Atomic Structure & Compounds (loosely based on Chapter 2 Sec 1 thru 4 of Jespersen 7 th ed)

1. Atomic Structure & Compounds(loosely based on Chapter 2 Sec 1 thru 4of Jespersen 7th ed) Dr. C. Yau Spring 2015 1

2. Dalton's Atomic Theory (p.7) • Matter consists of tiny particles called atoms. • In any sample of a pure element, all the atoms are identical in mass and other properties. • The atoms of different elements differ in mass and other properties. • When atoms of different elements combine they form compounds with the elements in a fixed ratio by mass. Reactions are merely rearrangement of atoms to form different compounds. • Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not themselves break apart. (?) Chemical (?) 2

3. The 3 major subatomic particles Know this well ! u (amu) = atomic mass unit (a very small unit of mass) 1 gram = 602,000,000,000,000,000,000, 000 u 3 3

4. Atomic Mass • Dalton’s atomic theory states that atoms of an element have a constant, characteristic atomic mass or atomic weightmeasured inu. • Atomic masses are based on a standard mass, that of an atom of C. • 1 atom of carbon-12 weighs 12 u exactly. • Thus 1 u = 1/12 the mass of a carbon-12 atom. This is the definition of u.

5. Isotopes • Most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses. • For example: There are 3 isotopes of hydrogen, 4 isotopes of iron and 10 isotopes of tin. • Isotopes are atoms of the same element with different masses. • Isotopes are atoms with the same #p but with different #n. • Chemically, isotopes have virtually identical properties. (There are slight differences that we will note be concerned with at this level of chemistry.) KNOW WELL

6. Atomic Notation • An element is a substance whose atoms all contain the identical number of protons, called the atomic number (Z) • Isotopes are distinguished by the mass number(A): • Atomic number, Z = number of protons • Mass number, A = (# of protons) + (# of neutrons) • Note that for atoms, A is greater than Z: the symbol is "top-heavy." • For neutral atoms, the number of electrons and protons must be equal.

7. Example: uranium-235 This indicates the mass number. From the nuclear symbol we can determine the # protons, electrons and neutrons: # protons = 92 # electrons = 92 (since no charge is shown) #neutrons = 235 – 92 = 143 235 U 92 Mass number, (protons + neutrons)  Chemical Symbol  Atomic number, (number of protons) 

8. Fill in the blanks: symbol neutrons protons electrons 60Co 81Br 36 29 29 27 27 33 35 35 46

9. This is how Fe appears in the periodic table. How many neutrons are there in Fe? Trick question! You can’t tell! How many neutrons are there in 53Fe? A) 26 B) 55 C) 27 D) none of these Ans. 53 – 26 = 27 mass number - # protons = # neutrons 26 Fe 55.85 This is NOT the atomic mass number.

10. Hydrogen has 3 isotopes: tritium deuterium protium 99.985% abundance 0.015 % abundance 0.000 % abundnace 1 proton 1 neutron 1 proton 2 neutrons 1 proton no neutron approx. mass of each atom = 1 u 2 u 3 u 10 10

11. What we call "heavy water" is D2O. (Regular water being H2O, with practically all the hydrogen atoms being protium.) One molecule of H2O weighs 18 u. One molecule of D2O weighs 20 u. Hence D2O is known as “heavy water.” Tritium is the only radioactive isotope of hydrogen. Such isotopes are referred to as a radioisotope. NOTE: The term "isotope" does NOT mean that it is radioactive. 11 11

12. Naturally occurring chlorine is a mixture of two isotopes. In every sample of this element, 75.77% of the atoms are chlorine-35 and 24.23% are chlorine-37. The measured mass of chlorine-35 is 34.9689 u and that of chlorine-37 is 36.9659 u. Calculate the average atomic mass of chlorine. 0.7577 x 34.9689 u = 26.495 u (4 sig. fig.) 0.2423 x 36.9659 u = 8.9568 u (4 sig. fig.) atomic mass = 35.4518 u (2 decimal places) 17 Cl 35.45 35.45 u This is the atomic mass or atomic weight.

13. There are 2 isotopes of element Z. The first is 56.5% in abundance and has a mass of 152.3 u. If the atomic mass is 155.5 u, what is the mass of the other isotope? Hint: Let x be the mass of the other isotope. What % in abundance is this other isotope? 159.6u = 160. u (3 sig.fig.)

14. Some Important Classifications: • A groups = representativeelements or main group elements I A = alkali metals II A = alkaline earth metals VII A = halogens VIII = noble (inert) gases • B groups =transition elements • Inner transition elements = elements 58 – 71 and 90 – 103 58 – 71 = lanthanide elements 90 – 103 = actinide elements

15. Metals, Nonmetals, And Metalloids

16. Properties Of Metals Metals... • reflect light (have metallic luster) • can be hammered or rolled into thin sheets (are malleable) andcan be drawn into a wire (are ductile) • are solids at room temperature(except Hg) • conduct electricity and heat

17. Nonmetals And Metalloids Nonmetals... • lack the properties of metals • tend to pulverize when struck with a hammer • Are non-conductors of electricity and heat • Many are gases, a few solids,and one liquid (Br2) • react with metals to form (ionic) compounds • Metalloids • have properties between metals and nonmetals

18. EXTREMELY IMPORTANT! nonmetal + nonmetal nonmetal + metal metal + metal covalentmolecules ionic compounds alloys Note: Alloys are not compounds. They are homogeneous mixtures of metals, sometimes with a small amount of nonmetals, such as C in steel.

19. 1 2 3 2 8 1 4 8 3 2 1 1 10 9 What exactly does this dot tell you? Learning Check: Count The Atoms In A Chemical Formula • Na2CO3 • (NH4)2SO4 • Mg3(PO4)2 • CuSO4•5H2O • ___Na, ___C, ___ O • ___N,___H, ___S, ____O • ___Mg, ___P, ____O • ___Cu, ___S, ___O, ___H

20. Covalent Molecules The subscripts in the formula of covalent molecules tell you exactly how many atoms of each element is present. They do not merely state a ratio. e.g. CO2 tells us the molecule contains one C atom and two O atoms.

21. Positively charged ions are called cations Negatively charged ions are called anions Subscripts in the formula always specify the smallest whole-number ratio of the ions needed to make a neutral combination (formula unit, or f.u.) It does not mean 2 Fe ions are bonded to 3 O ions. Subscripts tell us the ions are in a ratio of 2Fe to 3 O. 2 3 Fe3+ + O2- FeO 2 3 Ionic Compounds

22. The subscripts in the formula of • covalent molecules tell you the exactnumberof the atoms of each element is present. • ionic compounds tell you the ratioof the atoms of each element that is present.

23. Overview: Molecules vs. Formula Units • Electrically neutral, discrete particles called molecules. (H2O) • Neutral groups of charged particles calledformula units. (NaCl)

24. Why are ions charged? • # p+ = # e- if neutral • # p+ < # e- if negative • # p+ > # e- if positive • The number of p+ never changes when ions form. • Ions are formed when the atom gains or loses electrons. Protons are not affected. How does Ca form Ca2+? Ca lost 2 electrons How is N3- formed? N gained 3 electrons

25. Fill in the blanks: Symbol neutrons protons electrons 60Co3+ 81Br- 36 29 27 24 27 33 36 46 35

26. Summary of Properties Hardness and brittleness • Molecular compounds tend to be soft and easily crushed because the attractions between molecules are weak and molecules can slide past each other. • Ionic compounds are hard and brittle because of the strong attractions and repulsions between ions

27. Melting Points • To melt the a solid, there must be sufficient kinetic energy to overcome the attractions between particles • Molecular compounds have weak attractions between particles and so tend to have low melting points. • Many molecular compounds are gases at room temperature. • Ionic compounds tend to have strong attractions so they have high melting points. • Nearly all ionic compounds are solids at room temperature.

28. Electrical Conductivity • Requires the movement of electrical charge • Ionic compounds: • Do not conduct electricity in the solid state • Do conduct electricity in the liquid and aqueous states-The ions are free to move. • Molecular compounds: • Do not conduct electricity in any state. • Molecules are comprised of uncharged particles. • Exception: Strong acids are molecular substances that become ionic when dissolved in water.

29. Which of the following is likely true of NO2? • It conducts electricity well. • It has a low melting point. • It is likely a solid in its pure form. • None of these The formula tells you it’s a molecular compound, so it is not expected to conduct electricity, should have a relatively low mp, and therefore not likely to be a solid in its pure form. You should know how the formula tells you that it’s a molecular formula. See Slide 18!