Structure of Atoms

1. Structure of Atoms Chapter 3.2

2. Objective 1: • Describe the evidence for the existence of electrons, protons, and neutrons, and describe the properties of these subatomic particles. • Define atom: smallest particle of an element that retains the chemical properties of that element. • Subatomic particles: • Protons • Neutrons • Electrons

3. Discovery of the Electron • Late 1800’s – J.J. Thomson discovered the electron through experiments with cathode-ray tubes. • Electron Properties • Relative Charge Mass Number actual mass - 1 0 9.109 x 10-31 kg • Plum Pudding • Model of atom

4. Discovery of Atomic Nucleus • In 1911 – Ernest Rutherford – Gold Foil Experiment • Showed that there was a small – dense positively charged nucleus of an atom. • Proton Properties • Relative Charge Mass Number actual mass + 1 1 1.673 x 10-27 kg

5. Neutrons • Discovered in 1932 by James Chadwick • British Scientist • Neutron Properties • Relative Charge Mass Number actual mass 0 1 1.675 x 10-27 kg

6. The Atom Nuclear Forces – Short range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together • Sizes of Atoms • Nucleus compared to rest of atom - approximately size of marble compared to a large football stadium. • Atom is mostly empty space

7. Objective 2 • Discuss atoms of different elements in terms of their numbers of electrons, protons, and neutrons, and define the terms atomic number and mass number • Atomic Number (Symbol – Z) • Number of protons in nucleus of atom • Determines the identity of the atom!!!!! • C - 6 • He - ___ • F - ___ • Pb - ___ • Use periodic table to find these! • Since atoms are neutral, atomic number is also number of electrons • So C also has 6 electrons • He - ____ electrons

8. Objective 3 • Define isotope, and determine the number of particles in the nucleus of an atom. Isotopes • All Hydrogen atoms contain 1 proton • Many naturally occurring elements can contain different numbers of neutrons. • Isotope – at0ms of the same element that have different masses. n n n Protium 99.985 % Deuterium 0.015 % Tritium Trace Three isotopes of Hydrogen

9. Mass Number • Mass number – total number of protons and neutrons in the nucleus of an isotope • Symbol – A • Protium has 1 proton and 0 neutrons • Mass number = #p + #n • = 1 + 0 • = 1 • What is the mass number deuterium and tritium? • Find on periodic table

10. Determine the number of particles in an atom • How many protons, electrons, and neutrons are present in an atom of copper whose atomic number is 29 and whose mass number is 64? • How many protons and electrons are in an atom of sodium whose atomic number is 11? • An atom has 13 protons and 14 neutrons. What is its mass number? • Calculate the mass number for an atom that has 45 neutrons and 35 electrons.

11. Designating Isotopes • Hyphen Notation • Name of atom – mass number • Uranium - 235 • Nuclear symbol • A X A : mass number, Z : atomic number, X : symbol of element • 235U Z 92 • Number of protons Neutrons Electrons • 92 143 92 • Nuclide – general term for any isotope of any element

12. Determine the number of particles in isotopes. • Calculate the numbers of protons, electrons, and neutrons in oxygen – 17 and in oxygen – 18. • Chlorine has two stable isotopes, chlorine – 35 and chlorine – 37. The atomic number of chlorine is 17. Calculate the numbers of protons, neutrons, and electrons each isotope has. • Calculate the numbers of protons, electrons, and neutrons for each of the following isotopes of calcium : Ca and Ca 42 44 20 20