Chemistry Chapter 3 Atoms: The Building Blocks of Matter

Atom who? Atom The smallest particle of an element that retains the chemical properties of that element

Law of Conservation of Mass Mass is neither created nor destroyed during chemical or physical reactions. Total mass of reactants = Total mass of products Antoine Lavoisier

Dalton’s Atomic Theory (1808) All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties John Dalton Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged

Modern Atomic Theory Several changes have been made to Dalton’s theory. Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element.

Modern Atomic Theory Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Some Modern Cathode Ray Tubes

Mass of the Electron 1909 – Robert Millikan determines the mass of the electron. Mass of the electron is 9.109 x 10-31 kg and the charge is 1.67 x 10-19 C An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron. He also determined that there was a smallest 'unit' charge, or that charge is 'quantized'. He received the Nobel Prize for his work. We're going to explain that experiment here, and show how Millikan was able to determine the size of a charge on a single electron. What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 x 10 -19 C, the charge on a single electron. This meant that it was electrons carrying this unit charge. Here's how it worked. Have a look at the apparatus he used: An atomizer sprayed a fine mist of oil droplets into the chamber. Some of these tiny droplets fell through a hole in the upper floor. Millikan first let them fall until they reached terminal velocity. Using the microscope, he measured their terminal velocity, and by use of a formula, calculated the mass of each oil drop. Next, Millikan applied a charge to the falling drops by illuminating the bottom chamber with x-rays. This caused the air to become ionized, and electrons to attach themselves to the oil drops. By attaching a battery to the plates above and below this bottom chamber, he was able to apply an electric voltage. The electric field produced in the bottom chamber by this voltage would act on the charged oil drops; if the voltage was just right, the electromagnetic force would just balance the force of gravity on a drop, and the drop would hang suspended in mid-air. Now you try it. Click here to open a simulation of Millikan's chamber. First, allow the drops to fall. Notice how they accelerate at first, due to gravity. But quickly, air resistance causes them to reach terminal velocity. Now focus on a single falling drop, and adjust the electric field upwards until the drop remains suspended in mid-air. At that instant, for that drop, the electric force on it exactly equals the force of gravity on it. Some drops have more electrons than others, so will require a higher force to stop. When you've finished playing with the apparatus, close the window and we'll continue. O.K., let's look at the calculation Millikan was now able to do. When a drop is suspended, its weight  m · g is exactly equal to the electric force applied  q · E The values of  E, the applied electric field,  m the mass of a drop, and  g, tha acceleration due to gravity, are all known values. So you can solve for  q, the charge on the drop: Millikan determined the charge on a drop. Then he redid the experiment numerous times, each time varying the strength of the x-rays ionizing the air, so that differing numbers of electrons would jump onto the oil molecules each time. He obtained various values for  q. The charge  q on a drop was always a multiple of -1.6 x 10 -19 C,  the charge on a single electron. This number was the one Millikan was looking for, and it also showed that the value was quantized; the smallest unit of charge was this amount, and it was the charge on a single electron. The oil drop apparatus

Millikan's Oil Drop Experiment

Conclusions from the Study of the Electron Electrons are negative. Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass

Rutherford’s Gold Foil Experiment Alpha particles are positively charged Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Gold Foil Experiment

Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged

The Structure of the Atom Atoms consist of two regions Nucleus Very small region in the center. Contains protons & neutrons. Electrons Cloud Mainly empty space. Very large compared to the nucleus. Contains electrons. Subatomic particles Protons, neutrons, and electrons

Atomic Particles Particle Charge Mass (kg) Location Electron -1 9.109 x 10-31 Electron cloud Proton +1 1.673 x 10-27 Nucleus Neutron 1.675 x 10-27

Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Identifies the atom. Element # of protons Atomic # (Z) Carbon 6 Phosphorus 15 Gold 79

Mass Number Mass # = p+ + n0 Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p+ + n0 Nuclide p+ n0 e- Mass # Oxygen - 10 - 33 42 - 31 15 18 8 8 18 Arsenic 75 33 75 Phosphorus 16 15 31

Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons

Isotopes…Again (must be on the test) Hydrogen-2 (deuterium) Isotopes are atoms of the same element having different masses due to varying numbers of neutrons. Isotope Protons Electrons Neutrons Nucleus Hydrogen–1 (protium) 1 Hydrogen-2 (deuterium) Hydrogen-3 (tritium) 2

Atomic Masses Carbon = 12.0125 amu Atomic mass is the average of all the naturally isotopes of that element. On Periodic Table Carbon = 12.0125 amu Isotope Symbol nucleus % in nature Carbon-12 12C 6 protons 6 neutrons 98.89% Carbon-13 13C 7 neutrons 1.11% Carbon-14 14C 8 neutrons <0.01%

Writing Nuclear Symbols 3 He Mass # (proton + neutrons) Atomic Symbol Atomic # (proton) 2 How many protons, electrons, and neutrons? 2 protons, 2 electrons, 1 neutron Mass # - Atomic # = # Neutrons

Writing Isotopes Using Hyphen Notation Uranium-235, Helium-3, or Carbon-14 How many proton, electrons, neutrons? Name of atom Mass # 92 protons, 143 neutrons, 92 electrons

Convert these hyphen notation to nuclear symbols. Isotope problems Convert these hyphen notation to nuclear symbols. Uranium-235, Helium-3, or Carbon-14 235 U 3 He 14 C 92 2 6

Chapter 22 – Nuclear Chemistry

The Nucleus Contains nucleons Nuclear forces Protons & Neutrons Short-range proton-neutron, proton-proton, and neutron-neutron forces hold the nuclear particles together.

Nuclear Stability Kinetic Stability Describes the probability that a nucleus will decompose (radioactive decay)

Nuclear Stability Decay will occur in such a way as to return a nucleus to the band (line) of stability.

Number of Stable Nuclides Related to Numbers of Protons and Neurons

Types of Radioactive Decay 4 2+ alpha production (a): helium nucleus beta production (b): He 2 e - 1

Alpha Radiation Limited to VERY large nucleii.

Beta Radiation Converts a neutron into a proton.

Types of Radioactive Decay gamma ray production (g): positron production : electron capture: (inner-orbital electron is captured by the nucleus) e 1

Types of Radiation

Half-Life of Nuclear Decay

The Decay of a 10.0-g Sample of Strontium-90 Over Time

QUESTION

ANSWER 2) Sc Section 18.1 Nuclear Stability and Radioactive 46 21 Sc Section 18.1 Nuclear Stability and Radioactive Decay (p. 841) Make sure to memorize the abbreviations for the subatomic particles. HMCLASS PREP: Table 18.2

QUESTION

ANSWER 2) Th Section 18.1 Nuclear Sta bility and Radioactive 234 90 Th Section 18.1 Nuclear Sta bility and Radioactive Decay (p. 841) Just as chemical equations need the same number of each type of atom on each side, nuclear equations need the same number of each type of nucleon on each side.

QUESTION

ANSWER 2) b Section 18.1 Nuclear Stability and Radioactive + 2) b Section 18.1 Nuclear Stability and Radioactive Decay (p. 841) According to the band of stability graph (Figure 18.1) this nuclide is neutron poor, so it must do something to decrease the number of protons or increase the number of neutrons. -

QUESTION

ANSWER 5) b Section 18. 1 Nuclear Stability and Radioactive – 5) b Section 18. 1 Nuclear Stability and Radioactive Decay (p. 841) This process is the opposite of positron emission and allows the change of a neutron into a proton.

QUESTION

ANSWER 3) 4 Section 18.2 The Kinetics of Radioactive Decay (p. 846) 1 ® ½ ¼ 1/8 1/16 Each arrow indicates a half life of 1.0 hour. ® ® ® . -