atomic structure. scientific modeling a.scientists develop models to explain things they cannot...
TRANSCRIPT
Atomic Structure
Scientific Modeling
A. Scientists develop models to explain things they cannot directly observe
B. Models of Atoms are based on scientific discoveries
1. Models can be wrong and still use useful2. Scientists construct models based on the
data or evidence that is available at the time3. Early history regarding atoms4. The greek philosopher Democritus of
Abdera lived in the 4th centure B.C5. He was the first person to suggest the idea
of atoms ( he called them atomos)6. His ideas were not useful because they
were not based on experimental evidence
C. John Dalton’s Atomic therory (early 1800’s)
1. Dalton was the first person to develop a model of the atom
2. His model was based on experimental evidence which showed that elements combine in the same percent by mass each time the same compound in formed
3. Dalton thought the atom was indivisible uniformly dense, solid sphere that entered into chemical reactions, but was left unchanged by the reaction
II. Electrons, Protons, and Neutrons
A. Dalton’s theory that the atom is indivisible has been revised
1. There are dozens of subatomic particles
2. Electrons, protons, and neutrons are the particles important in chemistry
C. Electrons
1. Negatively charged subatomic particles
a. Sir Joseph J Thomson (1856 – 1940), English physicist, experimented with cathode ray tubes
1. A cathode ray tube is a closed glass tube with metal disks called electrodes at each end.
2. The tube contains a gas at low pressure
3. When connected to a high – voltage source, the tube glows.
a. The anode ( one electrode) becomes positively charged while the cathode ( the other electrode) becomes negatively charged
b. The glowing beam travels from the cathode the anode and thus is called the cathode ray
b. Thomson discovered that the cathode rays could be deflected (bent) by magnets or electrically charged plates.
c. Thomson showed that the cathode ray is a collection of very small negatively charged particles that are all alike. He named them electrons.
D. Protons1. Thomson and Goldstein further investigated
and found that when the CRT was subjected to high-voltage electricity, the negative beam of electrons (cathode rays) moved to the anode
2. A careful observation revealed another beam of particles traveling in the opposite direction toward the cathode
3. The new beam called (anode rays or canal rays) contains positive particles, because they move toward the cathode ( negative electrode)
4. The positively charged particles were called protons
5. Protons are 1840 times heavier than an electron
E. Thomson revised Dalton’s model of 1903.
1. Thomson’s model of the atom showed the bulk of the atom was made of the positive protons, with the negative electrons dispersed throughout to give an overall neutral charge.
F. Discovery of the nucleus
1. Ernest Rutherford in 1911 used an experiment called the Gold Foil Experiment
D. Neutrons
1. Sir James Chadwick – confirmed the presence of a third subatomic particle, the neutron
2. Neutrons have no charge
3. The mass of a neutron equals that of the proton
4. Neutrons are found in the nucleus
E. New discoveries of subatomic particles
1. These particles are unstable and do not exist in ordinary matter
2. There are 2 families of particles
a. Leptons
• Electrons – best known of the leptons
• Mu – mesons
• Tau-mesons
• Neutrinos (3 types)
b. Hadrons
1. Hadrons consist of smaller particles called quarks
2. Hundreds of hadrons are known. The most familiar hadrons are the following
• Protons
• Neutrons
• pion
Atomic Number and atomic Mass
A. Atomic number – the atomic number of an element is the number of protons in the nucleus of an atom of that element
1. Elements differ from one another only by the number of the protons the atoms contain in the nuclei
2. This difference ( in the number of protons) is the key to the following
• Arrangement of the elements in the periodic table
• Chemical properties of the elements
Because no two elements have the same atomic number, the atomic number (number of protons) defines the element
3. In a neutral atom, the number of protons must equal the number of electrons
• Protons and electrons have equal but opposite charges
• Hydrogen has one positive charge (one proton) and one negative charge (one electron). Together they balance the electrical charges to produce a neutral atom
B. Mass Number – the mass number of an element is the total number of protons and neutrons of an atom of that element
Mass number – atomic number = # of neutrons
Isotopes
• Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
• Isotopes of an element are chemically alike
Atomic MassA. Since both protons and neutrons have a mass
of 1 atomic mass unit (AMU) each it is reasonable to think that the mass of an atom should be expressed as a whole number
B. The atomic mass of an element is the weighted average of the masses of all the isotopes of that element
1. When calculating the average atomic mass you must take into account the relative abundance of each isotope
Chlorine has two isotopes both with 17 protons in their nuclei. One isotope has 18 neutrons and thus its atomic mass is 354 amu. This isotope is referred to as chlorine – 35. The other isotope is chlorine – 37. It has 20 neutrons. In nature the isotopes are found in a ratio of almost 3:1. Use (0.75) for the 3 and 25% (0.25) for the 1
Chlorine – 35 35 amu ∙ 0.75 = 26.25
Chlorine – 37 37 amu ∙ 0.25 = 9.25 +
Total = 35.5 amu
The mass spectrometer• The mass spectrometer separates atoms of
slightly different masses, thus it can separate isotopes of the same element