Department of Chemistry CHEM1010 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel: 931-6325 Email: [email protected] CHEM1010/General Chemistry _________________________________________ Chapter 3. (10)-Atomic Structure
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..More questions about atoms
..To see, and to believe?
..To see or not to see, that may not be the real question in science
..What to see and how to see, that is the question in science
..New experiments and new experimental observations Thomson’s experiment: Discovery of electrons
Goldstein’s experiment: Discovery of positive particles
Millikan’s oil-drop experiment: Determination of electron charge
Rutherford’s experiment: Nuclear model of the atom
Water: H2O, dihydrogen oxide
Baking Soda: NaHCO3, sodium bicarbonate
Vinegar: CH3COOH, HAc, acetic acid, plus H2O
Aluminum Foil: Al
Lime: CaCO3, calcium carbonate
..With the three fundamental laws of chemistry and the Periodic Table, chemistry seems to be fully grown, and almost perfect.
..Until we are asking the following questions:
How small is an atom?
Are atoms the smallest particles that form various things?
Does an atom have a structure? What is the structure of an atom?
How atoms combine together to form molecules?
Why the elements show these periodic regularity?
What is the rationale behind?
How can we explain the periodicities?
Can Dalton’s atomic theory be sufficient to offer the explanations?
How do batteries work?
Why does the Sun emit light?
And many more related questions.
..In science, questions are a very important driving force promoting the development of science.
..By trying to answer the questions, scientists got many many discoveries and obtained many many new observations, and in turn, developed many hypotheses and theories to explain the scientific observations and to predict and predicted many new observations.
..Before we go to see more observations in chemistry and physics, let’s take a look at some important traits of scientific observations.
..In everyday life, by intuition we all believe this is right: To see, and to believe
..Let’s see a few things, and then see if you may say, “wait a minute, and let’s have a second thought!”
..Demo (the trick of light and water)
..Railway experience and paradox
..Maurits Cornelis Escher (1898-1972), Dutch graphic artist
..In science, as in everyday life, we may not see, directly, with our eyes, many things.
Can you see air? Does air exist?
Can you see microorganisms? Do microorganisms exist?
Can you see the stars too far away that its light has to travel so long a time that we all already perish before the light carrying the information of their existence can reach the Earth? Do the stars exist?
Can you see the Earth? Does the Earth exist?
..In science, we use controlled experiments and measurements to “see” things, using instruments as well as reasoning.
..Observations and facts in science are not purely reflections of our senses, but instead, they are combination of experimental measurements and reasoning.
..In order to know what to see, scientists need some preliminary ideas (reasoning).
..Role of accidental discoveries in sciences
Many examples in the history of science.
Example: Roentgen’s discovery (to be discussed later, textbook, pp61-61)
..Imagination in sciences
Since in science, we are facing many things we cannot see with our eyes or direct senses, we need imagination in science to assist us to come up with models and theories and hypotheses to describe the nature and explain the experimental observations.
..Cathode ray tubes (TV screen, fluorescence tube)
Cathode, negatively charged electrode where positive particles would go
Anode, positively charged electrode where negatively charged particles would go
Cathode ray tube (see textbook pp.59), gas discharge tube (vacuum with a bit gas in it)
William Crookes (1832-1919, English chemist)’s experiment:
The beam of current in the powered cathode ray tube coated with ZnSO4 and cathode ray.
..Question: What is the nature of the cathode ray? Is it a beam of particles or a beam of light?
..English physicist, Joseph Thomson’s experiment in 1897
Experiment: Applied an electric field by two oppositely charged plates surrounding the tube
Observation: Cathode rays were deflected in an electric field, attracted to the positive plate and repelled by the negative plate.
The particles were the same regardless of the cathode materials and the gas filled
Conclusion: The cathode ray is negatively charged particles, which are the components of all kinds of atoms.
Thomson named the particle as electron.
..More observations: Cathode rays can also be deflected in magnetic fields.
..Thomson was able to determine the ratio of the mass of the electron over its charge by measuring the amount of deflection in the fields of known strength.
..But, with his experimental setup, he could not determine either the mass or the charge along.
..However, if either one is known, the other can be determined easily. Question: electron mass and charge?
..Thompson was awarded the Nobel Prize in 1906.
..Now, we know a new kind of particle in the atoms.
..Significance of Thomson’s experiment
-Atoms should contain electrons, which are negatively charged.
-Hence, atoms must also have positively charged components, because atoms are electrically neutral as we know.
-Now, the theory, or model, or hypothesis, is that atoms are composed of electrons and some unknown positively charged particles.
-Atoms are not ultimate particles and they can be further reduced into smaller particles at sub-atomic levels.
-Of course, now more questions pop out!
..In 1886, Eugen Goldstein (German scientist), using gas discharge tube, found positively charged particles shooting cathode.
..In 1907, the study of deflection of the positive particles in a magnetic field indicated that the particles have varying mass.
..The lightest mass of the particles is a mass 1837 times of the mass of an electron.
..This means the positively charged particles are much heavier than electrons.
Now, a new kind of sub-atomic particles were discovered, other than electron.
..American physicist, Robert Millikan (1868-1953) determined the charge of an electron in 1909.
..The experimental approach he used was to suspend tiny negatively charged droplets of oil with electrons captured by friction as a result of the balance between the electric attraction of the charged plates and gravity of the oil droplets.
..He used a microscope to view the oil droplet chamber.
..With his experimental setup, Millikan was able to determine the mass:charge ratio of the oil droplets by the voltage needed to suspend them.
..He then let the oil droplets fall without the electric field applied so as to determine the mass of the droplets. And then he was able to determine the charge of an electron captured by the oil droplet.
..Since now the charge of an electron is known, using the Thomson’s mass:charge ratio, the mass of an electron was found out to be 9.1×10-28 g. Extremely light.
..Millikan was awarded the Nobel Prize in 1923.
..Significance of Millikan’s experiment
-We now know that electron is extremely small and has an extremely small mass
-Hence, atoms must have something else that bear the major mass of an atom.
-Of course, now more questions pop out!
..The experiment of Ernest Rutherford (1871-1937, New Zealand physicist) under his supervision conducted by Hans Geiger (1882-1945, a German physicist) and Ernest Marsdon (1889-1970, an English undergraduate student)
Alpha particles: Mass 4 times of a hydrogen with a positive charge twice of that of an electron
Using alpha particles emitted from a radioactive source to bombard a thin gold foil (see textbook pp. 64 for detailed experimental setup).
..Hypothesis: An atom has its mass and positive charge uniformly distributed all over the space it occupies. And the positively charged inner components are possibly as small as electrons.
..Prediction 1: Most of the alpha particles should go right through the foil because alpha particles are big and small inner components cannot block them.
..Prediction 2: The alpha particles should spread in all possible directions, if deflected from the atoms by repelling of the positively charged inner unknown atomic components.
-Most of the alpha particles did go through the foil smoothly with little or no scattering.
-But, a few ones deflected sharply, and sometimes, some particles went back to where they came from originally.
This suggests that something as heavy as or as big as the alpha particles existing in an atom.
..Rutherford’s revised hypothesis or theory to explain the experimental observations:
Almost all the mass and charge of an atom are concentrated at the center of the atom in a tiny core, which was named as the nucleus. Most of the space in an atom is empty, but probably filled with electrons. This is because only a few particles were actually deflected. This is why most of the particles went through the atom.
Now, the atomic structure has this updated picture: An atom has negatively charged electrons and a heavy positively charged nucleus in the center. The electrons thus probably surround the nucleus so that an atom is uncharged overall.
..An analogy to visualize an atom with Rutherford’s atomic model:
You may picture an atom as a sphere as big as a huge indoor football stadium; the nucleus at the center of the sphere is as tiny as a pea with a mass of as large as several million tons; some flies are moving around the pea in the rest of the empty space.
Note: This is the Rutherford’s atomic model.
..Significance of Rutherford’s experiment and theory:
An atom is indeed dividable further. In other words, atoms are not the smallest particles of which matter is composed.
A brand new picture was created about the structure of an atom.
This theory is revolutionary in that mass and charge are not continuously distributed in an atom.
This theory is revolutionary.
..New question: Is the nucleus further dividable or it is the ultimate particle?
..In 1914, Rutherford proposed that the smallest positive ray particle as the one formed in the Goldstein experiment is the component of positive charge in the nucleus.
This particle is called proton, which has the charge equal to that of an electron and a mass almost like that a hydrogen atom.
All the positive charge of the nucleus results from protons.
Hydrogen atom has one proton, while other larger atoms have more than one protons. The number of protons in the nucleus increases with increasing of atom mass.
In short, nuclei are composed of protons. So, the nucleus is further dividable, but only to one kind of sub-nuclear level.
..However, new experimental observations indicated that the mass of the nuclei of the atoms other than hydrogen were larger than the mass as indicated only by the mass of protons contained in an atom.
Example: Helium has a mass four times the mass of hydrogen, but has only 2 positive charges, which means two protons, only twice the mass of hydrogen (recall: the mass of a proton is the mass of one hydrogen atom)
Question: How can the extra mass be explained?
..Discovery of James Chadwick (1891-1974, English physcist)
In 1932, Chadwick discovered a new kind of particle with a mass almost the same as that of a proton but without any charge, called neutron.
Now, the extra mass can be explained by the presence of neutrons.
In the case of helium, it thus has 2 protons and 2 neutrons. So, its overall mass is four times that of hydrogen (recall: mass of hydrogen = mass of proton = mass of neutron)
..A nucleus contains proton(s) and neutron(s), except for hydrogen, which has only one proton and no neutron.
..Proton mass = 1.0073 u
Neutron mass = 1.0087 u
electron mass = 0.00055 u
Hence, the mass of a nucleus is the mass of proton plus neutron, and the mass of electron can be ignored, regarded as zero practically.
..Now we can say an atom contains two major subatomic components: electron negatively charged and nucleus positively charged which is composed of proton(s) and neutron(s), except for hydrogen, which has only one proton and no neutron.
The mass of an atom is the mass of nucleus (the mass of a nucleus is the mass of proton plus neutron), because the mass of electron can be ignored, regarded as zero practically.
..The role of each sub-atomic particle in an atom:
Electron: Carrying negative charge, no contribution to mass of an atom
Proton: Carrying positive charge to balance the negative charge of electron, contributing to the mass of the atom
Neutron: Carrying no charge, contributing to the mass of the atom
Electron e- 1/1837 1- outside
Proton P+ 1 1+ nucleus
Neutron n 1 0 nucleus
The atomic number is the number of protons in the nucleus of an atom of a certain kind.
Significance: Atomic number determines the identity or kind of an atom.
One kind of atom is different from the other because of different numbers of protons, and then likewise because of different numbers of electrons.
Hence, protons and electrons play very important roles in chemistry.
We know currently there are 114 elements, that is, 114 different kinds of atoms.
This means: in the Periodic Table, the first atom/element has 1 proton, 2nd atom/element has two protons, and so forth, and the 114th atom/element has 114 protons.
The same is right for the number of electrons. The first atom/element has 1 electron, 2nd atom/element has two electrons, and so forth, and the 114th atom/element has 114 electrons.
In short, if you know the atomic number of an atom/element, you know the number of proton(s) in the atom and the number of electron(s) in the atom.
The mass of any atom is practically equal to the mass of its nucleus because
(a) the mass of proton nearly equals that of neutron;
(b) neutron has no charge;
(c) electron has negative charge;
(d) mass of electron is extremely small and can be regarded as zero and thus can be ignored in the overall mass of an atom.
An atom has no overall charge because
(a) neutron has no charge;
(b) electron nearly has no mass;
(c) the charge of an electron is the same as that of a proton, the two have just opposite charge, and the number of electrons equals that of protons;
(d) the mass of a proton is practically equal to that of a neutron.
The basic structure of an atom other than hydrogen is that
(a) it is composed of electrons only;
(b) it is composed of electron(s) and a nucleus containing neutron(s) only;
(c) it is composed of electron(s) and a nucleus containing proton(s) only;
(d) it is composed of electron(s) and a nucleus containing both proton(s) and neutron(s).
If you know the number of electrons in an atom, then you should know
(a) the number of neutrons of the atom;
(b) the temperature of the atom;
(c) the atomic number of the atom;
(d) the location of the atom.
The mass of a proton is almost equal to
(a) the mass of an electron;
(b) the mass of a water molecule;
(c) the mass of an alpha particle;
(d) the mass of a neutron.