Atomic Structure & Elements (Chapter 2: p 24-42, 46-48, 50-51. Problems (all 2.--): 3, 5b, 7, 15, 17, 19, 21, 23, 25, 27, 29, 63, 69

Picture of an Atom Nucleus

Electron Cloud I. Dalton’s Atomic Theory 1. All matter is made up of atoms. 2. Atoms can neither be created nor destroyed. 3. Atoms of a particular element are alike. 4. Atoms of different elements are different from one another. 5. A chemical reaction involves either the union or separation of individual atoms. (~1808)

II. Subatomic particles Particle protons neutrons electrons

Symbol

Charge

p+ n e-

+1 0 -1

Relative Mass (amu) 1.0073 1.0087 0.00054858

Compare charges and relative mass. An amu or atomic mass unit is a convenient “relative” mass unit, because a proton and

1 amu = 1.6606 x 10-24 g.

neutron each have a mass of about 1 amu.

III. How p+, n, and e− fit together in atoms & ions A. p+ & n are bound together in the nucleus, located in the center of the atom. Note: The p+ number = ______________________. What is in the nucleus of the most common form of the lithium (Li) atom? Note: 7Li means that the sum of the p+ and n in the nucleus is 7.

Key proton neutron

1

7Li

nucleus

B. Analogy for size of nucleus relative to the whole atom: The whole atom is the size of a major league baseball park. The nucleus would be like a marble sitting out past second base. This means: 1. Nucleus: very small & dense. (Does something about the nucleus bother you?) 2. Most of the atom’s space is occupied by e−, which have very little mass. C. Electrons (e−) are found in orbitals located outside of the nucleus.

e-

1.The Bohr model (planetary?) can be represented as: (Note: This drawing is not to scale.) 2. A Li atom has 3 e−. −

3. The circles that the e are located on are called _____________ 4. Electrons (e−) in orbits farther from the nucleus are less tightly bound.

e-

e-

= the nucleus

5. Energies of e− in the different orbitals were determined by observing light emission from atoms. (Like neon lights.) Based on your previous studies, what holds the e− near the nucleus? D. Symbolism:

A ZE

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Example for carbon with 6 neutrons is 6C 1. E = elemental symbol, 2. A = the mass number (sum of the number of protons + neutrons.) 3. Z = E. Isotopes 1. Atoms that have the same number of protons (ie the same atomic number), but a different number of neutrons (ie different mass number. 2. Many atoms have isotopes, some of which are more stable than others. 3. Example: isotopes of carbon: 12C, 13C, 14C 12 C is the most abundant isotope. 14 C is radioactive. (Used in determining age of old objects.) 12

C

13

C

14

C

p+ n e− 2

4. Neutrons are thought to act as a kind of glue that holds the protons and neutrons together. Otherwise the protons would repel each other.

IV. Looking at the elemental symbols on the periodic table.

6

C

A. For example, look at carbon on the Periodic Table:

12.011

1. The atomic number is 6. 2. The atomic weight is 12.011amu and is the weight of the average C atom on earth. Remember: The atomic mass unit (amu) is a convenient unit, defined relative to a 12C atom. One atom of 12C is defined to weigh 12.0000 amu. Are the atomic mass and mass number the same thing? B. Instruments like the mass spectrometer allow chemists to make accurate determinations of the weight and abundance of the different isotopic forms of an element. C. Carbon isotopic mass & abundance data: isotope abundance (%) mass (amu) 12 C 98.89 12.00000 13 C 1.11 13.00335 source: CRC Handbook, 59th ed.

D. Qualitative Atomic Mass: The weight of the average atom should be quite close to 12 (since most of the C atoms are 12C), but a little bit above 12 (because there are some 13C atoms which weigh more than 12 amu.)

E. Quantitative Atomic Mass: Calculation of the average weight 12 13

C component: C component:

12.00000 amu × 98.89/100 = 11.8668 amu 13.00335 amu × 1.11/100 = 0.1443372 amu + average weight is 12.0111372 amu

This is close to the 12.011 amu atomic weight value in Periodic Table. The % abundance is also called the natural abundance. Do you think the natural abundance values on earth are the same as those on other planets, meteors, asteroids, etc.?

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V. Quantum mechanics (views e− as waves instead of particles) describes atomic behavior better than Bohr model. A. Bohr model only works well for H atoms. B. Using a specific mathematical approach (that requires very advanced math) gives a model with much better predictive capabilities. C. We will use results from quantum mechanics. Don’t sweat the math.

VI. Atomic orbitals (where e− hang out) A. Principal quantum numbers: 1, 2, 3, etc.. Describes levels or “shells” around the nucleus (correlates to period numbers on the periodic table) Shells Orbitals 1 (smallest shell) one 1s 2 one 2s, three 2p 3 one 3s, three 3p, five 3d 4 one 4s, three 4p, five 4d, seven 4f B. Orbital shapes: (see the Orbitron at http://winter.group.shef.ac.uk/orbitron/) 1. s orbitals are one lobed and spherical 2. p orbitals are 2 lobed and are roughly dumbbell shaped. 3. d and f orbitals have relatively complicated shapes. C. Energies You might consider that all of the orbitals of an atom (up to and beyond 7f) always exist, but only become interesting when they are occupied by e−. Orbital occupancy by e-: 1. Lowest energy orbitals (those closest to nucleus) are occupied first. 2. An orbital can only contain two e−. 3. When orbitals of equal energy (ex.: 2px, 2py, 2pz) are being filled, put one e− in each first, then add start adding additional e−.

orbital

D. We describe the orbital occupancy of an atom (or ion) by writing its electronic configuration. In this class you will do this by direct application of the Periodic Table.

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Periodic Table 1A 1 H 1.00794 2A 3A 3 4