Chapter 3 Atoms: The Building Blocks of Matter

3-1 An Ancient Idea Water: atoms smooth and round so it flowed with no permanent shape Water: atoms smooth and round so it flowed with no permanent shape Fire: atoms thorny,making burns painful Fire: atoms thorny,making burns painful Earth: atoms rough and jagged so they held together to make hard stable substances Earth: atoms rough and jagged so they held together to make hard stable substances

3-1 Opposing View Did not believe in atoms Did not believe in atoms Believed matter was continuous Believed matter was continuous Very influential Very influential Neither Democritus nor Aristotle supported their ideas with experimentation. Neither Democritus nor Aristotle supported their ideas with experimentation.

3-1 Law of Conservation of Mass Antoine Laurent Lavoisier ( ) – the father of modern chemistry Antoine Laurent Lavoisier ( ) – the father of modern chemistry Mass is neither created nor destroyed during ordinary chemical reactions or physical changes Mass is neither created nor destroyed during ordinary chemical reactions or physical changes Arrested by French Revolutionary Tribunal for his membership in the Ferme Generale and executed Arrested by French Revolutionary Tribunal for his membership in the Ferme Generale and executed

3-1 Law of Definite Proportions (Constant Composition) A chemical compound contains the same proportions by mass regardless of the size of the sample or source of the compound A chemical compound contains the same proportions by mass regardless of the size of the sample or source of the compound Sodium chloride (NaCl) is always 39.34% sodium and 60.66% chlorine Sodium chloride (NaCl) is always 39.34% sodium and 60.66% chlorine

3-1 John Dalton English schoolteacher – 1808 English schoolteacher – 1808 Proposed explanation for these laws Proposed explanation for these laws

3-1 Dalton’s Atomic Theory All matter is composed of extremely small particles called atoms. 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. Atoms of a given element are identical in size, mass and other properties. Atoms of different elements differ in size, mass and other properties. Atoms cannot be subdivided, created or destroyed. Atoms cannot be subdivided, created or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated or rearranged. In chemical reactions, atoms are combined, separated or rearranged.

3-1 Dalton and Conservation of Mass In chemical reactions, atoms are rearranged – bonds are broken and new bonds are formed. In chemical reactions, atoms are rearranged – bonds are broken and new bonds are formed. Same number of atoms of each type exist before and after the reaction. Same number of atoms of each type exist before and after the reaction. No atoms are created or destroyed. No atoms are created or destroyed.

3-1 Dalton and Definite Proportions Each kind of atom has its own mass and atoms in a compound always exist in fixed whole number ratios. Each kind of atom has its own mass and atoms in a compound always exist in fixed whole number ratios. Sodium chloride always 1:1 sodium:chlorine Sodium chloride always 1:1 sodium:chlorine Na: 23, Cl: 35.5 Na: 23, Cl: 35.5 Na: 23/58.5 *100 = 39.34% Na: 23/58.5 *100 = 39.34% Cl: 35.5/58.5 *100 = 60.66% Cl: 35.5/58.5 *100 = 60.66%

3-1 Law of Multiple Proportions If two or more different compounds are composed of the same elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a whole number ratio. If two or more different compounds are composed of the same elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a whole number ratio.

3-1 Dalton and Multiple Proportions Each kind of atom has a unique mass and atoms in compounds are combined in fixed whole number ratios. Each kind of atom has a unique mass and atoms in compounds are combined in fixed whole number ratios.

3-1 Impact of Dalton’s Atomic Theory Related Democritus’s idea of atoms to the MEASURABLE property of mass – atomic theory could then be TESTED by EXPERIMENT Related Democritus’s idea of atoms to the MEASURABLE property of mass – atomic theory could then be TESTED by EXPERIMENT Dalton’s theory has been modified over the years as new information has become available, but the fundamental principles hold true today Dalton’s theory has been modified over the years as new information has become available, but the fundamental principles hold true today

3-1 Dalton’s Atomic Model

3-2 Structure of the Atom ATOM - The smallest particle of an element that retains the chemical properties of that element. ATOM - The smallest particle of an element that retains the chemical properties of that element.

3-2 Discovery of the Electron J. J. Thomson, English physicist J. J. Thomson, English physicist Did experiments with cathode ray tubes – glass tubes containing gases at low pressure Did experiments with cathode ray tubes – glass tubes containing gases at low pressure

3-2 Cathode Ray Tubes Cathode – negative electrode, anode – positive electrode Cathode – negative electrode, anode – positive electrode When a current is passed through the tube, the end opposite the cathode glows When a current is passed through the tube, the end opposite the cathode glows Glow caused by stream of particles called cathode ray because is originated at cathode – ray traveled from cathode to anode Glow caused by stream of particles called cathode ray because is originated at cathode – ray traveled from cathode to anode

3-2 CRT Experiments Observation – a paddle wheel placed on rails between electrodes rolled along rails from cathode to anode Observation – a paddle wheel placed on rails between electrodes rolled along rails from cathode to anode Conclusion – existence of cathode ray supported, cathode ray has MASS Conclusion – existence of cathode ray supported, cathode ray has MASS

3-2 CRT Experiments Observations – cathode rays deflected by a magnetic field in the same way as a wire carrying an electric current (cathode ray acts NEGATIVE); deflected away from negatively charged objects Observations – cathode rays deflected by a magnetic field in the same way as a wire carrying an electric current (cathode ray acts NEGATIVE); deflected away from negatively charged objects Conclusion – cathode ray is made of negatively charged particles Conclusion – cathode ray is made of negatively charged particles

3-2 Charge and Mass of the Electron J. J. Thomson called these tiny negatively charged particles “electrons” J. J. Thomson called these tiny negatively charged particles “electrons” He calculated the mass to charge ratio of the electron – it is always the same no matter what metal is used for the electrodes He calculated the mass to charge ratio of the electron – it is always the same no matter what metal is used for the electrodes

3-2 Charge and Mass of the Electron Robert Millikan, American physicist Robert Millikan, American physicist Showed that mass of electron is 1/2000 the mass of the simplest known atom (hydrogen) Showed that mass of electron is 1/2000 the mass of the simplest known atom (hydrogen) x kg x kg

3-2 Important Points Atoms contain tiny negatively charged particles called electrons. Atoms contain tiny negatively charged particles called electrons. Electrons are present in atoms of all elements. Electrons are present in atoms of all elements. Atoms are divisible and one of parts is negatively charged. Atoms are divisible and one of parts is negatively charged. Because atoms are neutral, there must also be a positive component. Because atoms are neutral, there must also be a positive component. Because electrons have such small mass, atoms must contain other parts that make up most of their mass. Because electrons have such small mass, atoms must contain other parts that make up most of their mass.

3-2 The Plum Pudding Model of the Atom

3-2 Discovery of the Atomic Nucleus Ernest Rutherford, New Zealand (with Hans Geiger and Ernest Marsden) Ernest Rutherford, New Zealand (with Hans Geiger and Ernest Marsden) Important experiment providing more detail into atom’s structure. Important experiment providing more detail into atom’s structure.

3-2 The Gold Foil Experiment Thin, gold foil bombarded with alpha particles (positively charged particles with 4x mass of hydrogen atom) Thin, gold foil bombarded with alpha particles (positively charged particles with 4x mass of hydrogen atom)

3-2 The Gold Foil Experiment - Results Most particles went right through gold foil Most particles went right through gold foil A few were slightly deflected A few were slightly deflected A few bounced off the gold foil! A few bounced off the gold foil!

3-2 The Gold Foil Experiment - Conclusions Particles that pass through foil – hit nothing Particles that pass through foil – hit nothing Particles slightly deflected – come close to a positive charge Particles slightly deflected – come close to a positive charge Particles that bounce back – hitting a positive charge Particles that bounce back – hitting a positive charge

3-2 Rutherford’s Explanation The atom has a densely packed bundle of matter with a positive charge (he called it the nucleus). The atom has a densely packed bundle of matter with a positive charge (he called it the nucleus). The nucleus contains all of the positive charge and most of the mass. The nucleus contains all of the positive charge and most of the mass. The nucleus has very little volume – the atom is mostly empty space. The nucleus has very little volume – the atom is mostly empty space.

3-2 Rutherford’s Atomic Model Small nucleus in the center Small nucleus in the center Electrons orbit nucleus like planets around the Sun. Electrons orbit nucleus like planets around the Sun. Sometimes called planetary atomic model. Sometimes called planetary atomic model.

3-2 Composition of the Atomic Nucleus Nuclei contain two kinds of particles – protons and neutrons Nuclei contain two kinds of particles – protons and neutrons Electrons are outside the nucleus, in the electron cloud Electrons are outside the nucleus, in the electron cloud Table 3-1 on p. 74 Table 3-1 on p. 74 ParticleSymbol Relative Charge Mass Number Relative Mass (amu) Actual Mass (kg) electron e-e-e-e x proton p+p+p+p x neutron n0n0n0n x10 -27

3-2 Forces in the Nucleus Like charges generally repel each other BUT at very close range there is an attraction between them Like charges generally repel each other BUT at very close range there is an attraction between them Nuclear Forces – short range proton- neutron, proton-proton, and neutron- neutron forces that hold the nuclear particle together Nuclear Forces – short range proton- neutron, proton-proton, and neutron- neutron forces that hold the nuclear particle together

3-2 The Sizes of Atoms Expressed as atomic radius – distance from center of nucleus to outer edge of electron cloud Expressed as atomic radius – distance from center of nucleus to outer edge of electron cloud Measured in picometers ( m) Measured in picometers ( m) Generally range from 40 pm to 270 pm Generally range from 40 pm to 270 pm Nuclear radius is ~0.001 pm (like a dime in a football field) Nuclear radius is ~0.001 pm (like a dime in a football field)

3-3 Counting Atoms Atoms of the same element all have the same number of protons – this is the atomic number Atoms of the same element all have the same number of protons – this is the atomic number Atomic number is found on the periodic table – elements arranged in order of increasing atomic number Atomic number is found on the periodic table – elements arranged in order of increasing atomic number

3-3 Isotopes Atoms of the same element that have different masses (different numbers of neutrons) – same chemical behavior Atoms of the same element that have different masses (different numbers of neutrons) – same chemical behavior

3-3 Isotopes Most of the elements consist of mixtures of isotopes. Most of the elements consist of mixtures of isotopes. To identify an isotope, must know atomic number AND mass number (protons + neutrons) To identify an isotope, must know atomic number AND mass number (protons + neutrons) Isotopes usually identified by specifying mass number Isotopes usually identified by specifying mass number

3-3 Isotopes Nuclide – general term for any isotope of any element Hyphen Notation neon-20, uranium-235 Nuclear Symbol

3-3 Relative Atomic Mass One atom has been chosen as a standard, masses of all other atoms expressed in relation to this standard One atom has been chosen as a standard, masses of all other atoms expressed in relation to this standard Atomic mass unit – exactly 1/12 the mass of a carbon-12 atom Atomic mass unit – exactly 1/12 the mass of a carbon-12 atom Atomic mass of any other atom is determined by comparing it with the mass of carbon-12 Atomic mass of any other atom is determined by comparing it with the mass of carbon-12

3-3 Average Atomic Mass Most elements occur naturally as mixtures of isotopes Most elements occur naturally as mixtures of isotopes The percentage of each isotope in an element is constant – same no matter where sample comes from The percentage of each isotope in an element is constant – same no matter where sample comes from Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element

3-3 Calculating Average Atomic Mass Copper has 2 isotopes – copper-63 and copper-65 Copper-63: 69.17%, amu Copper-65: 30.83%, amu (.6917)( amu) + (.3083)( amu) = amu

3-3 Average Atomic Mass Atomic masses that appear on the periodic table are AVERAGE atomic masses Atomic masses that appear on the periodic table are AVERAGE atomic masses

3-3 Relating Mass to Number of Atoms THE MOLE! THE MOLE! The amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12 The amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12 The mole is a counting unit, like a dozen or a ream The mole is a counting unit, like a dozen or a ream 1 mole of something is x mole of something is x 10 23

3-3 Avogadro’s Number 12 g of carbon-12 contains exactly x atoms. 12 g of carbon-12 contains exactly x atoms. Named Avogadro’s number after Amedeo Avogadro, Italian scientist Named Avogadro’s number after Amedeo Avogadro, Italian scientist

3-3 Molar Mass A mole is the amount of a substance that contains Avogadro’s number of particles A mole is the amount of a substance that contains Avogadro’s number of particles The mass of one mole of a pure substance is called the molar mass of that substance (g/mol) The mass of one mole of a pure substance is called the molar mass of that substance (g/mol)

3-3 Molar Mass To find the molar mass of an atom, find the atomic mass in amu on the periodic table and change the unit to g/mol To find the molar mass of an atom, find the atomic mass in amu on the periodic table and change the unit to g/mol The molar mass is numerically equal to the atomic mass The molar mass is numerically equal to the atomic mass element Atomic mass (amu) Molar mass (g/mol) carbon copper iron neon

3-3 Gram/Mole Conversions Molar mass can be used as a conversion factor Molar mass can be used as a conversion factor Can convert between mass and moles for any substance Can convert between mass and moles for any substance Moles to Grams Multiply (by molar mass) Moles to Grams Multiply (by molar mass) Grams to Moles Divide (by molar mass) Grams to Moles Divide (by molar mass)

3-3 Conversions with Avogadro’s Number If amount in moles is known, can calculate number of particles (and vice versa) If amount in moles is known, can calculate number of particles (and vice versa) Moles to Particles Multiply (by Avogadro’s number) Moles to Particles Multiply (by Avogadro’s number) Particles to Moles Divide (by Avogadro’s number) Particles to Moles Divide (by Avogadro’s number)

Mole Conversions