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The Structure of the Atom

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1 The Structure of the Atom
Chapter 4

2 The Roots of Atomic Theory
Democritus Aristotle John Dalton Ideas versus Science Why was it hard for Democritus to defend his ideas?

3 Dalton’s Atomic Theory
All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of different elements can chemically combine with one another in simple whole-number ratios to form compounds. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into another element as a result of a chemical reaction.

4 Defining the Atom An atom is the smallest particle of an element that retains the properties of that element. 100,000,000 atoms = 1 centimeter

5 The Electron Using a vacuum pump, scientists passed electricity through glass tubes with an anode at one end and a cathode at the other. Cathode rays led to the discovery of negatively charged particles called electrons.

6 Electrons JJ Thomson did experiments using cathode rays, discovered that opposite charges attract and like charges repel Robert Millikan determined electrons negative charge and very small mass found in cloud (sublevels) outside the nucleus; valence electrons are the outermost electrons Plum Pudding Model

7 Atomic Nucleus Ernest Rutherford proposed that the atom is mostly empty space (gold foil experiment) all the positive charge and almost all the mass is concentrated in a small region Rutherford called this region the nucleus nucleus is the center core of an atom composed of protons and neutrons

8 Rutherford’s Nuclear Model
The atom is composed of a dense, positively charged nucleus surrounded by negative electrons. What force causes the deflection of alpha particles?

9 Protons and Neutrons atoms are electrically neutral
protons are positively charged almost 2000 times the mass of an electron neutrons have no charge, but equal mass to a proton protons and neutrons are collectively called nucleons

10 Summary of Atom Anatomy
The nucleus of the atom is made of protons and neutrons. Electrons orbit the nucleus in a cloud. Protons + charge Neutrons no charge Electrons - charge

11 Atomic Number elements are different due to different numbers of protons number of protons is the atomic number number of protons equals number of electrons in a neutral atom

12 Mass Number total number of protons and neutrons in an atom is the mass number number of neutrons = mass # - atomic # oxygen has an atomic # 8, mass # 16 atomic number can be written as the subscript and mass number as the superscript can also designate element by mass number gold-197

13 Isotopes atoms with same number of protons but different number of neutrons are called isotopes different mass numbers; chemically alike due to same # of protons and electrons hydrogen-1 (mass number 1, no neutrons) hydrogen-2 (mass number 2, 1 neutron) deuterium

14 Atomic Mass Unit defined as one twelfth the mass of a carbon-12 atom (amu) mass of a single proton (or neutron) is about 1 amu Why isn’t atomic mass always a whole number? Atomic mass is the weighted average mass of the atoms in a naturally occurring sample of the element.

15 Isotope Abundance Analyzing an element’s mass can indicate the most abundant isotope for that element. Using the atomic mass, you can usually assume that the most common isotope is the atomic mass rounded to the nearest whole number.

16 Example Problems Find the weighted average of 3 assignments…
Term paper 20% A = 4.0 Midterm 30% B = 3.0 Final 50% C = 2.0 Student receives a “B” on term paper, “C” on midterm, and “B” on final. What is the overall grade for this student?

17 Example Problems Calculate the average atomic mass of nitrogen…
What is the average atomic mass?

18 Example Problems Three isotopes of Element X have atomic masses and relative abundances of amu (78.99%), amu (10.00%), and amu (11.01%). Calculate the atomic mass.

19 Radioactivity Nuclear reactions involve an atom of one element changing into an atom of another element. radioactive atoms undergo changes that alter their identities unstable nuclei lose energy by emitting radiation through radioactive decay

20 Radioactivity Alpha (helium-4 nucleus) Beta (fast moving electron)
Gamma (high energy, no mass)

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