Chemistry E18 - Electronic Structure of Atom & Periodic Table Bonding

1. This course is approximately at this level CHEMISTRYE182019 CH2 Electronic structure of atomPeriodic tableBonding Some pictures and texts were copied from www.wikipedia.com Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

2. ELECTRONIC STRUCTURE OF ATOM CH2 Electrons are arranged at different energy levels (as suggested by Niels Bohr – his model of atom consists of nucleus and electrons moving around at different orbitals and at discrete energy levels). Niels Bohr 1885-1962

3. Neon 1s2 2s2 2p6 Lowest energy level 1 Sublevel s 2 electrons 1 spherical orbit Higher energy level2 Sublevel s 2 electrons 1 orbit Sublevel p 6 electrons 3 orbits ELECTRONIC STRUCTURE OF ATOM CH2 Chemical properties of atom depend upon arrangement of electrons at different energy levels Energy sublevels s,p,d,f Principal energy level 1, 2, 3, … Capacity of sublevel s is 2 electrons, sublevel p 6, and sublevel d 10 electrons. Energy level 3 consists of 3 sublevels. Energy level 2 consists of 2 sublevels. Energy level 1 consists of 1 sublevel.

4. ELECTRONIC STRUCTURE OF ATOM CH2 Structure of energetic levels of electrons is closely related to spectra, emitted by excited atoms. This is the historical reason for naming sublevels according to the observed spectra s - sharp p - principal d - diffusive f - fundamental Examples: thermal radiation, Bremsstrahlung (decelerating of charged particle, e.g. electrons) Niels Bohr studied spectrum of hydrogen and observed 4 sharp emission/absorption lines, corresponding to transitions between different energy levels.

5. PERIODIC TABLE CH2 The Periodic Table of elements is based on the Electron shell structure. Electrons occupy discrete energy levels (1,2,...) and the i-th level has i sublevels (named s,p,d,f). Notation 3p4 means 4 electrons at energy level 3 and sublevel p. Filled sublevel s has 2, p 6, d 10 and f has 14 electrons (notice the difference of 4). Noble gases make full use of the capacity of these energy levels: He=1s2,Ne=1s2 2s2 2p6 (sum of superscripts 2+2+6 is the atomic number), Ar=1s2 2s2 2p6 3s2 3p6 ) Elements in a column of the periodic table (group) have the same configuration of electrons in the highest energy level (valence electrons) and therefore these elements exhibit similar chemical properties. E.g. all of the elements in group 13 have 3 valence electrons (B=1s22s2 2p1, Al=1s2 2s2 2p63s2 3p1,...)

6. AUFBAU PRINCIPLE CH2 describes the sequence of electrons filling increasing energetic levels. Why is it so important? Chemical properties are affected first of all by outer shell (VALENCE SHELL), by number of electrones at the highest principal energy level. At a first glance it seems to be logical that electrons will be arranged according to increasing principal energy level. Example: Potassium having atomic number 19 and therefore 19 electrons should have electronic structure (based upon the fact that energy level 1 has only one sublevel s, level 2 has sublevels s and p, level 3 has sublevels s,p,d, capacity of s-sublevel is 2, sublevel p-6, and sublevel d-10 electrons) K = 1s2 2s2 2p63s2 3p6 3d1 According to this arrangement there will be 2+6+1=9 electrons in the valence shell. Nevertheless, actual arrangement of electrons in potassium is K = 1s2 2s2 2p6 3s2 3p6 4s1 Therefore there is only one electron in the valence shell and only this electron determines chemical properties of potassium. 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p .... 7s Diagonal scheme of occupation of energy sublevels is called AUFBAU PRINCIPLE

7. VALENCE SHELL CH2 Wikipedia: The valence shell is the outermost shell of an atom. However, the truth is more complicated. The electrons that determine how an atom reacts chemically are those that travel farthest from the nucleus, that is, those with the most energy. As stated in Subshells, electrons in the inner subshells have less energy than those in outer subshells. This effect is great enough that the 3d electrons have more energy than 4s electrons, and are therefore more important in chemical reactions, hence making them valence electrons although they are not in the so-called valence shell.

8. PERIODIC TABLE CH2 Classical periodic table Rows - principal energy level, Columns - similar properties

9. IONIC - COVALENT BONDS CH2 Ionic bond is based upon the lend-borrow mechanism (one atom lends, the second atom borrows electrons). Ions are attracted by electrostatic forces. Example NaCl. Covalent bond is based upon sharing one, two or three pairs of electrons (single, double, triple covalent bonds) between atoms in a molecule. Atoms are bonded together because it is energetically most stable configuration. Example CH4 - methane. In both cases the aim is to completely fill the valence shell of all participated elements so that they become as stable as noble gases (noble gases have 2-Helium or 8-Neon, Argon,… electrons in the valence shell). Later on you will see that the difference between ionic and covalent bonds is not so sharp (depends upon the electronegativities of elements). free status of atoms (male and female ions can be freely replaced by different ions of the same sex) marital status of atoms, forming real molecule

10. IONIC - COVALENT BONDS CH2 the column 1 contains alkali metals, which have one more electron than the corresponding noble gases Non-metalsMissing electrons the column 18 contains noble gases which are inert . . MetalsSuperfluous electrons Covalent bond CH4(shared pairs) H has 1 electron in valence, C has 4 electrons in valence hydrogen needs 1 more electron to complete the valence (s-sublevel) carbon has only 4 electrons in valence shell and needs 4 more. 4 pairs of shared electrons complete valences of H, C Ionic bond NaCl (electrostatic attraction ions) Na-1 electron in valence, Cl-1 missing electron in valence sodium lends 1 electron chlorine borrows this electron (will have the same valence shell as stable Ar) Na+cationCl-anion (negative charge)

11. IONIC - COVALENT BONDS CH2 • Remark: The difference between ionic and covalent bonds is not sharp, and defined by a convention stating that the ionic bonds are characterised by difference of electronegativities of participating atoms greater than 1.7 (see next slides). • Ionic bonds are like a free sex (partners can be satisfied by any other ions) • Covalent bonds are like a conjugal (marital) sex

12. POLAR BONDS CH2 Why is water so specific substance? Why it has so high specific heat capacity, so high energy of evaporation (comparing with other molecules of a similar molecular weight)? Why is water heated so quickly in a microwave oven? And why is it partly dissociated to ions? Because it is a highly polar substance.

13. POLAR BONDS CH2 If the two covalently bonded atoms are identical (for example O2), the electrons are shared equally, the shared pair is just in the middle between the atoms, and the molecule is electrically nonpolar. On the other hand, if the atoms are different (HCl), the shared pair is pushed towards the "stronger" atom, which becomes negatively charged (Cl), and this is called a polar bond. The measure of the electron pair attraction by an atom is the electronegativity (see Pauling’s table) Polar molecule - + trend: increase in the diagonal direction Cl H Example, the electronegativity of Cl (3.16) is greater than the electronegativity of H (2.2) and this is the reason why Cl has a negative charge, and H has a positive charge in a molecule of HCl. Polar molecules exhibit similar behaviour as molecules with ionic bonds. Both molecules dissociate in water and form electrically conductive electrolytes.

14. POLAR BONDS CH2 So why is water a polar molecule? Because hydrogens are not located symmetrically with respect oxygen, but are inclined - + Electronegativity of oxygen (3.44) is much greater than hydrogen (2.2) therefore the pairs of shared electrons are shifted towards oxygen. Molecule becomes a dipole with negative charge at oxygen and positive charge at hydrogens.

15. pH – bases / acids CH2 H2O is decomposed (dissociated) to ions H+ cation (free proton) or hydronium 2H2O→(H3O)+ + (OH)- (OH)- anion Number of free protons H+ in 1 l of water [H+] is related to pH value pH = - log [H+] In 1 liter of pure water only few molecules are decomposed to ions, numerically 10-7 moles. Therefore pH=-log(10-7)=7 for water. pH < 7 acids(concentration of free protons > 10-7 mol/l) pH >7 bases(concentration of free protons < 10-7 mol/l) Example: HCl (gas) dissolved in water is hydrochloric acid. Ions H+ (cations) increase concentration of free protons, therefore pH<7.

16. Molar volume of water CH2 • In 1 l of water is 10-7 moles of cations H+ and anions (OH)- (see previous slide). But how many molecules of water (liquid) are in 1 liter? • We could proceed in different ways: • By measurement of electrical charge that is necessary to complete electrolysis of 1 l water. We could record electrical current. Or we could measure volume of produced hydrogen and oxygen (2 molecules of water are decomposed to 2 molecules of H2 and one molecule of O2). And one mol of ideal gas occupies 22.4 liters at standard state (Avogadro’s law). • By measurement of energy that is necessary for evaporation of 1 l water. We had to know energy of attraction between polar molecules of H2O. Or we could measure volume of produced steam and apply Avogadro’s law as previously. • Length of one molecule can be estimated 100 pm (see previous slide). Therefore volume of one molecule is 106 pm3. Volume of 1 l is 1 dm3=10-3 m3= 10-3 .1036 pm3=1033 pm3. Therefore in 1 liter should be 1033/106=1027 molecules of water, that is 1027/(6.1023)=1.7 kmol. • Straightforward way is to use periodic table. Molecular mass MH2O=2+16=18 g/mol. Mass of 1 l of water is 1000 g. Therefore in 1 l is 1000/18=55 moles of molecules.This is much less than calculated previously (1.7 kmol) – volume of molecule would have to be 31.106 pm3 (corresponding length 314 pm).

17. OXIDATION NUMBER CH2 Oxidation number is the electrical charge of an element if all bonds were ionic. This value is important for naming compounds (next lecture) or for decision whether some compound is realizable or not (for example KCl2 is a nonsense because a sum of oxidation numbers is not zero).

18. OXIDATION NUMBER CH2 OXIDATION NUMBERS +3 -2 -1 Variable oxidation numbers +2 +1 The oxidation number is equal to the electrical charge of an ion, or, in the case of covalent bonds, equals the charge of the "pseudoion" formed by the full attraction of the shared electron pairs to the "stronger" atoms. Oxidation number obviously depends on electronegativity of elements.

19. Summary - periodic table CH2 Elements are arranged according to their atomic number in the periodic table of elements (increasing Z in rows). The ability of elements to form a compound is determined by the number of electrons at the highest energy level (valence electrons). The elements with the same number of valence electrons and similar chemical behaviour are arranged in columns. Bonds can be ionic (attraction of cations /+/ and anions /-/), or covalent (sharing electrons by two atoms in a molecule). The oxidation number is equivalent to the electrical charge of the ion. Covalent bonds between atoms with different electronegativity are asymmetric (polar bonds). Even molecules containing polar bonds can be considered nonpolar from outside, if they are symmetric and the gravity centres of positive and negative charges coincide (e.g. CH4). H2O is a polar molecule, because the two hydrogen atoms are arranged asymmetrically. Some polar molecules dissociate in liquids to independently moving cations (+) and anions (-), e.g. 10-7 moles of H2O molecules dissociate to H+ (free proton) and (OH)- in 1 litre of neutral water. The molar concentration of protons is expressed by a pH value, pH= -log [H+] and pH=7 in neutral water. Acids increase the concentration of protons (pH<7), while bases cause a decreasing concentration of H+(pH>7).