1. HOW ATOMS DIFFER  If all atoms are made up of protons, electrons, and neutrons, what makes them different? They differ in the number of these particles

2. ATOMIC NUMBER  Atomic number is the number of protons in an atom  Since an atom is neutral, the number of protons must equal the number of electrons  Atomic number = # of protons = # of electrons

3. Isotopes and Mass Number Isotopes – atoms with the same number of protons but different number of neutrons - they differ in mass -essentially have same chemical behavior Mass Number = protons + neutrons potassium-39 = 19 protons + 20 neutrons Reference fig.4-15 p. 100

4. Atomic Mass and Atomic Mass Unit (amu) Chemists decided to develop a method of measuring the mass of an atom relative to the mass of a specifically chosen atom standard. That standard is the carbon-12 atom. Atomic Mass Unit (amu) = 1/12 the mass of carbon-12 atom Atomic Mass = the weighted avg. mass of the isotopes of that element

5. EXAMPLE Isotope 6 X: mass = 6.015 amu abundance = 7.50% Isotope 7 X: mass = 7.016 abundance = 92.5% 6 X: Mass Contribution = (mass)(% abundance) Mass Contribution = (6.015)(.0750) = 0.451 amu

6. Example Contd. For 7 X: Mass Contribution = (mass)(% abundance) Mass Contribution = (7.016)(0.925) = 6.490 amu Sum the mass contributions to find the atomic mass. Atomic mass of X = (0.451 amu + 6.490 amu) = 6.941 amu Use the periodic table to identify the element. The element with a mass of 6.941 amu is Lithium (Li)

7. Radioactivity Remember: chemical reaction – involves the change of one or more substances into new substances. Remember: chemical reaction – involves the change of one or more substances into new substances. Why don’t atoms of one element change into atoms of another element during this reaction? Why don’t atoms of one element change into atoms of another element during this reaction? Chemical reactions only involve the atom’s electrons – the nucleus remains unchanged Chemical reactions only involve the atom’s electrons – the nucleus remains unchanged There are reactions that involve one element changing into an atom of another element. There are reactions that involve one element changing into an atom of another element. nuclear reactions – involve a change in the atom’s nucleus

8. Radioactivity Contd. Radioactivity – substances spontaneously emitting radiation Radioactivity – substances spontaneously emitting radiation Radiation – the rays and particles emitted by the radioactive material Radiation – the rays and particles emitted by the radioactive material

9. TYPES OF RADIATION Alpha Radiation – deflected towards negatively charged plate Alpha Radiation – deflected towards negatively charged plate - made up of alpha particle - contains 2 protons and 2 neutrons - equivalent to helium-4 nucleus ( 4 2 He) 226 → 222 + 4 88 86 2 88 86 2 Radium-226 radon-222 alpha particle Radium-226 radon-222 alpha particle

10. TYPES OF RADIATION Beta Radiation – deflected toward positive charged plate. Beta Radiation – deflected toward positive charged plate. - consist of beta particles ( 0 -1 β) 14 → 14 + 0 6 7 -1 6 7 -1 carbon-14 nitrogen-14 beta particle carbon-14 nitrogen-14 beta particle Gamma Radiation – posses no electrical charge. Gamma Radiation – posses no electrical charge. Gamma Rays ( 0 0 γ)– high energy radiation that possess no mass - usually accompany alpha and beta radiation - account for most of the energy lost during radioactive decay radioactive decay

11. NUCLEAR STABILITY Atoms containing too many or too few neutrons are unstable. Atoms containing too many or too few neutrons are unstable. Unstable nuclei lose energy through radioactive decay in order to form a nucleus with a stable number of protons and neutrons. Unstable nuclei lose energy through radioactive decay in order to form a nucleus with a stable number of protons and neutrons.