The Structure of Atoms
We have examined the theoretical implications and practical applications of John Dalton’s ideas about atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. in our discussion on atoms, molecules and chemical reactions, and using chemical equations in calculations. Clearly the atomic theory is a powerful tool which aids our thinking about how much of one substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. can combine with (or be produced from) a given quantity of another. The theory is much less helpful, however, when we try to speculate about what holds the atoms together in molecules such as Br2, HgBr2 and Hg2Br2. As you have seen, techniques are available for experimental determination of the formula of a new compoundA substance made up of two or more elements and having those elements present in definite proportions; a compound can be decomposed into two or more different substances., but Dalton’s theory is of little value in predicting formulas. Neither does it tell us which elements are likely to combine with which, nor indicate what chemical and physical properties are to be expected of the compounds which form.
The ability to make predictions about chemical reactivity and properties is very important because it guides chemists’ efforts to synthesize new substances which are of value to society at large. Medicines, metals, transistors, plastics, textiles, fertilizers, and many other things that we take for granted today have been made possible by detailed knowledge of chemical and physical properties. Such knowledge also permits greater understanding of how the natural world works and what changes (favorable or detrimental) may be brought about by human activities.
Knowledge of chemical reactivity and properties may be approached on both the macroscopic and microscopic levels. Macroscopically this involves what is called descriptive chemistry. The person who first carries out a chemical reaction describes what happened, usually in terms of a balanced equationA representation of a chemical reaction that has values of the stoichiometric coefficients of reactants and products such that the number of atoms of each element is the same before and after the reaction., and lists properties of any new substances. This enables other scientists to repeat the experiment if they wish. Even if the work is not carried out again, the descriptive report allows prediction of what would happen if it were repeated.
The microscopic approach uses theory to predict which substances will react with which. During the past century Dalton’s atomic theory has been modified so that it can help us to remember the properties of elements and compounds. We now attribute structure to each kind of atom and expect atoms having similar structures to undergo similar reactions. Such work has led to the classification of groups of elements, for instance the alkali metals, halogens, alkaline earth metals, and many more. The additional complication of learning about atomic structure is repaid manyfold by the increased ability of our microscopic model to predict macroscopic properties.
In the following sections, you will see that a number of quite different kinds of experiments contributed to the extension of Dalton’s atomic theory to include subatomic particles and atomic structure. The periodic variation of valence and the periodic table’s successful correlation of macroscopic properties indicate that atoms must have certain specific ways of connecting to other atoms. It is reasonable to assume that valence depends on some underlying atomic structure. Atoms which are similar in structure should exhibit the same valence and have similar chemical and physical properties. While the periodicity was initially based upon atomic weightThe average mass of the naturally occurring isotopes of an element, taking into account the different natural abundances of the isotopes. Expressed relative to the value of exactly 12 for carbon-12; also called atomic mass., exceptions to periodic law based upon weight implied some other property led to periodicity.
The property on which periodicity is based is the electronic structure of atoms. Our model for electronic structure is both scientifically and philsophically interesting, because it is based on a wave model for electrons. Click on the "orbitalA mathematically defined region of electron density around one or more atoms; a wave function that defines the properties of a particular electron in an atom or molecule." notations 1s1, 1s2, 2s1 to see the spherical overall shapes of the 3 dimensional distributions for the first 3 electrons added to a lithium nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass.. The dots represent positions of electrons at various times.
The discovery of radioactivity and transmutation implied that one kind of atom could change into another. This can he explained if atoms have structure. A change in that structure may produce a new kind of atom. Experiments with cathodeThe electrode in an electrochemical cell where reduction occurs; the negatively charged electrode in a vacuum tube.-ray tubes indicated that electrons, which are very light and carry a negative charge, are present in all atoms. Rutherford’s interpretation of the Geiger-Marsden experiment suggested that electrons occupy most of the volume of the atom while most of the massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. is concentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). in a small positively charged nucleus. Moseley’s x-rayA high-energy form of electromagnetic radiation that has sufficient energy to ionize inner electrons from an atom. spectra and the existence of isotopes made it quite clear that Dalton’s emphasis on the importance of atomic weight would have to he dropped. The chemical behavior of an atom is determined by how many protons are in the nucleus. Changing the number of neutrons changes the atomic massThe average mass of the naturally occurring isotopes of an element, taking into account the different natural abundances of the isotopes. Expressed relative to the value of exactly 12 for carbon-12; also called atomic weight. but has very little effect on chemistry. The identity of an element depends on its atomic numberThe number of protons in the nucleus of an atom; used to define the position of an element in the periodic table; represented by the letter Z., not on its atomic weight. If the periodic law is restated as “When the elements are listed in order of increasing atomic number, their properties vary periodically,” there are no exceptions.