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Chapter 4
Atoms and Elements

04-03
Title
The plum-pudding model of the atom
Caption
The plum-pudding model of the atom was an early—and now obsolete—attempt to describe the interior structure of atoms.
Notes
Thomson's plum-pudding model arose from cathode ray experiments in the late 1800s.
Keywords
plum-pudding, Thomson, atom, electron, model
04-04
Title
Rutherford's gold foil experiment
Caption
Rutherford's experiment provided evidence that the positively charged part of the atom consisted of a tiny, dense object at the atom's center.
Notes
The history of Rutherford's experiment reveals a wonderful example of a careful, scrupulous scientist working hard to remain focused on observation as the basis for his conclusions: It would have been very easy for Rutherford to have dismissed the minor differences between what he saw and what he expected to see.
Keywords
Rutherford, atom, model, gold foil, electron, nucleus
04-05C
Title
Rutherford's expected and actual results
Caption
Rutherford expected the a-particles to pass undeflected through the atoms. But, Rutherford observed that a small fraction of the a-particles were deflected; to explain the observation, he proposed the nuclear model of the atom.
Notes
To deflect the energetic a-particles, the nucleus must be dense, with positive charge. For the fraction deflected to be small, the nucleus must be small, relative to the overall size of the atom.
Keywords
Rutherford, atom, model, nuclear atom, electron, nucleus
04-06
Title
The nuclear atom
Caption
Nearly all of the atom's mass is in the nucleus, which consists of protons and neutrons. Nearly all of the atom's volume is supplied by the electrons, which exist outside the nucleus.
Notes
Students can be challenged to explain how the electrons, with negative charge, remain outside of the nucleus, with its positive charge.
Keywords
Rutherford, atom, model, nuclear atom, electron, nucleus
04-07a,b,c
Title
Opposite charges attract; like charges repel
Caption
Positive and negative charges attract one another, but positive charges repel other positive charges, and negative charges repel other negative charges. Positive and negative charges have the same magnitude, so that the combination of a positive and negative charge is neutral.
Notes
With the discovery of positive and negative charges in atoms, chemists had a basis for chemical bonding: Atoms bond to one another through attraction of positive and negative charges.
Keywords
positive, negative, electron, proton
04-09
Title
The periodic table
Caption
The periodic table organizes the elements on the basis of similar chemical properties. Elements within the same group (column) tend to have the most similar properties.
Notes
The periodic table served much the same function as a scientific theory: through its ordering of the elements, predictions could be made about the properties of elements not yet discovered. If, when the elements are finally discovered, the observations match the predictions, we gain confidence in the atomic theory.
Keywords
periodic table, element, group, period, atomic number, atomic mass, chemical symbol
04-09-02gC
Title
Marie Curie, and a representation of curium from the periodic table
Caption
Marie Curie was a key figure in the development of our understanding of the elements. She discovered radium (Ra) and polonium (Po). Curium (Cm) was named in her honor.
Notes
Marie Curie discovered new elements that gave off radioactivity.
Keywords
Marie Curie, element, radium, polonium, curium, radioactivity
04-10
Title
Listing of the first twenty elements
Caption
These are the first twenty elements, by atomic number. The elements are color-coded to show similarities in properties.
Notes
The elements' properties follow a repeating pattern.
Keywords
periodic table, element, atomic number, chemical symbol
04-11
Title
The first twenty elements, in their periodic table groups
Caption
The periodic table groups elements according to similarities in their chemical properties. For example, He, Ne, and Ar belong to the same column (group) because they are known to be unreactive.
Notes
Elements are grouped by similar chemical properties, but with the discovery of the electron, the elements were also found to be grouped according to similar electron configuration.
Keywords
periodic table, element, atomic number, chemical symbol
04-12
Title
The periodic table, showing metal, metalloids, and nonmetals
Caption
The periodic table groups elements according to similarities in their chemical properties. The grouping also organized the elements into regions based on metallic character: metals, nonmetals, and metalloids.
Notes
Elements are grouped by similar chemical properties, but with the discovery of the electron, the elements were also found to be grouped according to similar electron configuration.
Keywords
periodic table, element, atomic number, chemical symbol, metal, metalloid, nonmetal
04-13
Title
The periodic table, showing the main group in yellow; transition metals in gray
Caption
The main group elements have s sublevels and p sublevels at highest energy; the transition metals have d sublevels and f sublevels at highest energy.
Notes
Elements are grouped by similar chemical properties, but with the discovery of the electron, the elements were also found to be grouped according to similar electron configuration.
Keywords
periodic table, element, atomic number, chemical symbol, main group, transition metal
04-13-01k
Title
Some groups in the periodic table are recognized by name
Caption
The more commonly encountered groups of elements are named: alkali metals, alkaline earth metals, halogens, and noble gases, for example.
Notes
Other groups are also named. For example, O, S, Se, Te, and Po are the chalcogens, and N, P, As, Sb, and Bi are the pnictogens. Groups are also known by number: For example, the halogens are also Group 7 elements.
Keywords
alkali metals, alkaline earth metals, halogens, noble gases, element, periodic table, chemical symbol
04-14
Title
The periodic table, showing commonly formed ions
Caption
The alkali metals reliably form +1 ions; alkaline earth metals form +2 ions; Group 6 elements form -2 ions, and halogens form -1 ions. Elements in Group 3 usually form +3 ions, and other elements vary in their ion charges.
Notes
Metals generally form positive ions, and nonmetals form negative ions. Main group elements tend to form only one type of ion, while the transition metals tend to form two or more types of ions.
Keywords
alkali metals, alkaline earth metals, halogens, element, periodic table, chemical symbol, ion, anion, cation
04-15-01un
Title
A-X-Z notation describes an atom's nucleus, and helps us to distinguish isotopes from one another.
Caption
The A-X-Z notation communicates the number of protons in an atom's nucleus (the atomic number, Z) and the sum of the protons and neutrons in the nucleus (the mass number, A).
Notes
The neutrons can be calculated by the formula: neutrons = A – Z. The mass number gets its name from the fact that protons and neutrons weigh about the same, and together account for virtually all of an atom's mass. Therefore, the sum of the protons and neutrons gives a good picture of an atom's overall mass.
Keywords
periodic table, element, atomic number, chemical symbol, proton, neutron, mass number, isotope
04-15-02un
Title
The X-A notation describes an atom's nucleus, and helps us to distinguish isotopes from one another.
Caption
The X-A notation is a common alternative to the A-X-Z notation, as a way to distinguish isotopes from one another.
Notes
We can translate X into Z using the periodic table. The neutrons can then be calculated by the formula: neutrons = A – Z. The mass number gets its name from the fact that protons and neutrons weigh about the same, and together account for virtually all of an atom's mass. Therefore, the sum of the protons and neutrons give a good picture of an atom's overall mass.
Keywords
periodic table, element, atomic number, chemical symbol, proton, neutron, mass number, isotope

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