Atomic Structure

1. Atomic Structure

2. Early Theories of Matter • Science as we know it did not exist several thousand years ago • Most philosophers thought that matter was made up of air, water, earth, and fire

3. Democritus (460-370 BC) • The first to propose that matter was not infinitely indivisible • Believed that matter was made up of atomos (or atoms) • He also said that atoms could not be created or destroyed

4. Democritus’ Theory • Matter is composed of empty space through which atoms move • Atoms are solid, homogeneous, indestructible, and indivisible • Different atoms have different sizes and shapes • The differing properties of matter are due to the size, shape, and movement of atoms • Changes in matter result from changes in the groupings of atoms and not the atoms themselves

5. Democritus • Democritus was on the right track, but had no scientific data to back up any of his claims • The foremost thinker of the day, Aristotle, rejected Democritus’ ideas because he believed that nothingness could not exist • No other theories of the atom came about until nearly 2000 years later

6. John Dalton • John Dalton was the next scientist to propose a theory about the atom in the 19th century

7. Dalton’s Atomic Theory • All matter is composed of extremely small particles called atoms • All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from other elements • Atoms cannot be created, destroyed, or broken into smaller particles • Different atoms combine in simple whole number ratios to form compounds • In a chemical reaction, atoms are separated, combined, or rearranged

8. Dalton’s Atomic Theory • This was a HUGE step toward the atomic model, but was not totally correct • Atoms ARE divisible into smaller subatomic particles • Atoms of a given element can have different masses

9. Basic Definitions • Atom– smallest unit of an element that retains the properties of that element • Atoms are made up of several subatomic particles called protons, neutrons, and electrons

10. Protons, Neutrons, & Electrons • Protons – have a +1 charge and are found in the nucleus of the atom • Neutrons – have no charge and are also found in the nucleus of an atom • Electrons – have a -1 charge and are found on the outside of the nucleus • Nucleus – made up of protons and neutrons, has an overall + charge

11. Atomic Structure

12. JJ Thomson • JJ Thomson used the cathode ray experiment to determine the charge to mass ratio of an electron. • He identified the first subatomic particle, the electron • He proposed the plum pudding model of the atom • Credited for discovering the electron

13. Robert Millikan • Millikan is noted for his famous Millikan’s Oil Drop Experiment • This experiment determined the charge and the mass of an electron

14. Earnest Rutherford • Rutherford’s Gold Foil Experiment helped to determine the existence of the nucleus • Rutherford proposed that the nucleus was small, dense and positively charged • Proposed the nuclear atomic model which stated that there was a nucleus with a positive charge and electrons around the outside

15. James Chadwick • Chadwick showed that the nucleus also contained neutrons • He is credited for the discovery of the neutron

16. Atomic Numbers • The atomic number of an element is the number of protons in the nucleus of an atom of that element. • It is the number of protons that determines the identity of an element, as well as many of its chemical and physical properties. • The number of protons for an element CANNOT be changed.

17. Atomic Numbers • Because atoms have no overall electrical charge, the number of protons must equal the number of electrons. • Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element. • The number of electrons can be changed when determining the charge of an ion.

18. Masses • The mass of a neutron is almost the same as the mass of a proton. • The sum of the protons and neutrons in the nucleus is the mass number of that particular atom. • Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number (number of protons)

19. Isotopes • When writing isotopes, the atomic number (or number of protons) will appear at the bottom left of the formula • The mass number (number of protons plus neutrons will appear at the top left of the formula. • The element symbol will appear to the right of the numbers • The different number of neutrons has NO bearing on chemical reactivity

20. Writing the Names of Isotopes • When writing the name of an isotope, you will write the name of the element – the mass number • For example 126 C would be named: • Carbon - 12

21. Try the following

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23. Atomic Mass • Atomic mass –the weighted average mass of all the naturally occurring isotopes of that element. • The number is usually located at the bottom of the periodic table and has decimal places

24. Calculating Atomic Mass

25. Calculating Atomic Mass • Copper exists as a mixture of two isotopes. • The lighter isotope (Cu-63), with 29 protons and 34 neutrons, makes up 69.17% of copper atoms. • The heavier isotope (Cu-65), with 29 protons and 36 neutrons, constitutes the remaining 30.83% of copper atoms.

26. Calculating Atomic Mass • First, calculate the contribution of each isotope to the average atomic mass, being sure to convert each percent to a fractional abundance.

27. Calculating Atomic Mass • The average atomic mass of the element is the sum of the mass contributions of each isotope.

28. Try this one… Calculate the atomic mass of germanium. 72.59 amu

29. You can tell many things from an isotope formula • Hydrogen has three naturally occurring isotopes in nature: Hydrogen – 1, Hydrogen – 2, and Hydrogen – 3. • Which is the most abundant in nature? • Hydrogen – 1 • Which is the heaviest? • Hydrogen - 3