Chemistry Chapter 4 Atoms: The Building Blocks of Matter

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 http://www.greatscientists.net/antoine-lavoisier/

Proust’s Law of Definite Proportions A given compound always contains the same proportions (by mass) of elements regardless of the source For example water H2O must Always have 8 g oxygen Combining with 1 g Hydrogen Hydrogen Peroxide H2O2 must Have 16 g Oxygen combining with 1 g Hydrogen

Law of Multiple Proportions In compounds atoms combine in simple whole number ratios. The mass ratios will be different for different compounds

Law of Definite Proportions H2O 2 atoms H to 1 atom O 2 gram H to 8 gram O 20% Hydrogen by mass and 80% Oxygen by mass Law of Multiple Proportions H2O 2 atoms H to 1 atom O 2 gram H to 8 gram O H2O2 2 atoms H to 2 atoms O 2 gram H to 32 grams O

If it takes 1. 000 g of S for every 1 If it takes 1.000 g of S for every 1.188 g F to make a compound SF what must be the mass ratio to make the compound SF2 ? A: 1.000 g S to 2.376 g F b) What must be the mass ratio for SF3 ? A: 1.000 g S to 3.564 g F If it takes 14.000g N to combine with 16.000g O to form NO, what is the mass ratio for NO2 ? A: 14.000g N to 32.000 g O

It takes 24. 000g C to combine with 4 It takes 24.000g C to combine with 4.000g H to form C2H4 what must be the mass of Hydrogen to combine with 72.000g Carbon? A; 12.000g H Hydrogen sulfide (H2S) is composed of two elements: hydrogen and sulfur. In an experiment, 6.500 g of hydrogen sulfide is fully decomposed into its elements. If 0.384 g of hydrogen is obtained in this experiment, how many grams of sulfur must be obtained? What law of chemical behavior does this experiment demonstrate? A: 6.500g- 0.384g= 6.116g S Law of conservation of mass

Assume that we used 5. 000g of Iron to heat up with 10 Assume that we used 5.000g of Iron to heat up with 10.000 g sulfur to make our pyrite (Fool’S gold). What is the mass of pyrite made? A; 15.000 grams pyrite What law does this illustrate? Law of conservation of mass

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

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce that the electrons are the subatomic negatively charged particles carrying electricity in a cathode ray tube. Thompson measured the mass of cathode rays, showing they were made of particles that were around 1800 times lighter than the lightest atom, Hydrogen Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Some Modern Cathode Ray Tubes

Conclusions from the Study of the Electron 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

Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Rutherford’s Gold Foil Experiment Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded http://www.youtube.com/watch?v=5pZj0u_XMb

Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The Answers Target #1 Target #2

Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like bullets bouncing off of tissue paper!” Conclusions: The nucleus size is small compared to the atomic size The nucleus is dense The nucleus is positively charged

The Atomic Scale Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space “q” is a particle called a “quark”

Atomic Mass Scale 1 atomic mass unit (u) has a mass of 1/12 of the mass of carbon-12 atom or 1.66054 *10-24g (1g= 6.02214*1023 u) Particle Charge Mass (u) Proton Positive (1+) 1.0073 Neutron None (neutral) 1.0087 Electron Negative (1-) 5.486*10-4

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

Mass Number Nuclide p+ n0 e- Mass # 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

Charged atoms and is not Included for neutral atoms THERE ARE TWO WAYS TO REPRESENT THE DIFFERENT TYPES OF ISOTOPES Mass number Or H-1 Atomic number Mass number Charge (is included for ions Charged atoms and is not Included for neutral atoms

An atom becomes an ion when it loses or gains electrons An atom becomes an ion when it loses or gains electrons. Now the electrons and protons are note balanced. -3e- 13p 13p 13p 13e 10e

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.

Average Atomic Mass (Atomic Weight) Average atomic mass is the average of all the naturally isotopes of that element Atomic mass= ∑ (mass number ∙ % abundance)

Composition of the nucleus Example Find the average atomic mass of Carbon. Isotope Symbol Composition of the 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% AAM= (.9889*12)+ (.0111*13)+ (.0001*14)= AAM (Carbon)= 12.011

Modern Atomic Theory #2 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!

Mass spectrometer The MS works by ionizing chemical compounds to generate charged molecules or molecule fragments and measurement of their mass to charge ratios