Chapter 3 Atoms and their structure

History of the atom n Democritus, a Greek philosopher, originally came up with the idea of an atom (around 400 BC) Atomos = Greek word meaning “cannot be cut”

n Aristotle - Greek philosopher who criticized Democritus’ idea of atoms n Aristotle believed all substances are made of 4 elements Fire Air Earth Water Fire Air Earth Water Blend these in different proportions to get all substances People believed him for over 2000 years!

n Late 1700’s - John Dalton n British Teacher who believed in atoms and set out to prove they were real n His experiments, along with the work of other scientists, led him to develop Dalton’s Atomic to develop Dalton’s Atomic Theory Theory

Dalton’s Atomic Theory  All matter is made of tiny indivisible particles called atoms.  Atoms of the same element are identical, those of different elements 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.

Parts of Atoms n J. J. Thomson - English physicist n Developed the cathode ray tube to study how electricity acted in a vacuum n It is a vacuum tube - all 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 +-

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

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

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

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

Thomson’s Experiment n By adding an electric field

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

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

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

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

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

Voltage source Thomson’s Experiment n By adding an electric field he found that the moving pieces were negative n He had discovered the electron! + -

Thomson’s Model n Since the atom has no overall charge, it must have positive things to balance out the negative electrons n Said it was like plum pudding with a bunch of negative electrons spread throughout a positive atom

Rutherford’s experiment n Ernest Rutherford-English physicist. (1910) n Believed in Thomson’s plum pudding model of the atom. n Tried to prove the model was true with the Gold Foil Experiment with the Gold Foil Experiment

Rutherford’s experiment n Shot alpha particles (with a positive charge) at very thin gold foil n A fluorescent screen showed how the particles came through the foil

Lead block Uranium Gold Foil Fluorescent Screen

Rutherford Expected: n The alpha particles to pass through without changing direction very much Because Because n The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

What he expected

What he got

How he explained it + n An atom is mostly empty space n There is a small dense, positive piece at the center n Alpha particles bounce off of the dense center if they dense center if they get close enough get close enough

+

Structure of the Atom n There are two regions n The nucleus has protons and neutrons, is positively charged and contains almost all the mass n The electron cloud- is most of the volume of an atom and is the region where the electron can usually be found

Subatomic particles Electron Proton Neutron NameSymbolCharge Relative mass Actual mass (g) e-e- p+p+ n0n x x

Size of an atom n Atoms are small. n Measured in picometers, meters n Nucleus is tiny compared to the entire atom n If the atom were the size of a stadium, the nucleus would be the size of a marble.

n Atomic Number = number of protons n # of protons determines the name of the atom n For a neutral atom (an atom with an overall charge of zero), the number of electrons is equal to the number of protons n Mass Number = protons + neutrons

n Atoms of the same element can have different numbers of neutrons n This means they have different mass numbers (Dalton was wrong) n Atoms with the same number of protons but different numbers of neutrons are called isotopes

2 Ways to Represent an Atom 1. Isotope symbol 2. Hyphen notation

Isotope Symbols n Use the symbol of the element, the mass number and the atomic number X Mass number Atomic number Charge, if any

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

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

n 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 –Atomic mass –Complete symbol

n if an element has 91 protons and 140 neutrons what is the –Atomic number –Mass number –number of electrons –Atomic mass –Complete symbol

n if an element has 78 electrons and 117 neutrons what is the –Atomic number –Mass number –number of protons –Atomic mass –Complete symbol

n 26 protons n 30 neutrons n 23 electrons

n 35 protons n 45 neutrons n 36 electrons

n Number of protons n Number of neutrons n Mass number n Atomic number n Number of electrons n Atomic mass Hg

Hyphen Notation n Put the mass number after the name of the element n carbon- 12 n carbon -14 n uranium-235

n Write the hyphen notation for the isotope with 51 protons and 69 neutrons

Atomic Mass of oxygen n Since there are different kinds of oxygen atoms, the atomic mass shown on the periodic table (the decimal number) is the average mass of all of the isotopes of oxygen n It is calculated by using the abundance of each isotope in nature and the mass of that isotope.

n Oxygen-16 = % n Oxygen-17 = 0.038% n Oxygen-18 = 0.200% n So the average mass based on percent abundance is:

Atomic Mass of Oxygen n (16 amu) n (17 amu) n (18 amu) amu amu

Atomic Mass n Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of amu and the rest has a mass of amu.

Atomic Mass n Magnesium has three isotopes % magnesium with a mass of amu, 10.00% magnesium with a mass of amu, and the rest magnesium with a mass of amu. What is the atomic mass of magnesium?

Atomic Masses n are not whole numbers because they are averages. n are the decimal numbers on the periodic table. n Atomic mass = the weighted average of the masses of the isotopes