Chapter 3: Atoms The Building Blocks of Matter An atom is the smallest particle of an element that retains the chemical properties of that element. Section 1 The Atom: From Philosophical Idea to
Scientific Theory Page 64 The Early Atom As early as 400 B.C., Democritus called natures basic particle the atomon based on the Greek word meaning indivisible. Aristotle succeeded Democritus
and did not believe in atoms. Instead, he thought that all matter was continuous. It was his theory that was accepted for the next 2000 years. (Read page 43 of your textbook.) Fast Forward to the early1700s: What scientists knew Definition of the word element was
widely accepted Elements combine to form compounds that have different properties from those elements Controversy Did elements always combine in the same ratio when forming a particular compound? How to answer the controversy:
Late 1700s, the study of matter was revolutionized by focusing on quantitative analysis of reactions These studies were made possible by developing and using newer, more precise balances Three Basic Laws of Matter: Law of Conservation of Mass
Law of Definite Proportions Law of Multiple Proportions Basic Laws of Matter Law of Conservation of Mass- mass is neither created nor destroyed during ordinary chemical reactions or physical changes. CH4 + 2O2 2H2O + CO2
16g + 64g 36g + 44g Antoine Lavoisier stated this about 1785 Basic Laws of Matter Law of Definite Proportions no matter how much salt you have, it is always 39.34% Na and 60.66% Cl by mass.
Example: Sodium chloride always contains 39.34% Na and 60.66% Cl by mass. 2NaCl 2Na + Cl2 100g 39.34g + 60.66g 116.88g ? + ?
Joseph Louis Proust stated this in 1794. Basic Laws of Matter Law of Multiple Proportions- Two or more elements can combine to form different compounds in whole-number ratios. Example
John Dalton proposed this in 1803. Daltons Atomic Theory In 1808, Dalton proposed a theory to summarize and explain the laws of conservation of mass, definite
proportions, & multiple proportions. I was a school teacher at the age of 12! Daltons Atomic Theory John Dalton - 1808
1. All matter is composed of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass, and other properties.** 3. Atoms cannot be subdivided, created, or destroyed.** 4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined,
separated, or rearranged. **Today, we know these parts to have flaws. Flaws of Daltons Theory 2. Atoms of a given element are identical inIsotopes size, mass, and other atoms with the
properties. same number of protons but a different number of neutrons 3. Atoms cannot be subdivided, created, or destroyed. Subatomic particles electrons, protons, neutrons, and more
Section 2 The Structure of the Atom Page 70 The Atom Atom - the smallest particle of an element that retains the chemical properties of that element.
CARBON Discovery of the Subatomic Particles The discovery of the subatomic particles came about from the study of electricity & matter. Benjamin Franklins kite experiment in 1752 demonstrated that lightning was electrical.
Charged Particles In 1832, Michael Faraday proposed that objects are made of positive and negative charges.
Discovery of the Electron In the late 1870s many experiments were performed in which electric current was passed through gases at low pressures due to the fact that gases at atmospheric pressure dont conduct electricity well. These experiments were carried out in glass tubes called cathode-ray tubes or tubes.
Crookes Sir William Crookes developed these tubes. Discovery of the Electron Cathode-Ray Tube or Crookes Tube Discovery of the Electron
When current was passed through the cathode ray tube, the surface of the tube, directly opposite the cathode, glowed. It was thought that this glow was caused by a stream of particles called cathode rays. The rays traveled from cathode (negative) to anode (positive). Discovery of the Electron
Negatively charged objects deflected the rays away. Therefore, it was determined that the particles making up the cathode rays were negatively charged. Joseph John Thomson In 1897 the English physicist Joseph John Thomson was able to measure the ratio of charge of the cathode ray particles to
their mass. He found that the ratio was always the same regardless of the metal used to make the cathode or the nature of the gas inside the cathode ray tube. Thomson concluded that cathode rays were composed of identical, negatively charged particles called electrons. Joseph John Thomson
Thomsons experiments revealed that the electron has a very large charge-to-mass ratio. Thomson determined that electrons were present in all elements because he noted that cathode rays had identical properties regardless of the element used to produce them. Cathode Ray Tube Experiment
Accomplishments Proved that the atom was divisible and that all atoms contain electrons. This contradicted Daltons Atomic Theory. This allowed a new model of the atom. Plum-Pudding Model of the Atom Discovery of X-Rays
In 1895 William Conrad Roentgen discovered X-rays, a form of radiation. Radioactivity In 1896, the French scientist Henri Becquerel was studying a Uranium mineral. He discovered it was spontaneously emitting
high-energy radiation. In 1898, Marie and Pierre Curie attempted to isolate radioactive components of the mineral. Radioactivity In 1899, Ernest Rutherford, a British
scientist, began to classify radiation: alpha (a), beta (b), and gamma (g). Radiation Look closely at the paths of radiation. Do you notice something about the amount of deflection of each type of particles?
Radiation Discovery of the Nucleus In 1911, Ernest Rutherford performed a Gold Foil Experiment. He and his colleagues bombarded a thin piece
of gold foil with fast moving, positively charged alpha particles. Alpha Particles Alpha (a) particles are Helium-4 nuclei. This means they are two protons and two neutrons (with no electrons). Thus, they are positive.
4 2 He +2 Gold Foil Experiment Gold Foil Experiment
As expected, most of the alpha particles passed straight through with little or no deflection. However, 1/8000 of the positively charged alpha particles were deflected, some back at the source. (Po) This slide is
animated! Check out the website! Gold Foil Experiment This slide is animated! Check out the
website! Gold Foil Experiment From this experiment, Rutherford discovered that there must be a very densely packed positively charged bundle of matter within the atom which caused the deflections. He called this positive bundle the nucleus. He tried this experiment with other
metals and found the same results. Gold Foil Experiment The volume of the nucleus was very small compared to the volume of the atom. Therefore, most of the atom was composed of empty space. Niels Bohr
later found that this empty space was where the electrons were located. Checking for Understanding Gold Foil Experiment Why did some of the alpha particles come straight back to the source or deflect
away from the nucleus? Why did he conclude that the nucleus must be positive? What three things did Rutherford
conclude from the gold foil experiment? Checking for Understanding Gold Foil Experiment If gold atoms were solid spheres stacked together with no space between them, what would you expect would happen to particles shot at them?
What year did Ernest Rutherford perform this experiment? Rutherford experimented with many kinds of metal foil as the target. The results were always similar. Why was it important to do this?
It was about as believable as if you had fired a 15inch shell at a piece of tissue paper, and it came back and hit you. -Ernest Rutherford Bohrs Model of The Atom 1913 Neils Bohr discovered that electrons orbit
around the nucleus at different distances. These orbits have different amounts of energy based upon how far from the nucleus they are located. More on this in Chapter 4! Discovery of the Proton - 1918 Henry Mosley found that he could use
alpha particles as bullets to knock positively charged particles from the nucleus of the atom. His conclusion was that the nucleus must be a collection of protons. Electro ns in orbit
Nucleus made of protons The Big Question: Similar charges repel, so how can the nucleus be made of all positively charged particles (protons)? The Answer:
The discovery of the neutron - 1932 James Chadwick discovered that the nucleus also includes neutral particles that he called, neutrons. Neutrons act like a glue to help hold the nucleus together. The electron cloud see next slide Nucleus made
of protons & neutrons The electron cloud In 1927, Werner Heisenberg proposed that electrons do not orbit the nucleus like planets orbit the sun. He proposed that electrons form an electron cloud around the nucleus More on
this in Chapter 4! The electron cloud see next slide Nucleus made of protons & neutrons
The electron cloud The Heisenberg Uncertainty Principle: In 1927, Werner Heisenberg proposed that there is no way to know both the location and the velocity of an electron at any given time. The electron cloud is a scatter plot of where you are most likely to find an electron at any given time.
The Simple Atomic Model This simple atomic model combines the big ideas that all of the scientists contributed to the atom. This is the model we will refer to because it is the simplest to understand
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