Chapter 4 The Structure of the Atom

1. Chapter 4The Structure of the Atom Big Idea: Atoms are the fundamental building blocks of matter.

2. Section 1: Early Ideas About MatterMain Idea: The ancient Greeks tried to explain matter, but the scientific study of the atom began in the early1800s. The Roots of Atomic Theory Science as we know it today did not exist thousands of years ago. There was no scientific methodology—no tools for scientific investigation. The power of the mind—intellectual thought—was considered the avenue to truth. These early thinkers about the nature of matter were philosophers, not scientists. There was no method available to test the validity of their theories.

4. Dalton’s Atomic Theory • Aristotle’s ideas about matter persisted for over 2000 years. • John Dalton revived and revised Democritus’s ideas about atoms in the early 1800s. • Unlike the Greeks, he performed numerous experiments that refined and supported his hypotheses. • Dalton’s atomic theory easily explained conservation of mass in a reaction as the result of the combination, separation, or rearrangement of atoms.

6. Not all of Dalton’s theory was accurate • He was wrong about atoms being indivisible. We now know they have parts. • He was also wrong about all atoms of a particular element having identical properties. • Atoms of the same element can have slightly different masses.

7. Section 2: Defining the Atom Main Idea: An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus. The Atom • Atoms are the smallest particle of an element that ___________________________________ • They are divisible into ____________________ ______________________________________

8. Discovery of the electron The cathode-ray tube • Used to investigate the association between matter and charge (1890s). • Scientists, such as Sir William Crookes, passed electricity through glass tubes from which most of the air had been removed. • As an electric current was passed through the tube a ray of light was observed travelling from the cathode (-) end to the anode (+) end. • The ray of light could be deflected by charged plates or a magnetic field.

9. Because they could be deflected, _________________ • The rays were determined to be composed of particles which were named ___________ • Since they were attracted to a positive charge the electrons carried a _____________ • The precise value of the negative charge was later determined by Robert Millikan in his oil-drop experiment (Fig 8 in text). https://www.youtube.com/watch?v=O9Goyscbazk

10. Mass and charge of the electron • J. J. Thomson determined that the mass of the charged particle was much less than that of a hydrogen atom. • This confirmed that atoms have parts. • Since atoms overall are neutral, Thomson’s model of the atom had the electrons embedded in a uniform, positively charged sphere. • Referred to as the

11. The Nucleus • In 1910 Ernest Rutherford studied how positively charged alpha particles (helium nuclei) interacted with solid matter. • His gold foil experiment showed that atoms do not have the same consistency throughout. • -- A narrow beam of alpha particles were aimed at a sheet of gold foil. • -- Some passed through but others were deflected as though they had hit something very hard and dense. -- Rutherford concluded that all the mass of an atom was ____________ ____________________ __________________________________________

12. Rutherford’s model placed electrons in orbit around the very small, ______________. • The nucleus contained positively charged particles called _________. • A neutral particle in the nucleus, called the neutron, was discovered in 1932. It had nearly the same mass as the proton. https://www.youtube.com/watch?v=u7xrDhpY-oA

13. Rutherford’s model of the atom

14. 4.6 The Modern Concept of Atomic Structure Summary of Atomic Structure in 1910 Atoms are the fundamental units of which elements are composed. The smallest unit of matter that retains all properties of an element. Atoms are composed of three main parts: protons, neutrons and electrons. The nucleus contains protons (+) and neutrons (0). The electrons (-) are orbiting outside the nucleus. In a neutral atom, # protons = # electrons

15. Mass of electron is insignificant! • Masses of parts of an atom Electron = 1 (relative mass) Proton = 1836 Neutron = 1839 • Size of atom • Diameter of nucleus ~ 10-13 cm • Diameter of overall atom ~ 10-8 cm • Diameter of an atom is about 100,000 times the diameter of the nucleus! • Atoms are mostly empty space!

16. 4.7 Isotopes Section 3: How Atoms Differ Main Idea: The number of protons and the mass number define the type of atom Atomic Number All atoms of an element have the same number of protons. The number of protons in an atom is referred to as the _______________. On the periodic table, the atomic number is located _______ ____________________________

17. Isotopes and Mass Number • Atoms of an element with different numbers of neutrons are called ___________. • All isotopes of an element are chemically identical and behave the same in chemical reactions. • Isotopes of an element have ______________________. • Isotopes are identified by their _______________ mass number = protons + neutrons • Isotope symbols

18. 4.8 Introduction to the Periodic Table Mass of Atoms Mass of protons and neutrons is about 1.67 x 10-24 gram. Electron is about 1/1840 mass of proton or neutron. Because atoms are so small, the mass of an atom is measured in the atomic mass unit (amu) which is 1/12 the mass of a carbon-12. The atomic mass of an element is the _______________ _____________________________________________ 18