1. Chapter 5 Atomic Structure & the Periodic Table

2. Sec 1: Atoms Democritus was the first to suggest the idea of atoms. The modern idea of the atom started with John Dalton.

3. Dalton’s Atomic Theory Dalton’s Atomic Theory: 1. All elements are composed of tiny particles called atoms 2. Atoms of the same element are identical, atoms of one element are different than other elements. 3. Atoms of different elements can physically mix or chemically combine in whole-number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of 1 element are never changed into another element.

4. Dalton’s Atomic Theory

5. Atoms A sample of an element can be broken down into smaller and smaller pieces— eventually it can’t be broken down any more. Atom—smallest particle of an element that retains the properties of that element.

6. Sec 2: Nuclear Atom Atoms are now known to be divisible. Atoms are composed of electrons, protons, and neutrons. Electrons—negatively charged subatomic particles. Electrons were discovered by J.J. Thomson using a cathode ray. Negative electrons are attracted to positively charged metal plates.

7. Protons & Neutrons Atoms are neutral; they have no net charge. Protons—positively charged subatomic particles. Neutrons—subatomic particles with no charge. Mass of neutron is about equal to proton. Mass of neutron is about equal to proton.

8. Nucleus Nucleus—the central core of an atom composed of protons & neutrons. The nucleus was discovered by Ernest Rutherford using the gold foil experiment.

9. Sec 3: Distinguishing Between Atoms Atomic Number—the number of protons in the nucleus of an atom of an element. Every element has a different number of protons. Remember : atoms are neutral. The number of electrons equal the number of protons in a neutral atom.

10. Mass Number Mass number—the total number of protons and neutrons in an atom Example: a helium atom has 2 protons and 2 neutrons. What is the mass number? Example: a helium atom has 2 protons and 2 neutrons. What is the mass number? 4 To find the number of neutrons in an atom: Number of neutrons = mass # – atomic # Number of neutrons = mass # – atomic # Shorthand notation: You can use the mass number to name specific atoms. Example: Helium-4

11. Isotopes Isotopes—Atoms that have the same number of protons, but different numbers of neutrons. Isotopes have different numbers of neutrons, so they have different mass numbers.

12. Atomic Mass The mass of a proton or neutron is 1.67 x 10 -24 g. The mass of an electron is 9.11 x 10 -28 g. This is negligible in comparison. This is negligible in comparison. A mass spectrometer can be used to determine the tiny masses of atoms.

13. Atomic Mass Atomic mass—a weighted average mass of the atoms in a naturally occurring sample of the element. The atomic mass is usually closest to the most abundant isotope of that element.

14. Atomic Mass To calculate atomic mass, you need to know 1. The number of stable isotopes of the element 2. The mass of each isotope 3. The natural percent abundance of each isotope. Multiply the atomic mass of each isotope by its abundance, then add the results. Example: Element B has 2 isotopes. The isotope with a mass of 10.012 amu has an abundance of 19.92%. The isotope with a mass of 11.009 amu has an abundance of 80.09 %. What is the atomic mass?

15. Calculating Atomic Mass 10.012 amu x 0.1991 = 1.993 amu 11.009 amu x 0.8009 = 8.817 amu atomic mass = 10.810 amu Done with Section 3