II∙3/2 = 3 x∙1 = II ∙1 x II x = 2 x∙2 = II ∙7 x II x = 7

9. How to define chemical formula of molecule? An element with smaller electronegativity usually has maximum valency, but the element with high electronegativity has minimum. 17 X : 13 Y: … 3s 2 3p 5 = 1, 3, 5, 7 … 3s 2 3p 1 = 3 Y X III I b a III∙a = I∙b a = 1; b = 3 YX 3 looses e – obtains e – 2 8 7 +1 e – looses e – Define the formula of the compound that consist of 17 X & 13 Y. 17 X has higher electronegativity than 13 Y.

10. How to define chemical formula of more complex molecule? (PO 4 ) y   II III x ∙ II = y ∙ III The same way: In compound X x Y y valency X intiger x = valency Y intiger y   Check-up Determine following formulas : & O Н – & S 2- &   x∙ III = y ∙ I III I x = 3; y = 2 Fe 3 (PO 4 ) 2 Fe(OH) 3 x∙ I = y ∙ II I II (NH 4 ) 2 S x∙ I = y ∙ II I II (Na) 2 HPO 3 Sometimes X & Y might be polyatomic particles like with valency I, OH – with valency I and etc.  

11. Thank You!