Chapter 3 Atomic Structure: Images of the Invisible

Electricity and the Atom Volta invented the first battery in 1800 Electric current is supplied by chemical reactions Electrolysis Splitting of compounds using electricity Electrolyte conducts electricity Electrodes –Cathode negatively charged –Anode positively charged Ions –Cation is positively charged; moves to cathode –Anion is negatively charged; moves to anode

Cathode Ray Tube Pass current through a tube at low pressure Used in TV and computer monitors  J. J. Thomson’s Experiment Used cathode ray tube to discover negatively charged particles

Cathode Rays Emitted by cathode Same thing came from different metals Electrons –Negatively charged –Found their mass-to-charge ratio –Could not determine mass or charge separately

Goldstein’s Experiment Positive ions could flow in the opposite direction –Mass dependent on gas present in the tube

Millikan’s Oil-Drop Experiment Able to determine the mass of the electron –9.1 x 10 –28 g –Would take ~1 x to make 1 gram of electrons Determined the charge on the electron

Roentgen Studied the glow caused by the cathode ray tube –Chemically treated paper to glow Even through walls! Put his hand between the rays and the paper Discovery of Radioactivity Becquerel found that uranium ores would fog photographic plates in the dark Marie and Pierre Curie isolated other elements that behaved like uranium Radioactivity – spontaneous emission of radiation from certain unstable elements

Types of Radioactivity Three commonly found types

Behavior of Radioactivity

Rutherford’s Experiment Model Explaining Rutherford’s Experiment Originally assumed all particles in an atom were evenly spread out –Cannot explain results of his experiment Needed new model

Structure of Atom Rutherford suggested: –Most of mass in nucleus –All the positive charge Nucleus: protons and neutrons –Neutrons have mass but no charge –Very small size compared to the rest of the atom The rest of the atom contains the electrons Subatomic Particles Particles smaller than the atom Number of protons in element = atomic number Element: all atoms having the same atomic number

Isotopes Atoms having the same atomic number BUT differing in number of neutrons Symbols for Isotopes Z AXAX A = mass number = number of protons + number of neutrons = number of nucleons Z = atomic number X – symbol of the element Isotopes of hydrogen 1 1H1H 1 2H2H 1 3H3H

Example 3.1 Number of Neutrons How many neutrons are there in the U nucleus? Solution Simply subtract the atomic number (number of protons) from the nucleon number (number of protons plus neutrons). A rubidium isotope has 50 neutrons in its nucleus. What is its nucleon number? Exercise 3.1B Nucleon number – atomic number = number of neutrons 235 – 92 = 143 There are 143 neutrons in the nucleus. How many neutrons are there in the Sr nucleus? Exercise 3.1A 90 38

Conceptual Example 3.3 Isotopes (a) Which of the following are isotopes of the same element? (We are using the letter X as the symbol for all elements so that the symbol will not identify the elements.) (b) Which of the five isotopes have the same number of neutrons? XXXXXXXXXX Solution a. and are isotopes of nitrogen (N). and are isotopes of carbon (C). and have the same nucleon number. The first is an isotope of oxygen, and the second is an isotope of nitrogen. and have the same nucleon number. The first is an isotope of nitrogen, and the second is an isotope of carbon X 14 7 X 14 6 X 12 6 X 16 8 X 16 7 X 14 7 X 14 6 X b. and each have eight neutrons (16 – 8 = 8 and 14 – 6 = 8, respectively) X 14 6 X Which of the following are isotopes of the same element? Exercise 3.3 XXXXXXXXXX

Flame Tests Various elements placed in a flame will change the color of the flame –Different colors present in fireworks Typically see several colors Implies something about the structure of the atom Bohr’s Explanation Light can have only discrete amounts of energy –Energy is quantized Electron can have only these values and no others

Ground and Excited States Electrons “prefer” to be in the lowest energy level –levels closest to the nucleus –Ground state Excited state –electron goes from the lowest energy level to a higher energy level Energy Levels Specified energy value for an electron Shifts to lower energy levels give rise to light

Shells Elements may have more than one electron Placed into shells –Shells numbered 1, 2, 3, … –Have 2n 2 electrons/shell How many electrons in third shell?

Example 3.4 Electron Shell Capacity What is the maximum number of electrons in the fifth shell (fifth energy level)? What is the maximum number of electrons in the sixth shell (sixth energy level)? Exercise 3.4 Solution For the fifth level, n = 5, and so we have 2 x 5 2 = 2 x 25 = 50

Filling Shells Add electrons to the lowest shell until filled, then go to the next shell

Orbitals Schrödinger’s model: probability of finding electron in a given volume –Orbitals –Electron clouds Different shapes for different types of orbitals

Subshells Each orbital can contain two electrons Orbital shape determines subshell –Can have s, p, d, f, g, … sublevels

Example 3.6 Subshell Notation Without referring to Table 3.3, use subshell notation to write out the electron configuration for (a) oxygen and (b) sulfur. What similarity of features do the electron configurations exhibit? Without referring to Table 3.3, use subshell notation to write out the electron configuration for (a) fluorine and (b) chlorine. What similarity of features do the electron configurations exhibit? Exercise 3.6 Solution a.Oxygen has eight electrons. Place them in the lowest unfilled energy sublevels. Two go into the 1s orbital and two into the 2s orbital. That leaves four electrons to be placed in the 2p subshell. The electron configuration is 1s 2 2s 2 2p 4. b.Sulfur atoms have 16 electrons each. The electron configuration is 1s 2 2s 2 2p 6 3s 2 3p 4. Note that the total of the superscripts is 16 and that we have not exceeded the maximum capacity for any sublevel. Both O and S have electron configurations with four electrons in their highest energy sublevel (outermost subshell).

Electron Configuration Shells and subshells are filled from the lowest shell/subshell Electron configuration of nitrogen

To fill for an element, follow this flow chart

Electron Configurations and Periodic Table Each column is a group or family –Elements in each group have similar properties –Common groups: alkali metals, alkaline earth metals, halogens, and noble gases Each row is a period –Properties vary across each Outer Electron Configurations Valence electrons – electrons in outermost shells –Determines chemistry Elements in the same group have the same number of valence electrons Examples –Alkali metals – 1 valence electron –Alkaline earth metals – 2 valence electrons –Halogens – 7 valence electrons

Periodic Table Blocks –Correspond to different subshells –s and p block: Main group elements –d block: transition metals –f block: inner transition metals

Periodic Table Metals –Characteristic luster –Good conductors of heat and electricity –Solid at room temperature, except mercury Nonmetals –Dull in appearance –Poor conductors of heat and electricity Metalloids –Properties between the other classes