Atoms and their structure Chapter 4 Atoms and their structure

History of the atom Not the history of atom, but really the idea of the atom The original idea - Ancient Greece (400 B.C..) Democritus and Leucippus Greek philosophers who were debating whether matter was continuous or discontinuous. Continuous – divide matter forever and always have a smaller piece of matter. Discontinuous – divide matter only so far and can go no farther.

History of Atom Start with a box of marble divide in half eventually you get down to one marble which if you divide again you no longer have a marble. The Greek word for “can not cut” is atomos – thus atom.

Another Greek Aristotle – another famous Greek philosopher All substances are made of 4 elements Fire - Hot Air - light Earth - cool, heavy Water - wet Aristotle and others believed in 4 elements of matter, combined in different proportions rather than indivisible particles

Who Was Right? Greek society was slave based and so it was beneath the famous to work with their hands. The Greeks did not experiment, they settled disagreements by argument (debates). Aristotle was more famous so his ideas carried through to the middle ages. During the middle ages Alchemists tried to change lead to gold

Re-emergence of the Atomic Theory Late 1700’s - John Dalton- England (a major contributor to today’s Atomic Theory) A teacher who summarized results of his experiments and those of other’s In Dalton’s Atomic Theory he combined ideas of elements with that of atoms

Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical, those of different atoms are different. Atoms of different elements combine in whole number ratios to form compounds Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

Law of Definite Proportions Each compound has a specific whole-number ratio of elements; ratio is by mass [Definite Proportions] Water H2O Carbon dioxide CO2 Methane CH4 8.0 g oxygen reacts with 1.0 g hydrogen (H2O) Ratio = 8:1 by mass

Law of Multiple Proportions if two elements form more that one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number. In hydrogen peroxide 16.0 g oxygen reacts with 1.0 g hydrogen (H2O2) Ratio of the masses of oxygen in hydrogen peroxide and water is 16:8 = 2:1 Therefore H2O2 contains twice as many oxygen atoms per hydrogen atom than H2O [Multiple Proportions]

What? Water is 8 grams of oxygen per gram of hydrogen. (H2O) Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen. (H2O2) 16 to 8 is a 2 to 1 ratio Therefore H2O2 contains twice as many oxygen atoms per hydrogen atom than H2O [Multiple Proportions] True because you have to add a whole atom, you can’t add a piece of an atom.

Parts of Atoms J. J. Thomson - English physicist. 1897 As scientists began to develop methods for more detailed probing of the nature of matter, the atom (supposedly indivisible) began to show signs of a more complex structure J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode ray tube which was used to study the electrical conductivity of gasses. The cathode ray tube is a vacuum tube - the air has been pumped out.

Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment Voltage source By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field he found that the moving pieces were negative because the rays were attracted to the positive electrode in the external field.

Cathode Ray Tube

Thompson concluded that: Cathode rays consist of beams of particles The particles have a negative charge Based on his findings, a new fundamental particle of matter was discovered – The Electron!

Thomsom’s Model Couldn’t find positive (for a while) Said the atom was like plum pudding or (blueberry muffin) A bunch of positive stuff, with the electrons embedded (able to remove the embedded electrons)

Other pieces Proton - positively charged pieces 1840 times heavier than the electron Neutron - no charge but the same mass as a proton. Where are the pieces?

Rutherford’s experiment Ernest Rutherford English physicist. (1910) Believed in the plum pudding model of the atom. Used radioactivity Alpha particles - positively charged pieces given off by uranium Shot them at gold foil which can be made a few atoms thick

Rutherford’s experiment When the alpha particles hit a florescent screen, it glows. Here’s what it looked like

Florescent Screen Lead block Uranium Gold Foil

He Expected Rutherford believed that the a particles would pass through unhindered. (The alpha particles to pass through without changing direction very much.) Because the positive charges were spread out evenly within the atom. The positive charge in the atom was not enough to stop the alpha particles

What he expected

Because

Because, he thought the mass was evenly distributed in the atom a particles

Because, he thought the mass was evenly distributed in the atom

What he got A small percentage of the particles were being reflected at unexpected angles, inconsistent with the “muffin model”

How he explained it Atom is mostly empty Small dense, positive piece at center Alpha particles are deflected by it if they get close enough +

+ a particles Rutherford explained his observations as follows: Atom is mostly empty space Small, dense, and positive at the center Alpha particles were deflected if they got close enough + a particles

Modern View The atom is mostly empty space Two regions Nucleus- protons and neutrons Electron cloud- region where you might find an electron

Density and the Atom Since most of the alpha particles went through, the atom is mostly empty. Because the alpha particles turned so much, the positive region of the atom is heavy. Small volume, big mass, big density This small dense positive area is the nucleus

Subatomic particles Relative mass Actual mass (g) Name Symbol Charge Electron e- -1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron nº 1 1.67 x 10-24

Structure of the Atom There are two regions The nucleus – with protons and neutrons so that it has a Positive charge and almost all the mass Electron cloud- Most of the volume of an atom and is the region where the electron can be found (extra nuclear)

Size of an atom Atoms are small and are measured in picometers, 10-12 meters Hydrogen atom, 32 pm radius Nucleus tiny compared to atom. If the atom was the size of a stadium, the nucleus would be the size of a marble. Radius of the nucleus near 10-15m. Density near 1014 g/cm3

Counting the Pieces Atomic Number = number of protons (p+) The number of protons determines kind of atom – 2 protons in the nucleus means that this is a Helium atom. Chemists use Z as a symbol for atomic number. In a neutral atom there is the same number of electrons (e-) and protons (atomic number) Mass Number = number of protons + neutrons [Sum of p+ and nº (p+ + nº)] The symbol used for mass number is A. The neucleons (p+ and nº) make up the mass of the atom.

Isotopes Dalton was wrong when he said that all atoms of one element are the same. Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers and different masses. The atoms of the same element that differ in the number of neutrons are called isotopes of that element.

Nuclear Symbols Contain the symbol of the element, the mass number and the atomic number

E Z Nuclear Symbols A Contain: the mass number the atomic number the symbol of the element the atomic number Z

F Symbols 19 9 Find the number of protons number of neutrons number of electrons Atomic number Mass Number 19 F 9

Br Symbols 80 35 Find the number of protons number of neutrons number of electrons Atomic number Mass Number 80 Br 35

Symbols if an element has an atomic number of 34 and a mass number of 78 what is the number of protons number of neutrons number of electrons Complete nuclear symbol

Symbols if an element has 91 protons and 140 neutrons what is the Atomic number Mass number number of electrons Complete nuclear symbol

Symbols if an element has 78 electrons and 117 neutrons what is the Atomic number Mass number number of protons Complete nuclear symbol

Naming Isotopes Put the mass number after the name of the element carbon- 12 carbon -14 uranium-235

Atomic Mass How heavy is an atom of oxygen? There are different kinds of oxygen atoms. More concerned with average atomic mass. Based on abundance of each isotope in nature. Don’t use grams because the numbers would be too small

Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) One twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass we need the average from percent abundance.

Calculating averages You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? Total mass = 4 x 50 + 1 x 60 = 260 g Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5

Calculating averages Average mass = .8 x 50g + .2 x 60g = 52g Average mass = 4 x 50g + 1 x 60g = 260 g = 52g 5 5 5 Average mass = .8 x 50g + .2 x 60g = 52g 80% of the rocks were 50 grams 20% of the rocks were 60 grams Average = % as decimal x mass1+ % as decimal x mass2 + % as decimal x mass3 +…

Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest (30.9%) has a mass of 64.93 amu. 0.691 x 62.93amu 43.48463amu + 0.309 x 64.93amu = + 20.06337amu = 63.548amu

Atomic Mass Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 26 with a mass of 25.9826 amu. What is the atomic mass of magnesium? If not told otherwise, the mass of the isotope is the mass number in amu

Atomic Mass is not a whole number because it is an average (are the decimal numbers on the periodic table). Isotopes - atoms of the same element can have different numbers of neutrons and therefore have different mass numbers When naming, write the mass number after the name of the element H 1 Protium 2 Deuterium 3 Tritium