2. EARLY MODELS OF THE ATOM

3. Models of Matter A model is a tentative description of a system or theory that accounts for all of its known properties Models are invented (usually) to interpret the invisible and relate experimental results

4. The Four Elements An Ancient Greek model of the atom was based on logic Matter is composed of four elements: air, fire, earth and water An atom is a single indivisible particle

5. John Dalton’s Atomic Theory (1803) Based on experimentation Experimental data showed: Elements combine in the same percent by mass each time the same compound is formed Atoms of different elements combine in fixed proportions

6. Dalton’s Theory 1. All matter is made of small spherical particles called atoms 2. Each element is made of its own type of atom 3. Atoms of different elements have different properties

7. 4. Atoms of different substances can combine in constant ratios to form new substances 5. Atoms cannot be created or destroyed during chemical or physical changes 6. Atoms can neither be subdivided nor changed to one another Dalton’s Theory Continued…

8. Law of Constant Composition: Atoms of different elements can be chemically combined in a fixed whole number ratio to form compounds. Example: H 2 O Law of Conservation of Mass: The mass of the reactants in a chemical reaction is equal to the mass of the products

9. The Atom The smallest indivisible particle of an element that has the properties of that element

10. Thomson’s Atomic Theory (1897) Used a cathode ray tube to energize electrons Experimental data showed: Charged particles originated from the gas inside the tube Charged particles are deflected away from a negatively charged object, but towards a positively charged object.

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

12. Thomson’s Experiment n By adding an electric field, he found that the moving pieces were negative + -

13. Thomson’s Model Found the electron. Couldn’t find positive (for a while). Said the atom was like plum pudding. A bunch of positive stuff, with the electrons able to be removed.

14. Atoms are composed of smaller particles. These particles are the same for all different types of atoms. These particles are negatively charged and are called electrons. Electrons are embedded throughout the uniform sphere of positive charge to make up a neutral atom. Matter is naturally neutral except when it gains or loses electrons “Plum Pudding Model”

15. Rutherford’s Gold Foil Experiment (1911) Rutherford used very small positive particles (alpha particles) and a fluorescent screen detector to see the particles Since the mass is evenly distributed in gold atoms alpha particles should go straight through.

16. Gold Foil Experiment Experimental data showed: Alpha particles (+ve) were shot at a thin sheet of gold Most alpha particles passed through unaffected Some alpha particles were deflected at large angles. Some are even reflected back.

17. What he got

18. Atom is mostly empty space Small dense positive region at the center (a nucleus?) Alpha particles are deflected by this region if they get closeenough. Electrons surround but do not fill the rest of the atom Rutherford’s Theory…

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

20. Symbolic Representation of Sub Atomic Particles NameSymbolChargeMassLocation proton p+ +11 a.m.u.nucleus neutron n 01 a.m.u.nucleus electron e - 1/1837 a.m.u. energy levels around nucleus

21. Sub-atomic Particles Z - atomic number = number of protons determines type of atom. A - mass number = number of protons + neutrons. Number of protons = number of electrons if neutral. Number of neutrons = atomic mass – atomic number

22. Symbols X Z A Na 11 23

23. IONS Charged atoms formed by a neutral atom to attain a stable structure on the last shell (octet or doublet) Two types of ions: Positive ions (cations) are formed when an atom loses electrons Negative ions (anions) are formed when an atom gains electrons

24. Ionic symbols Cation Anion X +N X -N X = Symbol (+) = losing (-)= gaining N= # of electrons lost N= # of electrons gained

25. Subatomic particles for ions CationsAnions protons p+ = Z electrons e- = Z - N e- = Z + N neutrons n = A - Z

26. Practice! Calculate the subatomic particles for the following particles: Cu Fe +3 N -3