FSN 1500 Week 3

1. FSN 1500 Week 3 Atoms, Elements and Chemical Bonding: An Introduction to Chemistry

2. Introduction • Why is chemistry often called the “central science”? • An understanding of chemistry enables us to better understand ourselves and the surrounding universe • The chemistry you’ll review today will be a component of multiple other topics we discuss this semester

3. Atoms and Elements • Atom - smallest particle of matter that retains all the properties of a particular chemical element • Element - basic chemical building block of matter; can’t be altered into other substances by chemical or ordinary physical means • Presently 118 chemical elements identified (114 formally named); typically 92 elements are identified as naturally occurring

4. Partial Periodic Table of Elements • Under what extraordinary physical circumstances can atoms be altered (even split apart)?

5. Atoms and Elements • The most powerful electronic microscopes (Scanning Tunneling Microscope) can “see” the external surfaces of some atoms • Atoms can be thought of as spherical bodies having approximate diameters of one ten-billionth of a meter 37 Xenon atoms arranged into the IBM logo

6. Subatomic Particles and Atom Structure • All atoms consist of smaller particles of matter called subatomic particles; subatomic particles bear none of the characteristic properties of any chemical element • Perhaps as many as 300 different subatomic particles; conceptually much chemistry can be explained by knowledge of three subatomic particle types

7. Subatomic Particles and Atom Structure • How do we deduce the atom’s structure, and the existence of subatomic particles and their properties since we can’t see them? • Ingenious experimentation provides the answers! (e.g., closed can analogy, and the 1909 Rutherford Gold Foil Experiment) The Rutherford Gold Foil Experiment deduced the existence of the atomic nucleus and its extraordinary density

8. Subatomic Particles • The masses and electrical charges of subatomic particles are conventional expressions of very small actual values • The atomic mass unit (amu) is the conventional expression of the approximate mass of one proton • The three subatomic particles we must understand: protons, neutrons and electrons

9. Subatomic Particles • Proton - positive electrical charge (1) and mass of 1 amu • Neutron - no electrical charge and mass of 1 amu • Electron - negative electrical charge (-1) and negligible mass

10. Atomic Structure • Atoms of all elements have the same basic structure; atoms of different elements have different numbers of protons, neutrons and electrons • The protons and neutrons cluster together in a dense, central core area called the nucleus • The protons and neutrons constitute nearly all the atom’s mass

11. Atomic Structure • The electrons move continuously through differently shaped clouds of space that lie at specific distances from the nucleus (see figure)

12. Crude Atom Model

13. Atomic Structure • The specific levels outside the nucleus that host the electrons are often referred to as “energy” levels (see figure) Carbon Atom Example • Why aren’t all the atom’s electrons found at one distance from the nucleus?

14. Figure 3.5A

15. Atomic Structure • The comparable sizes of the nucleus and the entire atom can be considered by thinking of a dime (nucleus) placed in the center of a major-league baseball park (the circular confines of the stadium are equivalent to the entire atom’s boundaries)

16. Ongoing Atomic Structure Research • During spring 2010, the Large Hadron Collider in Europe, the most powerful atomic accelerator in the world, began colliding ions or subatomic particles (protons) together at nearly the speed of light to further investigate the nature of the atom and matter – hoping to detect the Higgs boson particle.

18. Ongoing Atomic Research • In July, 2012 the Large Hadron Collider scientists announced they were highly confident their experiments had detected the Higgs boson and validated existence of the “Higgs field” • The way particles interact with the Higgs field determines their mass according to the Standard Model of particle physics

19. Element Symbols • All chemical elements can be represented by one, two, or three letter symbols (e.g., C = Carbon)

20. Element Symbols and Atomic Structure • In text form, a superscript to the left of the symbol and a subscript to the left of the symbol allow us to mentally reconstruct the atomic structure • Example: 23892U

21. Element Symbols and Atomic Structure • The subscript number is called the atomic number and represents the number of protons in the atom’s nucleus • The superscript number is called the atomic mass and represents the sum of the neutrons and the protons in the nucleus • E.G., 23892U

22. Element Symbols and Atomic Structure • Electrically neutral atoms contain the same number of protons and electrons • Example: What is the name of the element and the number of protons, neutrons and electrons represented by the symbol 16 8O?

23. Chemical Compounds • Compound - a substance composed of two or more atoms chemically combined in definite proportions (e.g., water - H2O) • Why do H and O (or any other elements) combine to form water? The answer lies within the atom structure • There is a maximum number of electrons that can occupy each energy level

24. Chemical Compounds • The maximum number of electrons that can occupy an energy level is calculated from the formula 2n2 , where n equals the energy level; the energy levels are numbered consecutively outward from the nucleus beginning with the number 1 (see figures) • The energy levels are defined by the presence of electrons; no electrons, no energy level

25. What is the maximum number of e- which can reside in each energy level?

26. Chemical Compounds • Atoms that contain the maximum number of electrons in their energy levels, or eight electrons in their outermost energy level, are considered energetically stable • Energetically stable elements have no tendency to chemically react; energetically unstable elements will react to become stable (see board examples)

27. Chemical Bonding Background • Is carbon a chemically reactive or unreactive element?

28. Chemical Compounds • The mechanisms by which energetically unstable elements combine to achieve energetic stability is called chemical bonding; chemical bonding is necessary to form a chemical compound • In nature, four primary bonding mechanisms operate: ionic, covalent, metallic and Van der Waals bonding

29. Chemical Bonding Types • Ionic - involves ions; an ion is an atom or group of atoms that carries an electrical charge • Two Ion types: cation (+ charge); # of protons > # electrons ; anion (- charge); # of electrons > # protons

30. Chemical Bonding Types • Ions form when an atom structure loses or gains electrons; cations are represented with a + sign used as a superscript to the right of the chemical symbol, this is usually preceded by a numeral indicating the magnitude of the charge (e.g., Na 1+ )

31. Chemical Bonding Types • Anions are represented by a - sign used as a superscript to the right of the chemical symbol, this is usually preceded by a numeral indicating the magnitude of the charge (e.g., Cl1- ) • Ionic bonding involves the complete transfer of one or more electrons from one atom structure to another

32. Chemical Bonding Types • The atom structure that loses electron(s) becomes a cation, the atom structure that gains electron(s) becomes an anion • The oppositely charged ions are attracted to each other (by electrical forces) in the proportions needed to produce an electrically neutral combination (see figure)

33. Sodium Chlorine Sodium Chloride 18 e- 11 e- 17 e- 10 e- Ionic Bonding Example

34. Ionic Bonding Example

35. Chemical Bonding Types • Review Na + Cl = NaCl example • Important points: all ionic chemical compounds are electrically neutral, even though their constituents are electrically charged; the properties of compounds are substantially different than the properties of the compound’s constituent elements alone

36. Chemical Bonding Types • Covalent bonding - bonding mechanism involving a sharing of electrons between adjacent atom structures; the sharing is accomplished by the overlap of energy levels between the atom structures • Review water example (see figure)

37. Covalent Bonding Model

38. Chemical Bonding Types • Metallic bonding - variation of the covalent bond; more electrons present than needed to meet the energy level requirements; the “extra” electrons are free to migrate across the bonded atom structures • The mobility of electrons conveys many of the characteristic properties of metals (e.g., good conductors of electricity and heat)

39. Metallic Bond Model N Cu atoms mobile electron e- N e- N e- N N= nucleus

40. Chemical Bonding Types • Van der Waals - very weak electrical attraction between the electrons of one atom structure and the protons of another atom structure’s nucleus due to distorted energy levels and sublevels (see figure)

41. Van der Waals Bond Model Van der Waals bonds never exist exclusively in a chemical compound - one or more other bond types is always present

42. Relative Bond Strengths • On average the strength of chemical bonds is (strongest to weakest): covalent, ionic, metallic, Van der Waals • Relevance? Since energy is required to break bonds, how strongly a compound’s constituents are bonded will partially determine the stability (i.e., reactivity) of that compound • Example: what really happens when salt (NaCl) is placed into water?

43. Important Considerations • The chemical and physical properties of compounds are determined by their chemical composition, types of bonding and orientation of the bonding • Review graphite and diamond example (see figures)

46. Additional Terminology • molecule - smallest electrically uncharged unit of matter that retains all the properties of a chemical compound • A molecule has the same relationship to a compound that an atom has to what?