3.2: Atomic Structure

You’ve probably heard of Occam’s Razor, a common phrasing of which is “the simplest explanation for a given event is usually the correct one". There is no evidence, as far as we know, that John Dalton was directly influenced by this principle but he certainly employed a similar approach in formulating a physical model of the atom. Unlike some Greek philosophers, he did not assume atoms had complex geometric shapes like cubes or tetrahedra; rather, he chose the simplest shape, the sphere, as being the most likely form of atoms. He had no knowledge of subatomic particles and had no reason to make the physical model of atoms any more complicated than they needed to be to explain the physical evidence described by others. His featureless, spherical model of the atom became known as “The Billiard Ball Model”, for obvious reasons (Figure 3-5). He envisioned atoms as hard spheres that could attach to other atoms and become unattached in the course of chemical reactions.

Figure 3-5. John Dalton speculated that atoms were indestructible spheres and that molecules consisted of atoms of different elements that are connected in specific ratios. During chemical reactions, the spheres rearrange into new compounds, but the total number of atoms and their essential nature remain the same.  (Photo by by Curatorial Research Centre is licensed under CC BY-NC 2.0)

Atoms are still frequently represented as simple spheres, and bulk samples of elements, like the gold bars depicted in Figure 3-3, are pictured as vast collections of these identical spheres, like trillions upon trillions of orderly arranged cannon balls (Figure 3-6). But atoms have considerably more complex structures than that suggested by  billiard balls and cannon balls and it did not take scientists long to find evidence for this. By the mid-1800s a number of experiments began to suggest that atoms had “parts” and, what’s more, these parts were common amongst atoms of different elements. In other words, atoms are not the simplest form of matter after all but are, instead, composed of even simpler particles, what we now (quite reasonably) call subatomic particles. These include positively charged protons, negatively charged electrons, and neutrons, which, as the name implies, have no electric charge (Table 3-1). All but the simplest type of atoms, and we’ll specify that notable exception shortly, contain varying numbers of all three of these three subatomic particles. [6]

Figure 3-6. The gold atoms in the ingots shown in Figure 3-3 pack together in a manner that is similar to that seen in the stacking of cannonballs. Fun fact: if gold atoms were the size of these cannonballs, one mole of gold would have a volume about 50 times greater that of planet Earth. (Cannon ball pyramid at Fort Delaware on Pea Patch Island; photo by WorldIslandInfo.com is licensed under CC BY 2.0)

A successor to the Billiard Ball Model was called the Plum Pudding Model, proposed in the late 19th century, and depicted the negatively charged particles, the electrons, embedded in a positively charged matrix, like raisins in a coffee cake or the chocolate chips in mint chocolate chip ice cream. But an ingenious experiment carried out by students working in the laboratory of physicist Ernest Rutherford revealed that the atoms consists instead of an extremely dense, positively charged center, surrounded by a diffuse “electron cloud” of very low density. [7] Our current understanding of the atom is based largely on this experimental evidence: the center, or nucleus, consists of the atom’s neutrons and protons, the sum of which is referred to as the mass number, while the electrons move rapidly and (sort of) chaotically around the nucleus according to the laws of quantum mechanics. You will note from the information in Table 3-1 that the masses of the proton and neutron are similar to each other but greater than that of the electron by more than three orders of magnitude. Yet the radius of the nucleus is about five orders of magnitude smaller than that of the electron cloud. [8] Thus, nearly all of the mass of any atom is due to the protons and neutrons in the nucleus, yet its volume is nearly entirely due to the electron cloud. The atom is therefore sometimes described as being mostly “empty space”. [9]

The distinguishing feature of an element, that which makes it physically and chemically distinct from other elements, relates to the composition of its atoms, specifically of their nuclei. Every element has a unique atomic number, defined as the number of protons in the nuclei of its atoms. The atomic number of gold is 79, thus every atom of gold, anywhere in the universe, has 79 protons (and every atom in the universe with 79 protons is, by definition, a gold atom). The Periodic Table of the Elements lists the elements, using each of their unique one- or two-letter symbols, in order of increasing atomic number, beginning with hydrogen, H, with one proton, up to oganesson, Og, an artificial element with 118 protons that is the current title-holder of the element with the greatest atomic number. Thus, a quick scan of the Table (Figure 3-7), reveals that every carbon atom has six protons, every sodium atom (symbol, Na) has eleven, etc.

For a given element the number of protons in its atoms is invariant, but the same cannot be said of its neutrons or electrons. As we already described, the number of protons defines an element, and therefore must have a large influence on its chemical and physical properties. It is, after all, the difference of a single proton in their respective nuclei that distinguishes the element with an atomic number of six, carbon, which can exist in its pure form as diamond or graphite, from nitrogen, atomic number seven, that exists as the colorless gas that makes up 75% of Earth’s atmosphere. Electrons are a different story: most chemical reactions involve electrons being transferred, shared, or otherwise redistributed. As we will see very soon, neutral atoms can gain electrons to become negatively charged, or lose electrons to become positively charged. Chlorine, Cl, readily accepts an electron to become the negatively charged Cl^-, called chloride. This new species, called an ion, has eighteen electrons but only seventeen protons in its nucleus; the charges are no longer balanced. It remains a form of chlorine but is no longer elemental, or pure, chlorine. In stark contrast, the number of protons in an atom’s nucleus is never affected by the ordinary chemical reactions, the sort that defines all of biochemistry, for example.

Figure 3-7. The Periodic Table of the Elements. Each element is shown with its symbol and atomic number and is color-coded according to its metallic or non-metallic nature (see Chapter 4 for more about the properties of metals and non-metals); note that elements 57-70, the Lanthanide metals, and elements 89-103, the Actinide metals, as well as elements 104 and above are omitted for clarity in this rendition of the Periodic Table.

If the number of protons define what element a particular atom is, and the electrons are responsible for the chemical reactivity of an atom, what do role do neutrons play? What do they do? How do they affect the properties or reactivity of atoms? A discussion of their role veers into the realm of physics, but it is worth a brief detour at this point. You are undoubtedly aware that particles or objects having the same electrical charge repel each other. As just described, however, current understanding of atomic structure holds that all of the positive charge in atoms exists in the form of protons, which are packed into the tiny nucleus at the center of each atom. According to classical physics, one of two things must be true: the protons will repel each other, forcing each other apart and thereby blasting the nucleus apart, or there must be some compensating force that is stronger than the repulsive forces exerted by the protons against each other, serving as a sort of “nuclear glue” to preserve the structural integrity of the nucleus. [10] Since most atoms don't, in fact, readily blow themselves apart, option two seems the most viable: neutrons serve to stabilize the nucleus by overcoming the mutual repulsion exerted by the protons. Hydrogen, with only a single proton, has no need for any such nuclear adhesive because there is no proton-proton repulsion that needs to be overcome; it is the only element that does not require neutrons to maintain its nuclear integrity. Atoms of all other elements, because they have more than one proton in their nuclei, must have neutrons to hold the protons together in a stable nucleus. 

For most elements, there is a range in the number of neutrons in their nuclei. For example, returning to chlorine as an example, while every atom of chlorine has seventeen protons, roughly 75% of them have 18 neutrons and 25% have 20. Atoms of the same element that differ in the number of neutrons in their nuclei are called isotopes and these particular isotopes are referred to as chlorine-35 and chlorine-37, respectively.  This common way of naming isotopes gives the element name first followed by the mass number of the particular isotope. In this case, the mass numbers are 35 and 37, respectively, from the sum of 17 protons plus either 18 or 20 neutrons. This scheme allows you to determine the number of neutrons by difference. For example, consider an atom of carbon-12. Because it is carbon, it must have six protons, and the “12” in carbon-12 indicates that the sum of protons and neutrons is twelve. Thus, this atom must have six neutrons (12 – 6 = 6). Other isotopes of this element include carbon-10, carbon-11, carbon-13, and carbon-14 all have six protons, but four, five, seven, and eight neutrons, respectively. 

With respect to predicting roughly how many neutrons an element will have, the following is a useful guide. For lighter elements, the number of neutrons is usually similar in number to protons, but the ratio gradually increases as with increasing atomic number. The most common isotope of carbon, for example, has a 1:1 ratio of neutrons to protons. In contrast, the most abundant isotope of uranium, uranium-238, has 146 neutrons and 92 protons, a ratio slightly in excess of 1.5:1. The nucleus of this isotope, as well as many others, is unstable and undergoes radioactive decay, processes by which high energy nuclei fragment into smaller pieces.  It is in this realm, referred to as radiochemistry, where the influence of neutrons is most apparent; “normal” reaction chemistry of a given element is, for the most part, unaffected by the number of neutrons in its isotopes [11], but the nuclear stability of those isotopes is keenly dependent on it: too many or too few neutrons is destabilizing and gives rise to radioactivity. We discuss the various modes of radioactive decay later in this text.

With the above description of atomic structure in mind, we are almost ready to address the question we began this chapter with, namely, why do atoms come together to form stable molecules? Why don’t they simply stay as individual entities, randomly bouncing off of each other like agitated billiard balls? In actuality, the atoms of some elements do just that, that is, nothing, except to move about randomly as isolated entities. These elements, the so-called inert gases, do not succumb to the tendency to “clump” together in molecular aggregates. But to explain why these elements do not form compounds, as well as to explain why other elements do, we need to introduce some elementary concepts of energy. The importance of these ideas cannot be overstated as they govern all known physical and chemical transformations.

Notes and References.

[6]. You may at this point ask why these particles are not considered the basic building blocks of elements, rather than the atom. The reason for doing so has more to do with the nature of one's questions about matter than anything else: chemists are interested in understanding and predicting the behavior of compounds that can be manipulated or transformed, by human hands or via biochemical, geologic or other naturally occurring processes, and that requires atoms because they are the smallest particles of elements that retain the chemical properties of a given element. But because all atoms, regardless of which element it is, consist of the same subatomic particles, these particles, in and of themselves, do not offer insight into the behavior of compounds and elements in the same way that atoms do. One could, in fact, continue to divide matter, splitting these subatomic particles into more fundamental species such as quarks, an endeavor that is well outside the realm of chemistry and is part of physics. The separation between these disciplines can be murky but lies roughly at the atomic level: chemists are interested in the behavior of atoms and molecules, especially how they react, while physicists focus more on matter at the subatomic level, although they too are often interested in the structure and bonding of atoms and molecules.

[7] The following resources are good starting points to learn more about the Rutherford’s Gold Foil Experiment:

https://www.youtube.com/watch?v=zUtIrO3fUgg

https://www.youtube.com/watch?v=1EdTw4I6L0U

https://history.aip.org/exhibits/rutherford/sections/alpha-particles-atom.html

https://en.wikipedia.org/wiki/Geiger%E2%80%93Marsden_experiments

 http://chemed.chem.purdue.edu/genchem/history/gold.html 

[8] The radius of a “typical” atom is on the order of about 1×10^-10 m, whereas that of the nucleus is about 1×10^-15 m; using this proportion, if an atom were the size of a football field, 100 yards across, the nucleus would be like a dime sitting on the 50-yard line; the rest of the surrounding stadium would be the electron cloud.

[9] So why don’t atoms simply pass through each other if they are mostly empty? The electrons effectively occupy the entire volume of the cloud because of the speed at which they are moving: just as two rapidly spinning fan blades cannot pass through each other’s space, even though the blades may be small compared to the volume they sweep through, electron clouds cannot overlap in space owing to the very strong electron-electron repulsion that will result.

[10] This is a crude way of describing one manifestation of the so-called strong nuclear force, one of the four known types of physical forces, along with gravity, electromagnetism, and the weak nuclear force.

[11] This is not quite true; the rate of some reactions slows down when performed with heavier isotopes. This is called an isotope effect and cane be used to investigate the details of reaction pathways.