Chemistry AI Chapter 4 A. Atom (Section 4.1) 1. Democritus (460B.C.-370B.C.) The smallest part of an element that retains its identity during a chemical reaction 1st to suggest the idea of an atom. He believed they were indestructible and indivisible. He never experimentally tested his idea.

2. Dalton’s Atomic Theory ( ) 12 year old English teacher and chemist Atomic Theory 1. All elements are composed of tiny indivisible particles called atoms. 2. All atoms of one element are exactly alike, but they differ from the atoms of a separate element. 3. Atoms of different elements can physically mix or chemically combine. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

3. Size The radii of most atoms fall in the range of pm. Scanning tunneling microscopes are used to observe atoms. (Page 100, Pg103 Figure 4.3)

A.Structure of the atom 1. J.J.Tomson ( ) M odification of the Atomic Theory He used a cathode ray tube to show the presence of negatively charged particles called electrons Electrons are negatively charged subatomic particles cathode ray tube in TV cathode demonstration and explanation 2. Eugen Goldstein ( ) Used the canal rays in a cathode ray tube to discover protons. Protons are positively charged subatomic units

3. James Chadwick ( ) Confirmed the existence of neutrons Neutrons are subatomic particles with no charge but with a mass equal to that of a proton

4. Ernest Rutherford ( ) Previously Thomson’s model of the atom resembled chocolate cookie dough with the dough containing the protons and neutrons and the chocolate chips being the electrons. a. The Gold Foil Experiment Steps of the experiment (Figure 4.7) 1. Released positively charged alpha particles from a radioactive source towards a very thin sheet of gold foil 2. the beam of particles struck the gold foil

a. The Gold Foil Experiment Results: 1. Most of the particles went straight through the foil 2. Some of the particles were deflected to the side 3. Some of the particles reflected directly back at the source animation of experiment

1.The atom is mostly empty space where the electrons are located. (#1) 2.The atom has a nucleus which is a tiny dense central core. (#2) 3.The nucleus is positive containing the protons and neutrons. (#3) Conclusions: Based of the results from his experiment, Rutherford developed the following theory of the atom: b. Rutherford’s Atomic Model

Bohr modified Rutherford’s model to include newer discoveries about how the energy of an atom changes. Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. Bohr said that each possible electron orbit had fixed energy and these fixed energies of an electron are called energy levels. 5. Niels Bohr model (1913)

The fixed energy levels are much like the rungs on a ladder and an electron cannot exist between these energy levels. For an electron to move to another energy level they have to gain the correct amount of energy. The energy levels are not equally spaced and are closer together the further the electron gets from the nucleus. Bohr’s model worked well for the hydrogen atom but failed to explain the behavior of many other atoms. animated Bohr model The amount of energy required to move from one energy level to the next. a. quantum

6. Schrödinger (1926) He proposed a mathematical equation to describe the behavior of electrons in atoms. From his research the Quantum Mechanical Model was proposed. a. atomic orbital These are the regions of space (fuzzy clouds) that describe the probability of finding an electron.

C. Atomic Terminology 1.Atomic Number The number of protons in the nucleus of an atom, for a particular element (Table 4.2) It is also the number of electrons in a neutral atom. The number of protons identifies the element

The sum of the protons plus the neutrons in the nucleus. Most of the mass of an atom is in the nucleus. #neutrons = Mass # - atomic # Short hand for a particular atom 197gold Au 2.Mass Number

3. IsotopesAtoms of the same element that are chemically alike but differ in their number of neutrons and mass

1.Atomic mass a. atomic mass unit (amu) Weighted average mass of all the isotopes for a particular element. 1amu = 1/12 the mass of a carbon -12 atom 1proton = 1amu 1 neutron = 1 amu The mass of an electron is negligible in comparison.

To calculate the atomic mass you will need: 1.# of isotopes 2.mass # of the isotopes 3. natural abundance of each isotope in decimal form. Atomic Mass = (mass # * abundance) + (mass # * abundance ) + ……

5. Periodic Table a. Period b. Group An arrangement of elements into groups based on their chemical properties Horizontal rows; 7 periods; patterns of properties repeat from one period to the next. Vertical column; elements within a group have similar chemical properties.