Atomic theory

In 1808 Dalton published New System of Chemical Philosophy in which he presented his theory of atoms  

These statements are a modern paraphrase of Dalton s Ideas. 

Each element is made up of tiny particles called atoms. 

The atoms of a given element are identical the atoms of different elements are different in some fundamental way or ways. 

Chemical compounds are formed when atoms of different elements combine with each other. A given compound always has the same relative numbers and types of atoms. 

But in order to gain a deeper understanding of how fountains work it is necessary to grasp the nettle of atomic theory and also of the electron. 

Imagine an atom as a bubble magnified to an immense size (about the size of a small town). If you had the magical and perceptive powers of Alice in Wonderland you might penetrate the outer cover of the empty bubble and wander through its atmosphere which, on closer inspection, would be seen to be not quite empty . 

Now the protons carry positive charges, while the neutrons are electrically neutral. But nature has arranged things so that the negative 

The rest of the atom is sparsely populated but also vibrant and dynamic. The ghostly electrons are arranged in vague clouds and have no clearly defined position. Heisenberg s Uncertainty Principle (1927) tells us that we can t pin-point their positions. Instead, we have to talk in terms of the probability of there being electrons of a certain energy in certain positions (or orbits) around the nucleus at certain times. 

The most significant feature from a firework maker s point of view is that the outermost electrons (furthest from the nucleus) posses higher energies than their innermost cousins and are also reactive. In fact, they are so reactive that they can be made to rearrange their positions in the hierarchy of the atom. 

The idea that all matter is made of atoms is a familiar concept that that can be recited by the average first grader. However, the general acceptance of the atomic theory dates to the early part of the twentieth century, when Albert Einstein published a paper explaining Brownian motion. The notion of atoms dates back to ancient times, but these models are largely based on philosophical arguments, rather than empirical evidence. John Dalton first formulated an atomic theory 

3 Actually, it depends on the mass-to-charge ratio of the ion. However, it is unusual for the ionization sector to remove more than one electron. Therefore, the charge on the molecular ion is almost certain to be +1. Therefore, the mass/charge ratio is equal to the mass/1, which is numerically equal to the mass of the particle. 

Compounds are formed by bonding atoms together in a fixed ratio. 

Chemical reactions do not create, destroy, or change atoms into atoms of other elements. Chemical reactions cause atoms to recombine into new substances. 

The English chemist John Dalton was responsible for reviving the theory of atoms during the first decade of the nineteenth century. Dalton argued that the law of conservation of mass in chemical reactions and the definite proportions of chemical compoimds were convincing evidence for the existence of atoms. In recognition of his work, Dalton is often called the father of modern atomic theory. 

With a theory of atoms, chemists began to have a more exact way to define elements and compounds elements are pure substances that contain only one kind of atom compounds are pure substances that contain more than one kind of atom and that are present in very definite proportions. Mixtures, on the other hand, were recognized as impure substances that consist of more than one element or compoimd, but which are present in variable proportions. 

Laboratory experiments shed light on the chemical composition of compounds. The determination of crude atomic weights of elements gave rise to listing elements in order of weight. But fundamental questions persisted How many elements are there  

Are the chemical and physical properties of elements completely random or does some logical framework exist in which to arrange elements so that their properties make sense It took more than a century to answer these questions. 

Planck s Quantized Energies and Einstein s Photoelectric Effect 

In order to better understand how our existing model of the atom evolved, we must step back to about 1900, when physicist Max Planck discovered an unusual property of atoms. While studying the spectra emitted from glowing objects, Planck concluded that energy could be emitted or absorbed from atoms only in fixed amounts, or quanta. He proposed that this amount of energy (E) was directly proportional to the frequency (v) of the electromagnetic wave. Mathematically, this is expressed in the formula  

Shortly after Bohr s proposal, Louis de Broglie made an important proposal. He said that if waves have matter-like properties, then matter should have wave-like properties. It is not necessary for you to know the de Broglie equation, but you should understand that it predicted that matter of normal mass would create infinitesimally small waves. It is only matter with an extremely small mass, like an electron, and traveling at high speed that will emit appreciable wavelengths. 

TABLE 4.1 SUMMARY OF THE MAJOR CONTRIBUTORS TO MODERN ATOMIC THEORY 

John Dalton Dalton s atomic theory First quantitative evidence for discrete particles (atoms) 

In 1804, John Dalton proposed the existence of atoms. He not only postulated that atoms exisL as had ancient Greek philosophers, but he also attributed to the atom certain properties. His postulates were as follows  

Elements are composed of indivisible particles, called atoms. 

All atoms of a given element have the same mass, and the mass of an atom of a given ele from the mass of an atom of any other element. 

When elements combine to form a given compound, the atoms of one element combine I other element(s) in a definite ratio to form molecules. Atoms are not destroyed in this prop 

Atoms of two or more elements may combine in different ratios to form different compoJ The most common ratio of atoms is 1 1, and where more than one compound of two or mo the most stable is the one with 1 1 ratio of atoms. (This postulate is incorrect.) 

Thomson Plum Pudding Model Charge-to-mass ratio of electron Work with cathode rays discovered the positive and negative nature of the atom also determined the charge-to-mass ratio for electrons 

Max Planck Quantized energy Energy is released from atoms in discrete packets, or quanta. 

To see a WorCcC in a Qrain of Sand JAnda J-Ceaven in a y iCd jCower J-CoCdInfinity in the j aCm of your hand lAnd Eternity in an hour 

This chapter offers a brief, qualitative introduction to the mathematical description of electrons and describes the highly utilitarian model of atomic structure that chemists have constructed from it. 

Because we are reaching beyond the world of our senses, we should not be surprised that the model we create is uncertain and, when described in normal language, a bit vague. In spite of these limitations, however, you will return from your journey into the strange, new world of the extremely small with a useful tool for explaining and predicting the behavior of matter. 

Chemists try to see the structure of matter even more closely than can be seen in any photograph. 

The presentation of information in this chapter assumes that you can already perform the tasks listed below. You can test your readiness to proceed by answering the Review Questions at the end of the chapter. This might also be a good time to read the Chapter Objectives, which precede the Review Questions. 

Dalton s liypothesis of the existence of atoms as definite quantities did not, however, meet with general acceptance. Davy, Wollaston, and others considered the quantities in Avhich Dalton had found the elements to unite with each other, as m oo proportional numbers or equivalents, as they expressed it, nor is it probable that Dalton s views would have received any further recognition until such time as they might have been exhumed from some musty tome, had their publication not been closely followed by that of the results of the labors of Humboldt and of Gay Lussac, concerning the volumes in which gases unite with each other. 

In the form of what are known as Gay Lussac s laws, these results are  

—There exists a simple relation between the volumes of gases which combine with each other. 

Second.—There exists a simple relation between the sum of the volumes of the constituent gases, and the volume of the gas formed by their union. For example .  

1 volame chlorin unit 8 with 1 volume bydrog en to form 2 volumee hydrochloric acid. 

A given compound always contains elements in exactly the same proportion by mass. 

As scientists of the eighteenth century studied the nature of materiais, several things became clear  

Most natural materials are mixtures of pure substances. 

Pure substances are either elements or combinations of elements called compounds. 

A given compound always contains the same proportions (by mass) of the elements. For example, water always contains 8 g of oxygen for every 1 g of hydrogen, and carbon dioxide always contains 2.7 g of oxygen for every 1 g of carbon. This principle became known as the law of constant composition. It means that a given compound always has the same composition, regardless of where it comes from. 

---

The dawn of the nineteenth century saw a drastic shift from the dominance of French chemistry to first English-, and, later, German-influenced chemistry. Lavoisier s dualistic views of chemical composition and his explanation of combustion and acidity were landmarks but hardly made chemistry an exact science. Chemistry remained in the nineteenth century basically qualitative in its nature. Despite the Newtonian dream of quantifying the forces of attraction between chemical substances and compiling a table of chemical affinity, no quantitative generalization emerged. It was Dalton s chemical atomic theory and the laws of chemical combination explained by it that made chemistry an exact science. 

For two thousand years atoms were considered the smallest and indivisible units of nature. At the beginning of the nineteenth century Dalton got chemistry on the path of atomic theory with his book, A New System of Chemical Philosophy, in which he argued that unbreakable atoms form compounds by linking with other atoms in simple... 

The concept that all substances are composed of elements and atoms goes back at least 2000 years. Originally, only four elements were recognized air, earth, fire, and water. Each substance was thought to consist of very small particles, called atoms, that could not be subdivided any further. This early mental concept of the nature of matter was extremely prescient, considering there were no experimental results to indicate that matter should be so and none to verify that it was so. Modern atomic theory is much more rigorously based, and we even have the ability to see atoms with special tunneling microscopes. All of chemistry is based on how atoms react with each other. 

Whereas zirconium was discovered in 1789 and titanium in 1790, it was not until 1923 that hafnium was positively identified. The Bohr atomic theory was the basis for postulating that element 72 should be tetravalent rather than a trivalent member of the rare-earth series. Moseley s technique of identification was used by means of the x-ray spectra of several 2ircon concentrates and lines at the positions and with the relative intensities postulated by Bohr were found (1). Hafnium was named after Hafma, the Latin name for Copenhagen where the discovery was made. 

This system of nomenclature has withstood the impact of later experimental discoveries and theoretical developments that have since the time of Guyton de Morveau and Lavoisier greatiy altered the character of chemical thought, eg, atomic theory (Dalton, 1802), the hydrogen theory of acids (Davy, 1809), the duahstic theory (Berzehus, 1811), polybasic acids (Liebig, 1834), Periodic Table (Mendeleev and Meyer, 1869), electrolytic dissociation theory (Arrhenius, 1887), and electronic theory and modem knowledge of molecular stmcture. 

Hume-Rothery (1946) Atomic Theory for Students of Metallurgy (The Institute of Metals, London). 

The oxides of nitrogen played an important role in exemplifying Dalton s law of multiple proportions which led up to the formulation of his atomic theory (1803-8), and they still pose some fascinating problems in bonding theory. Their formulae, molecular structure, and physical appearance are briefly summarized in Table 11.7 and each compound is discussed in turn in the following sections. 

The discovery of hafnium was one of chemistry s more controversial episodes. In 1911 G. Urbain, the French chemist and authority on rare earths , claimed to have isolated the element of atomic number 72 from a sample of rare-earth residues, and named it celtium. With hindsight, and more especially with an understanding of the consequences of H. G. J. Moseley s and N. Bohr s work on atomic structure, it now seems very unlikely that element 72 could have been found in the necessary concentrations along with rare earths. But this knowledge was lacking in the early part of the century and, indeed, in 1922 Urbain and A. Dauvillier claimed to have X-ray evidence to support the discovery. However, by that time Niels Bohr had developed his atomic theory and so was confident that element 72 would be a... 

E. Schrodinger (Berlin) and P. A. M. Dirac (Cambridge) discovery of new productive forms of atomic theory. 

Atom-kem, m. atomic nucleus, -kette,/. chain of atoms, atomic chain, -lage, /. atomic layer atomic position, -lehre, /, doctrine of atoms, atomic theory, -mechanik, /. mechanics of the atom, -modell, n, atomic model, -nummer, /, atomic number, -ord-nung, /. atomic arrangement, -refraktion, /. atomic refraction, -rest, m. atomic residue (= Atomrumpf). -ring, m. ring of atoms, -rumpf, m. atomic residue or core (remainder of an atom, as after removal of some electrons), -schale, /, atomic shell, -strabl, m. atomic ray, -tafel, /, atomic table, atomtbeoretisch, a. of or according to the atomic theory,... 

Urstoff, m. primary matter (formerly) ele ment initial material, -lehre,/. the theory of a primary matter of which the elements are composed atomism, atomic theory. 

In 1808, an English scientist and schoolteacher, John Dalton, developed the atomic model of matter that underlies modem chemistry. Three of the main postulates of modem atomic theory, all of which Dalton suggested in a somewhat different form, are stated below and illustrated in Figure 2.1. 

Dalton s atomic theory explained three of the basic laws of chemistry The law of conservation of mass This states that there is no detectable change in mass in an ordinary chemical reaction. If atoms are con-... 

The law of multiple proportions This law, formulated by Dalton himself, was crucial to establishing atomic theory. It applies to situations in which two elements form more than one compound. The law states that in these compounds. the masses of one element that combine with a fixed mass of the second element are in a rath of small whole numbers. 

The validity of this law depends on the fact that atoms combine in simple, whole-number ratios (postulate 3). Its relation to atomic theory is further illustrated in Figure A. 

Dalton, John, 25,27,266 Dalton s atomic theory, 25 Dalton s law A relation stating that the total pressure of a gas mixture is the sum of the partial pressures of its components, 115,117... 

This proposal is called the atomic theory. As with any theory, its value depends upon its ability to aid us in explaining facts of nature. There is no more valuable theory in science than the atomic theory. We shall use it throughout this course. Later, in Chapter 14, we shall review many of the types of experiments which cause chemists to regard the atomic theory as the cornerstone of their science. 

---