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10/15/15 Chapter 3 Atoms: The Building Blocks of Matter This power point condenses the main ideas in Chapter 3 of your textbook. Don’t forget that you can get a broader explanation of these topics by reading the chapter! Chapter 3 Atoms: the building blocks of matter Bellwork: 1. Pick up 2 handouts, 1 piece of colored paper, and 1 pair of scissors. 2. Cut out the title “The Evolution of the Atom” and the 6 atom models. Examine the models closely. 3. Lay the colored paper “landscape”. Place the title at the top of the page. Then arrange the models in the order in which you think they were developed. 1 10/15/15 Chapter 3 Atoms: the building blocks of matter Evolution of the Atom 1. While you watch the video: ! Rearrange your models, if necessary ! Note the NAMES, DATES, and BIG IDEAS on your notebook paper. 2. Glue the models to the colored paper in the correct order. Write the info you collected under/next to each model. Chapter 3 Atoms: the building blocks of matter Atoms " How did scientists recognize the existence of something so small? " How small? ! 1 g of H gas contains 6.022 x 1023 atoms (six hundred thousand billion-billion) ! This is the number of: " popcorn kernels needed to cover the U.S. 9 miles deep " cupfuls of water in Pacific Ocean " stacked soft drink cans that would cover the Earth 200 miles deep 2 10/15/15 Chapter 3 Atoms: the building blocks of matter First ideas about matter date back to Greek philosophers (not scientists) " ! Democritus (400 BC) developed an idea that matter is made of indivisible particles he called atoms; (atomos = uncuttable) ! Aristotle (350 BC) disagreed with Democritus; he believed that matter was continuous and that all matter was made of 4 elements (Earth, Fire, Water, Air) ! Neither of them had experimental methods to prove their ideas ! Aristotle’s ideas prevailed for the next 2000 years Chapter 3 Atoms: the building blocks of matter " By the late 1700’s, scientists had gathered evidence favoring the idea that matter is made of atoms. ! Evidence: law of conservation of mass – mass is neither created nor destroyed during ordinary chemical reactions or physical changes " law of definite proportions – a chemical compound always contains the same elements in exactly the same proportion regardless of how much of the compound you have or where it came from " law of multiple proportions – if different compounds are made of the same 2 elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers " 3 10/15/15 Chapter 3 Atoms: the building blocks of matter Law of Conservation of Mass Chapter 3 Atoms: the building blocks of matter Law of Multiple Proportions 4 10/15/15 Chapter 3 Atoms: the building blocks of matter In 1808, John Dalton proposed an explanation for the three laws in his Atomic Theory: Dalton’s Atomic Theory 1. All matter is composed of extremely small particles called atoms. Atoms can’t be subdivided, created, or destroyed. 2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. 3. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 4. In chemical reactions, atoms are combined, separated, or rearranged. Chapter 3 Atoms: the building blocks of matter Modern Atomic Theory Not all aspects of Dalton’s atomic theory have proven to be correct. We now know that: Atoms are divisible into even smaller particles. A given element can have atoms with different masses. • Some aspects of Dalton’s theory remain unchanged. All matter is composed of atoms. • Atoms of any one element differ in properties from atoms of another element. 5 10/15/15 Chapter 3 Atoms: the building blocks of matter Modern Atomic Theory " All atoms consist of two regions: ! Nucleus – tiny, very dense, positively charged region made up of protons (+) and neutrons (o) ! Electron cloud – region occupied by extremely tiny, negatively charged electrons " But how did we discover this????? Chapter 3 Atoms: the building blocks of matter Discovery of the Electron " Cathode Rays and Electrons Experiments in the late 1800s showed that cathode rays had mass and probably had a negative charge. 6 10/15/15 Chapter 3 Atoms: the building blocks of matter Discovery of the Electron " Charge and Mass of the Electron In 1897, JJ Thomson’s cathode-ray tube experiments measured the charge-to-mass ratio of an electron. He was able to conclude that all cathode rays are made of identical negatively charged particles. These particles were later named electrons. Chapter 3 Atoms: the building blocks of matter 7 10/15/15 Chapter 3 Atoms: the building blocks of matter Discovery of the Electron, continued " Charge and Mass of the Electron Once the charge-to-mass ratio was known (Thomson), scientists could measure either the charge or the mass of one electron which would also yield the other. In 1909, Robert A. Millikan’s oil drop experiment measured the charge of one electron. With this information, scientists were then able to determine the mass of one electron. Chapter 3 Atoms: the building blocks of matter Discovery of the Atomic Nucleus • More detail of the atom’s structure was provided in 1911 by Ernest Rutherford and his associates Hans Geiger and Ernest Marsden. • The results of their gold foil experiment led to the following major conclusions: • The atom is mostly empty space. • Atoms contain a very dense, tiny, positively charged bundle of matter at the center. Rutherford called this positive bundle of matter the nucleus. Later, in 1919, Rutherford discovered the proton. • 8 10/15/15 Chapter 3 Atoms: the building blocks of matter Gold Foil Experiment Chapter 3 Atoms: the building blocks of matter Gold Foil Experiment on the Atomic Level 9 10/15/15 Chapter 3 Atoms: the building blocks of matter Rutherford’s model of the atom Chapter 3 Atoms: the building blocks of matter In 1913, physicist Niels Bohr improved on Rutherford’s model by proposing that electrons can circle the nucleus only in allowed paths he called orbits. This model of the atom was called the Bohr model. 10 10/15/15 Chapter 3 Atoms: the building blocks of matter In the 1920’s, Erwin Schrodinger further refined the model of the atom. He proposed that electrons exist in three-dimensional regions around the nucleus that indicate the probable location of an electron. He called these regions “orbitals”. Chapter 3 Atoms: the building blocks of matter In 1932, British physicist James Chadwick discovered the neutron. The neutron is a neutral (no charge) particle that is located in the nucleus of an atom. 11 10/15/15 Chapter 3 Atoms: the building blocks of matter Chapter 3 Atoms: the building blocks of matter Modern Atomic Model: the Quantum Mechanical Model • • • • • • • Except for the nucleus of the simplest type of hydrogen atom, all atomic nuclei are made of protons and neutrons. A proton has a positive charge equal in magnitude to the negative charge of an electron. Atoms are electrically neutral because they contain equal numbers of protons and electrons. A neutron is electrically neutral. The nuclei of atoms of different elements differ in their number of protons and therefore in the amount of positive charge they possess. Thus, the number of protons determines that atom’s identity. Electrons exist in the electron cloud surrounding the nucleus. 12 10/15/15 Chapter 3 Atoms: the building blocks of matter Forces in the Nucleus When two protons are extremely close to each other, there is a strong attraction between them. A similar attraction exists when neutrons are very close to each other or when protons and neutrons are very close together. The short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces. Chapter 3 Atoms: the building blocks of matter The Sizes of Atoms • The radius of an atom is the distance from the center of the nucleus to the outer portion of its electron cloud. • Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms. • This unit is the picometer, pm. 13 10/15/15 Chapter 3 Atoms: the building blocks of matter Properties of Subatomic Particles Chapter 3 Atoms: the building blocks of matter Atomic Number • Atoms of different elements have different numbers of protons. • Atoms of the same element all have the same number of protons. • The atomic number (Z) of an element is the number of protons of each atom of that element. 14 10/15/15 Chapter 3 Atoms: the building blocks of matter Atomic Number Chapter 3 Atoms: the building blocks of matter Isotopes • Isotopes are atoms of the same element that have different masses. • The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. • Most of the elements consist of mixtures of isotopes. 15 10/15/15 Chapter 3 Atoms: the building blocks of matter Mass Number • The mass number is the total number of protons and neutrons that make up the nucleus of an isotope. Chapter 3 Atoms: the building blocks of matter Mass Number 16 10/15/15 Chapter 3 Atoms: the building blocks of matter Designating Isotopes • Hyphen notation: The mass number is written with a hyphen after the name (or symbol) of the element. • uranium-235 • U-235 • Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number. 235 92 U Chapter 3 Atoms: the building blocks of matter Designating Isotopes • The number of neutrons is found by subtracting the atomic number from the mass number. " " • mass number − atomic number = number of neutrons 235 (protons + neutrons) − 92 protons = 143 neutrons Nuclide is a general term for a specific isotope of an element. 17 10/15/15 Chapter 3 Atoms: the building blocks of matter Designating Isotopes Sample Problem: How many protons, electrons, and neutrons are there in an atom of chlorine-37? Solution: atomic number = number of protons = number of electrons mass number = number of neutrons + number of protons An atom of chlorine-37 is made up of 17 protons, 17 electrons, and 20 neutrons. Chapter 3 Atoms: the building blocks of matter Atomic Mass • • • • • • The mass of any atom comes from the protons and neutrons in the nucleus (electrons have negligible mass). The mass of one atom in grams is extremely small. • One atom of oxygen weighs 2.657 x 10-23 g So scientists came up with a different unit for measuring the mass of atoms. One atomic mass unit, or 1 amu, is exactly 1/12 the mass of a carbon-12 atom. Since the mass of a carbon-12 atom comes from the 12 particles in the nucleus (protons + neutrons), 1 amu is generally the mass of 1 proton or 1 neutron. Therefore the atomic mass of any atom is its mass number in amu’s. 18 10/15/15 Chapter 3 Atoms: the building blocks of matter Average Atomic Masses of Elements Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element. " Calculating • Average Atomic Mass The average atomic mass of an element depends on both the mass and the relative abundance of each of the element’s isotopes. Chapter 3 Atoms: the building blocks of matter Average Atomic Masses of Elements " Calculating Average Atomic Mass Copper consists of: • 69.15% copper-63, which has a mass of 63 amu • 30.85% copper-65, which has a mass of 65 amu The average atomic mass of copper can be calculated by multiplying the mass of each isotope by its relative abundance (expressed in decimal form) and then adding the results. 19 10/15/15 Chapter 3 Atoms: the building blocks of matter Average Atomic Masses of Elements " Calculating Average Atomic Mass, (0.6915 × 63 amu) + (0.3085 × 65 amu) = 63.62 amu The average atomic mass of copper is 63.62 amu. Chapter 3 Atoms: the building blocks of matter Relating Mass to Numbers of Atoms " The • • • • • Mole The mole (mol) is the SI unit for amount of substance Mole is a counting unit similar to a dozen One mole of anything = 6.022 x 1023 of those things (just like a dozen of anything = 12 of those things) The number 6.022 x 1023 is called Avogadro’s number Fun fact: • If every person living on Earth worked to count the atoms in one mole of an element, and if each person counted continuously at a rate of one atom per second, it would take about 4 million years for all the atoms to be counted. THAT’S A LOT OF ATOMS!! 20 10/15/15 Chapter 3 Atoms: the building blocks of matter Relating Mass to Numbers of Atoms 1 mole of 6.022 x 1023 atoms atomic mass in = = any element of that element grams Examples: 1 mole of oxygen = 6.022 x 1023 atoms of oxygen = 16.00 g of O 1 mole of nitrogen = 6.022 x 1023 atoms of nitrogen = 14.00 g of N 1 mole of chlorine = 6.022 x 1023 atoms of chlorine = 35.45 g of Cl Chapter 3 Atoms: the building blocks of matter " Molar Mass The atomic mass in grams of any element is called the molar mass (mass of one mole). Molar mass is written in units of g/mol. Examples: The molar mass of oxygen = 16.00 g/mol The molar mass of nitrogen = 14.00 g/mol The molar mass of chlorine = 35.45 g/mol 21 10/15/15 Chapter 3 Atoms: the building blocks of matter So what’s the difference between atomic mass and molar mass?? Chapter 3 Atoms: the building blocks of matter Atomic Mass Mass of one atom Molar Mass Mass of one MOLE of atoms Numerically equivalent Unit: amu Unit: g/mol 22 10/15/15 Chapter 3 Atoms: the building blocks of matter Relating Mass to Numbers of Atoms 1 mole of any element 23 = 6.022 x 10 atoms = molar mass of that element The relationship between moles, atoms, and molar mass makes them useful as conversion factors: Chapter 3 Atoms: the building blocks of matter Solving Mole Problems 23 10/15/15 Chapter 3 Atoms: the building blocks of matter Relating Mass to Numbers of Atoms " " Sample Problem ! What is the mass in grams of 3.50 mol of the element copper, Cu? Solution ! Given: 3.50 mol Cu ! Unknown: mass of Cu (g) ! Conversion factors to use: Chapter 3 Atoms: the building blocks of matter A chemical reaction produced 11.9 g of Al. How many moles of Al were produced? 11.9 g Al × 1 mol Al = 0.441 mol Al 26.98 g Al 24 10/15/15 Chapter 3 Atoms: the building blocks of matter How many moles of Ag are in 3.01 x 1023 atoms of Ag? Chapter 3 Atoms: the building blocks of matter What is the mass in grams of 1.20 x 1023 atoms of Cu? 25