2. isotopes

3. The Periodic Table

4. An atom consists of a nucleus nucleus – (of protons and neutrons) electrons in space about the nucleus. electrons in space about the nucleus. The Atom Nucleus Electron cloud

5. Dalton’s Atomic Theory John Dalton (1766-1844) proposed an atomic theory While this theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today instead of alchemy Thus, it’s an important landmark in the history of science.

6. Dalton’s Atomic Theory - Summary 1.matter is composed, indivisible particles (atoms) 2.all atoms of a particular element are identical 3.different elements have different atoms 4.atoms combine in certain whole-number ratios 5.In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.

7. Problems with Dalton’s Atomic Theory? 1. matter is composed, indivisible particles Atoms Can Be Divided, but only in a nuclear reaction 2. all atoms of a particular element are identical Does Not Account for Isotopes (atoms of the same element but a different mass due to a different number of neutrons)! 3. different elements have different atoms YES! 4. atoms combine in certain whole-number ratios YES! Called the Law of Definite Proportions 5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements. Yes, except for nuclear reactions that can change atoms of one element to a different element

8. Atomic Number, Z All atoms of the same element have the same number of protons in the nucleus, Z 13 Al 26.981 Atomic number Atom symbol AVERAGE Atomic Mass

9. Mass Number, A C atom with 6 protons and 6 neutrons is the mass standard C atom with 6 protons and 6 neutrons is the mass standard = 12 atomic mass units = 12 atomic mass units Mass Number (A) = # protons + # neutrons Mass Number (A) = # protons + # neutrons NOT on the periodic table…(it is the AVERAGE atomic mass on the table) NOT on the periodic table…(it is the AVERAGE atomic mass on the table) A boron atom can have A = 5 p + 5 n = 10 amu A boron atom can have A = 5 p + 5 n = 10 amu

10. Isotopes Atoms of the same element (same Z) but different mass number (A). Atoms of the same element (same Z) but different mass number (A). Boron-10 ( 10 B) has 5 p and 5 n Boron-10 ( 10 B) has 5 p and 5 n Boron-11 ( 11 B) has 5 p and 6 n Boron-11 ( 11 B) has 5 p and 6 n 10 B 11 B

11. Isotopes are two forms of an element with the same atomic number but different mass number. All atoms contain three kinds of basic particles: protons, neutrons, and electrons. The protons and neutrons in an atom are found in the atomic nucleus, while the electrons are found in the space around the nucleus.

12. The number of protons in a nucleus defines an atom. Hydrogen atoms all have one proton in their nucleus; helium atoms all have two protons in their nucleus; lithium atoms all have three protons in their nucleus; and so on. The number of protons in an atom's nucleus is called its atomic number. Hydrogen has an atomic number of 1; helium, an atomic number of 2; and lithium, an atomic number of 3.

13. But atoms of the same element can have different numbers of neutrons. Some helium nuclei, for example, have two neutrons; others have only one. The mass number of an atom is the total number of protons and neutrons in the atom's nucleus. The two-neutron atom of helium has a mass number of four (two protons plus two neutrons) The one-neutron atom of helium has a mass number of three (two protons plus one neutron).

14. Figure 3.10: Two isotopes of sodium.

15. Representing isotopes Isotopes are commonly represented in one of two ways. First, they may be designated by writing the name of the element followed by the mass number of the isotope. The two forms of helium are called helium-4 and helium-3. Second, isotopes may be designated by the chemical symbol of the element with a superscript that shows their mass number. The designations for the two isotopes of helium are 4 He and 3 He.

16. Learning Check – Counting Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12 C 13 C 14 C 6 6 6 #p + _______ _______ _______ #n o _______ _______ _______ #e - _______ _______ _______

17. Answers 12 C 13 C 14 C 6 6 6 #p + 6 6 6 #n o 6 7 8 #e - 6 6 6

18. Learning Check An atom has 14 protons and 20 neutrons. A.Its atomic number is 1) 142) 163) 34 B. Its mass number is 1) 142) 163) 34 C. The element is 1) Si2) Ca3) Se D.Another isotope of this element is 1) 34 X 2) 34 X 3) 36 X 16 14 14

19. AVERAGE ATOMIC MASS Because of the existence of isotopes, the mass of a collection of atoms has an average value. Because of the existence of isotopes, the mass of a collection of atoms has an average value. Boron is 20% 10 B and 80% 11 B. That is, 11 B is 80 percent abundant on earth. Boron is 20% 10 B and 80% 11 B. That is, 11 B is 80 percent abundant on earth. For boron atomic weight For boron atomic weight = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu 10 B 11 B

20. Isotopes & Average Atomic Mass Because of the existence of isotopes, the mass of a collection of atoms has an average value. Because of the existence of isotopes, the mass of a collection of atoms has an average value. 6 Li = 7.5% abundant and 7 Li = 92.5% 6 Li = 7.5% abundant and 7 Li = 92.5% – Avg. Atomic mass of Li = ______________ 28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10% 28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10% – Avg. Atomic mass of Si = ______________

21. Radioactive isotopes A radioactive isotope is an isotope that spontaneously breaks apart (decay), changing into some other isotope. As an example, potassium has a radioactive isotope with mass number 40, 40 K. This isotope breaks down into a stable isotope of potassium, 39 K. Radioisotope, have an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached The stable end product is a nonradioactive isotope of another element, i.e., radium-226 decays finally to lead-206