The Nucleus 1. The existence of a nucleus

Transcription

1 Atomic Structure

2 The familiar picture we use to describe atoms is a theory of what atoms look like. It is called the Bohr model of the atom after the physicist Niels Bohr who proposed it. A model is just a representation of a theoretical structure it is in a sense a theory. There are numerous observations of chemical and physical behavior and structure of materials that suggest this structure. We will not go into them in any detail, but they may be available if you are interested and if I can get the presentation finished.

3 The Nucleus 1. The existence of a nucleus

4 Atoms have nucleii. There are observations that suggest the existence and placement of a nucleus deep inside the atom, at its center. Some of the same suggestions indicate that it is exceedingly tiny in comparison to the region beyond it. This diagram shows it nowhere near its actual proportional size. To scale it would not be visible even with a good microscope. As with other models, we enlarge parts so we can see the structure. The observations also suggest that essentially all the mass of the atom is concentrated in the nucleus. I reiterate: this is not mere guesswork, it is based on observations of things interacting with the nucleus. We know the mass of substances is concentrated in these tiny bundles as surely as it is possible to know something you cannot directly observe. The same is true of all the claims in this presentation.

5 The nucleus is unaffected by ordinary chemical reactions. Many observations suggest that the nucleus does not participate in ordinary chemical reactions that the mass of the atom is not changed measurably by being bonded to other atoms. Nuclei can only be changed by very high energy, physical nuclear reactions which either require or produce huge amounts of energy, or both. Chemical reactions do change other attributes of an atom involved in them, as we will see. Thus the attribute of the atom that allows bonding and chemical reactions must lie in the region outside the nucleus. Not only are the nuclei unaffected by such reactions, they remain very far apart when bonded together. To paraphrase (or twist) a common saying (usually involving Las Vegas) whatever happens in chemistry stays outside the nucleus.

6 The Nucleus 1. The mass of a nucleus, part 1

7 Mass of atoms exists in unit values. There are several ways to calculate or to measure (directly or indirectly) the mass of an atom. When this was done for a large number of atoms an interesting observation was made. Every atom has a mass that is an even multiple of the mass of a hydrogen atom. If we assign a mass of 1 unit to hydrogen (H) then heavier atoms are always whole number multiples of that. For example, the most common mass for the next heavier element (helium He) is 4 units 4x hydrogen s mass. Some of the atoms are 3x or 5x, but the vast majority are 4x. (Actually, an occasional atom of H has 2x or 3x the normal mass). No atom has ever been found that is 2.5 or 3.1 or x the mass of hydrogen. To reiterate and reinforce: there can be atoms 2x, 3x, 4x, 5x, but nothing in between. Mass on the atomic scale comes in units. The pattern of mass after helium is very roughly an increase of two units with each successive element. The pattern is far from perfect, and gets less and less correct with heavier and heavier elements, but for our purposes it works pretty well to assume that one atom on the periodic table weighs 2 units mor or less than the ones beside it. We will come back to this after we have discussed charge and bonding.

8 The Electrons

9 Chemical bonds are electrical in nature. Many observations suggest that the bonding (or unbonding) of atoms in chemical reactions is electrical in nature. The evidence for this is of two main forms: 1)When we break the bonds of a substance the electrical properties of the surroundings change, as if somehow electrical conductivity has been released from the bonds. Solid rock salt does not conduct electricity; pure water conducts almost no electricity, salt water conducts like crazy, and the conductivity is proportional to the amount of dissolved salt. 2)When we apply electrical current to a compound (like water) the substance is broken down into simpler substances with different properties from the original (like hydrogen and oxygen gasses). Some are attracted to and accumulate at the positive pole of the current and some at the negative pole. This is called hydrolysis.

10 The electrical charges are carried by particles with virtually no mass. The particles that carry this charge are typically treated as massless, though the mass has actually been calculated for them. They have roughly 1/50,000 th the mass of a single nuclear particle. So, no matter how many there are we can safely pretend that they have zero mass when talking about atomic mass overall.

11 The charge particles occur in discrete shells, clouds, or orbitals. Observations of two sorts suggest that the electrons do not swarm all through the space between the nuclei, but rather occupy specific ( discrete ) zones concentric to the nuclei. These observations are of two sorts: 1) How energy is absorbed and radiated by the atoms when they are exposed to it (discrete layers explain the quanta of energy involved), and 2) The measurable sizes of the atoms when they are bonded into a crystal structure. An atom can be of different sizes (that is, it can occupy spaces of different dimensions) but each size is distinctly different from the others. Furthermore, if a crystal have openings of a certain size and an atom cannot be that size then it will never occur in that crystal in that position. The sizes of atoms, like other things we ve seen, seem to come in approximate units (sizes) and these result from distinct concentric layers of possible sites for electrons.

12 The ratio in which two atoms will naturally bond is constant and easily predictable. It is always a small whole number ratio. There are numerous ways to calculate or determine the numbers of atoms involved in a particular compound. Under normal conditions hydrogen and oxygen bond in the ratio 2:1, giving us H 2 O. In less normal conditions they make hydrogen peroxide, which still has a whole number ratio (1:1) of the atoms (H 2 O 2 ). The ratio is part of the formula for the compound: CO 2 (1:2), NaCl (1:1), NH 3 (1:3 ammonia) and CH 4 (1:4 methane) are all examples. Ratios in which the terms are 4 or fewer are fairly common. Ratios with one term more than 4 are not. Of course we could express many ratios without a whole number, but when all ratios are reduced to the lowest common denominator, the ratio is of whole numbers. For example, we could say that two things occur in a 1:1-½ ratio, but this is the same as a 2:3 ratio. The inference we draw is that the charges moving around during bonding, that is, the charges of electrons, come in units, just like mass of atoms and position of electrons. We say that one electron has a charge of -1 it is negative and carries one unit of charge.

13 The Nucleus 3. The particles in a nucleus

14 Complete atoms are electrically neutral. That is, the negative charges of electrons are balanced by charges in the nucleus, which also come in units. An atom by itself is a bit like one hand clapping more of a theoretical model than a tangible object. Such an atom, if we could find it, would be electrically neutral. That is not to say that we cannot find atoms occurring by themselves that are not neutral. It s pretty easy to do, but they are not whole atoms. This indicates that there must be something balancing the electrons charges, allowing neutrality. Because only electrons occur in the spaces between nuclei, the carrier of positiveness must be in the nuclei. The + charges in the nucleus must also come in units. This will be more obvious as we talk about bonding, so we will return to this point then. For now assume that the best explanation for balancing the (-)charge of an electron in the atom is to have a single (+)particle in the nucleus. This particle also has mass as we will see. We call it a proton. Each one is shown as a red ball in the nucleus.

15 Not all particles in the nucleus have a charge. Mass increases faster than charge in progressively heavier elements. Remember that both the chemically active (-) charges in atoms and the balancing (+) charges in them come in units. Also remember that mass comes in units as well. If we compare the normal mass of the lightest element (hydrogen H) with the next lightest one (helium He) the two behave chemically as if they are one charge unit different. This, in fact, holds for every pair of adjacent elements: each heavier element is one charge unit different from the next lighter one. Increased charge in atoms comes one unit at a time, or, as you can probably see, one electron at a time. Mass does not do this. H normally (in practically all cases) has a mass of 1. That shouldn t be a surprise since H is the basis for the units. (The rarer atoms of H that do not weigh 1 unit weigh either 2 or, VERY rarely, 3. exactly 2 or 3. The next heavier element normally weighs 4 units (but can be 3 or 5), and so on. As a very rough rule of thumb the mass increases in steps of two beyond this. The heavier the element, the more likely the increase from one to the next is more than two. The increases are in unit values, but not in single steps.

16 There must be a separate mass particle in the nucleus that has no charge. This pattern makes sense if we hypothesize another particle in the nucleus with mass but without charge. We call this a neutron (N). He The diagrams show a He atom and the next heavier one (lithium Li). He ordinarily has a mass of 4 units, remember. Li is one of the exceptions to our rule of thumb. It s typical mass is 7. Atoms of Li with masses of 5,6, and 8 also occur, but most have a mass of 7. Notice that there is one charge unit difference between them. He has 2 electrons (e-) and two protons (P+) and Li has three of each. (the next heavier element would have four of each, and so on. Li The masses of the two elements however is not just one unit apart. In this case it their masses are 3 units apart. The nuclear mass particle without charge is the culprit. It allows the atom to gain mass without modifying its charge. Add a P+ to balance an additional e- and mass would only increase by one unit. Add an N as well and the mass increases by two units, still leaving the electrical charges balanced. In this case there is normally another N added increasing mass by three (1P+ and 2N).

17 Can you spot the difference in these two atoms? Li Both atoms are lithium. That is, they both behave in an identical fashion chemically because they both have the same electrical properties: three e- and 3 P+. The difference is the number of protons (P) and, consequently, the mass of the two. The upper atom has a mass of 6 (3P+ and 3N) the lower a mass of 7 (3P+ and 4N). The lower one is the more typical form of lithium, but for most atoms low on the mass scale the number of P+ is generally the same as the number of N, making the mass 2x the number of P+ (the atomic number). With heavier atoms the proportional number of N+ increases so they are more than, sometimes far more than, twice the atomic number in mass. (The mass also becomes more variable.) Li Different atoms of the same element that weigh different amounts are called isotopes.

18 Summary of basic structure. The ideas presented above, based upon observations about the behaviors of atoms, are the basis for the atomic structure proposed in the Bohr model. Let s summarize the structure. Electrons: Electrical forces cause chemical bonding, and the electrical properties of atoms are found in the outer part of the atoms, where they can easily interact. The electrical charge that binds atom to atom comes in units of equal charge. That charge is carried by nearly massless particles called electrons (e-). They are negative. We assign a value of -1 as their charge. ( 1 and unit are essentially synonymous.) The electrons occur at discrete distances away from the center of the atom that we call shells or orbitals. I reiterate: all these statements are based on observations, usually multiple independent observations of different kinds, of atom behavior. As we explore bonding we will enlarge this picture of the elecrtrons because of observations we ll make then.

19 Summary of basic structure. The existence and properties of the nucleus in the Bohr model are likewise meant to conform to multiple observations. Nucleus: The nuclei are quite small most of the atom is empty space but they carry essentially all the mass of the atom, even when there are a large number of electrons. The nucleus has two types of particles in it. One carries a charge of +1, balancing the -1 charge of electrons. This is the proton (P+, or just P). The other has virtually the same mass as the proton but carries no charge. This is the neutron (N). All atoms of an element have the same number of protons. This number is the atomic number. The numbers of the other particles can vary under different conditions but the proton number is constant. Neutron number varies with different isotopes, all of which retain the same chemical properties. Electron number in a balanced whole atom is the same as the atomic number, but in bound or ionized atoms the number can vary. This is why the proton number is the atomic number and not the electron number.

20 So there you have it. You ve all seen this model of atomic structure in earlier science classes. Now you have some idea not only of what the model looks like, but why it looks that way.