Atomic Structure

4-1 Studying Atoms What was Dalton’s theory of the structure of matter? What contributions did Thomson and Rutherford make to the development of atomic theory?

Ancient Greek Models of Atoms Democritus believed that all matter consisted of extremely small particles (Atoms)that could not be divided. He thought atoms had specific sets of properties. Atoms in liquids were round and smooth Atoms in solids were rough and prickly Aristotle believed that ALL SUBSTANCES WERE BUILT FROM FOUR ELEMNTS AND THAT there was a limit to the number of times matter could be divided. By the 1800s, the atomic model of matter was the supported by data from experiments.

Dalton’s Atomic Theory John Dalton was a teacher who spent his spare time doing scientific experiments, especially ones on the behavior of gases in air. He determined gas consists of individual particles. Evidence for Atoms Burning magnesium combines with oxygen and forms magnesium oxide The ratio of the masses is constant: 1– gram sample of magnesium combines with 65.8 grams of oxygen. A 1- gram sample of magnesium combines with 658 grams of oxygen. The wooden spheres on the right were made by Dalton to represent the atoms of different elements. To review: What did Dalton notice that all compounds have in common? Using logic: Why do you think there are holes in Dalton’s wooden spheres?

Dalton’s Atomic Theory Cont. Dalton’s Theory explains why all the elements in a compound always join in the same way. Remember that a theory must explain the data from many experiments. All matter is made up of individual particles (atoms) which can’t be divided Four main points: All elements are composed of atoms. All atoms of the same element have the same mass and atoms of different elements have different masses. Compounds contain atoms of more than one element. In a particular compound, atoms of different elements always combine in the same way. Also widely accepted in Dalton’s time, scientists found that not all of his ideas were correct. What did Dalton notice that all compounds have in common?

Thomson’s Model of the Atom Thomson’s Experiments Used an electric circuit to learn more about atoms Hypothesized that the beam was a stream of charged particles that interacted with the air in the tube and caused the air to glow

Thomson’s Model of the Atom Continued Evidence for Subatomic Particles Experiments provided the first evidence that atoms are made of smaller particles Thomson’s Model Like a scoop of chocolate chip ice cream Chips are negatively charged particles Chips are spread through a mass of positively charged mater How do objects with the same charge act when they come close to each other?

Rutherford’s Atomic Theory Ernest Rutherford (1871-1937) was actually surprised by the results of his own experiments. Rutherford’s Hypothesis: the mass and charge at any location in the gold would be too small to change the path of an alpha particle The Gold Foil Experiment: the equipment pictured was used to test his hypothesis some of the locations of the flashes on the screen did not support Rutherford’s prediction Some of the alpha particles behaved as thought they had struck an object and bounced straight back Discovery of the Nucleus: dense, in the center of the atom, positively charged

4-2 The Structure of an Atom What are three subatomic particles? What properties can be used to compare protons, electrons, and neutrons? How are atoms of one element different from atoms of other elements? What is the difference between two isotopes of the same element?

Properties of Subatomic Particles Protons, electrons and neutrons are subatomic particles. Protons: positive charge varies among elements…each nucleus must contain at least one particle with a positive charge (1-100) Electrons: negatively charged…outside the nucleus Neutrons: neutral charge…in the nucleus…mass almost equal to mass of protons

Comparing Subatomic Particles Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom.

Atomic Number and Mass Number Atomic Number: equal to the number of protons in an atom of that element Hydrogen only has one proton Different elements have different numbers of protons Mass Number: sum of the protons and neutrons Al = 13 protons + 14 neutrons = 27 S = sulfur = atomic number = 16 Fe= iron = atomic number = 26 Ag = silver = atomic number = 47

Isotopes Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons. Isotopes are atoms of the same element that have different numbers of neutrons and different mass numbers Oxygen: 8 protons, 8 neutrons and mass number of 16 Oxygen-16 But some have 9 neutrons and a mass number of 17 Oxygen-17 And some have 10 neutrons and a mass number of 18 Oxygen-18 Check your understanding: How are the compositions of heavy water and ordinary water similar? What type of hydrogen atoms does ordinary water contain? What type of hydrogen atoms does heavy water contains? Water that contains hydrogen-2 atoms (instead of hydrogen-1 atoms)is called heavy water Compare physical properties of ordinary water and heavy water.

4-3 Atomic Theory What can happen to electrons when atoms gain or lose energy? What model do scientists use to describe how electrons behave in atoms? What is the most stable configuration of electrons in an atom?

Chronological Models of the Atom

Bohr’s Model of the Atom -constant speeds and fixed orbits, like planets around the sun Bohr’s models correctly introduced the concept of energy levels, but energy levels cannot be used to describe the actual location of an electron.

Electron Cloud Model The electron cloud model can be used to model the probability that an electron is in a certain location but the exact speed and location of a single electron cannot be determined. How is a moving airplane propeller similar to an electron cloud? What other examples can you think of hat could model the concept of an electron cloud?

Class Participation Opportunities Build or draw models that represent the changes over time in scientists’ understanding of the atomic structure. A three dimensional version of the time line could be displayed as a mobile or a diorama Don’t forget to not the time scale on the time line. Research the life of one of the following scientists: Dalton, Thomson, Nagaoka, Rutherford, Bohr, de Broglie, Schrodinger, or Chadwick Research what was known about planetary obits during the same period of time that modern atomic theory was evolving

Atomic Orbitals An electron cloud is a good approximation of how electrons behave in their orbitals.

Electron Configurations The arrangement of electrons in the orbitals of the atom The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies. When all electrons in the atom have the lowest possible energies, the atom is said to be ground stable. (lithium, atomic number of 3, 2 in first orbital and 1 in second orbital) If lithium absorbs energy, one of it’s electrons on the first orbital can move to the second orbital. This is lithium in an excited state, not very stable.)

Analogy When is the configuration of the gymnast like an atom in an excited state? When is the gymnast most like an atom in its ground state?