The nature of science

The Big Bang hypothesis (Chapter 2) regarding the sudden expansion of the universe around fourteen billion years ago has now been corroborated by so many experimental observations that it is now an accepted model. Some scientists have reservations about certain parts of the model. It is not a theory because it cannot explain what happened earlier than a fraction of a second during the Big Bang, and it cannot predict with certainty the future fate of the universe. However, it is currently the most popular model available to explain where we are in the evolution of the observable universe. On the other hand, the law of gravity allows everyone to predict exactly the attractive gravitational forces between all objects from tennis balls to planets. There are no scientists who publicly dispute this law. There have been no exceptions to this law! If there were, it would cease to be a scientific law.

Chemistry, natural sciences, and medicine

Chemistry is the study of the properties, composition, and structure of matter, and the changes matter undergoes. Matter is anything that possesses mass when at rest and is observable by the senses. Mass is a measure of a body’s resistance to being accelerated. Therefore, a truck is more difficult to move than a small car because there is more resistance to accelerating the larger mass of the truck.

The closely related property of matter, weight, is the gravitational force with which the Earth, or any other mass attracts another mass.The gravitational force exerted upon an object is directly proportional to the mass of the object. If the mass of a father is twice that of his daughter, the weight (on Earth) of the father will be twice the weight of his daughter. Both her weight and that of her father on the Moon will be less than their respective weights on Earth because the Moon has a smaller mass than the Earth. However, in outer space, where there is only a small gravitational pull from the Earth because of the distance from the Earth, the weights of both father and daughter would both be nearly zero, that is, theyare “weightless” in space. However, their masses in outer space would be the same as they are on Earth. This is because in outer space objects with different masses will still have different resistances to acceleration.

Mass
vs.
Weight

The Chemical Building Blocks

One of the building blocks of matter is the atom. An atom is the smallest component of matter

Figure 1-1 Depiction of a hydrogen atom containing one positively charged nucleus (proton) and one negatively charged electron which is attracted to the proton because of its opposite charge.

Atoms can be broken down into their component parts. For example, if we heat a hydrogen atom (Fig. 1-1) to temperatures comparable with those on the Sun, we obtain an exceedingly small, exceedingly light, negatively charged (–) particle called an electron and an exceedingly small nucleus consisting of a single, positively charged (+) proton. All atoms have nuclei and all nuclei have at least one proton. The mass of a proton is several thousand times greater than the mass of an electron.

Any atom whose nucleus contains one proton (Z = 1) is chemically identified as hydrogen (H); any atom whose nucleus contains two protons (Z = 2) is helium (He); and one whose nucleus contains three protons (Z = 3) is lithium (Li), etc. A listing of all the known elements is contained in a table of elements known as the periodic table(Table 2-1), consisting of all the known elements arranged in rows (periods) with increasing atomic numbers and columns containing elements with similar chemical characteristics. The atomic number is listed below the element’s symbol.

Periodic Table

Table 1-1 Periodic table of the chemical elements. Elements are arranged in horizontal rows or periods with increasing atomic number (the number of positively charged protons in the nucleus shown below the symbol of the element) and vertical groups that have similar chemical properties. (The14 lanthanide and 14 actinide elements following La and Ac, respectively, are not shown.)

Exercise 1-2      Atomic numbers and elements in the periodic table

Locate each of the following elements in the periodic table (Z = atomic number) :
carbon (C), Z = 6; lithium (Li), Z = 3; fluorine (F), Z = 9;
sodium (Na), Z = 11; sulfur (S), Z = 16;
nitrogen (N), Z = 7; oxygen (O), Z = 8.

The symbol H is used to represent a neutral hydrogen atom, which is composed of an intimately bound negative electron and positively charged proton. Each substance containing only atoms with the same atomic number (Z) is designated as an element. Elemental carbon is a substance that is composed of atoms that contain 6 protons in each of the atomic nuclei making up the elemental substance. There may be several different forms of an element called allotropes. For example, there are three forms of elemental carbon: diamond, graphite, and recently discovered forms called “buckyballs” and “carbon nanotubes.” The differences in the external properties among these forms are explained by the different internal spatial arrangements of the carbon atoms.

Superscripts in the upper right hand corner of an element represent the net charge of the chemical species, – for an excess unit of negative charge and + for an excess unit of positive charge. The hydrogen atom, H, containing one proton and one electron, has no superscript because the single positive and the single negative charges cancel each other’s charge and the atom is electrically neutral. The symbols representing the electron and proton are e– and H+, indicating each has a net charge of one, but also indicating that they are oppositely charged.

Attraction and repulsion of charged particles

Experimental measurements in the laboratory demonstrate that:

1. all negatively charged (–) particles repel negatively charged (–) particles;
2. all positively charged (+) particles repel positively charged (+) particles.
3. all positively charged (+) particles attract negatively charged (–) particles,

These observations lead to the general principle that, at room temperature, oppositely charged particles attract each other and similarly charged particles repel each other. Thus, when free electrons encountered free electrons, and free protons encountered free protons in the Big Bang plasma, there should have been, at least initially, a strong repulsion.

However, it also has been experimentally established that when temperatures are exceedingly high, as they must have been in the Big Bang plasma, and particle speeds are very high, positively charged particles in a plasma can undergo forceful enough collisions to overcome like-charge repulsion at very short distances between nuclei. When nuclei get this close, different nuclear forces take over so that attractive nuclear forces are dominant and nuclear reactions are now possible. Because of this, new nuclei and therefore new atoms can form during such high-energy nuclear collisions.

Symbols used in nuclear reactions

In order to accurately represent nuclear reactions, it is convenient to provide the following information regarding particles in the proposed Big Bang plasma:

As an example of this naming system, we examine three different kinds of
hydrogen ions, each containing a single proton, and, for the heavier ions, one and two
neutrons, respectively. These different nuclei are called isotopes, defined as one of
two or more nuclei or atoms having the same atomic number but different mass
numbers.

The proton is a hydrogen atom nucleus with both an atomic number and a mass number of one and therefore has a single positive charge. The proton is one of three isotopes of hydrogen. According to the rules outlined above, the appropriate symbol for the proton is represented as:

₁¹H⁺

A deuteron is a nucleus containing one proton combined with one neutron. The atomic number of the deuteron is the same as that of the proton (Z = 1) because the deuterium nucleus is still chemically identified as hydrogen. However, the deuteron has a mass number equal to 2 (M = 1 proton mass + 1 neutron mass = 2). Again, the deuteron nucleus contains no electrons and therefore has a positive charge. A deuteron is therefore represented as ₁²H⁺. The deuterium atom, containing one electron and one deuteron nucleus, would be neutral ( ₁²H ), i.e., no + charge in the upper right hand corner of the symbol, because the single negative electron neutralizes the single positive charge of the deuteron nucleus to yield a neutral atom.

The tritium atom contains a radioactive nucleus. Nuclei are radioactive when the composition of these nuclei makes them unstable, causing them to transform (decay) into new, more stable nuclei, usually giving off particles such as electrons or even nuclear fragments such as helium nuclei from more massive nuclei. The time period during which radioactive nuclei undergo radioactive decay ranges from fractions of a second to many millions of years, depending on the type of radioactive nuclei. For example, tritium has a half-life of 12 years. This means that half of a large population of H-3 nuclei will undergo radioactive decay during an interval of 12 years, and half of the remaining population will decay away in another 12 years, and so forth. During each half-life period, the population of similar radioactive nuclei is reduced by one half.

When discussing hydrogen isotopes, the symbolism H-1, H-2, and H-3 is often used to describe hydrogen isotopes containing one proton in combination with 0, 1(deuterium - D), and 2(tritium - T) neutrons, respectively. Physical properties of chemical species containing different hydrogen isotopes differ. For example, heavy water (D₂O) containing all deuterium atoms has different properties from light water (H₂O). D₂O has a density (see below for a definition of density) that is larger than H₂O and thus has been given the name heavy water. Heavy water is used in some types of nuclear reactors because of its heavy hydrogen isotope.

A neutron, represented in nuclear equations by the symbol ₀¹n , has a mass number = 1, contains no protons, and is electrically neutral.

Nuclear and chemical reactions

Chemical reactions and nuclear reactions are events in which a substance is or substances are transformed into one or more new substances over a period of time that can range from fractions of a second to years.

In a chemical reaction, all atoms retain their identities during the reaction, but are transformed into new arrangements of the same atoms in space forming new substances called products. The substances that react with each other to form products are called reactants. An example of a chemical reaction between two hypothetical molecules, designated A and B, is shown on the next page. During a collision between A and B, a chemical reaction takes place and the atoms in A and B are rearranged into two new molecules (C and D) if the collision is sufficiently energetic.

Both chemical and nuclear reactions can be represented as equations. In a mathematical equation, e.g., (3 + 2) = 5, the equals sign represents an identity. In a chemical or nuclear equation, the equals sign is replaced by an arrow generally pointing from left to right. To the left of the arrow are the chemicals present before the reaction, the reactants, and to the right of the arrow are the chemicals resulting from the chemical or nuclear reaction, the products. Thus, in a hypothetical chemical or nuclear reaction in which reactants A and B react to form the products C and D, the reaction is written as reaction (1-1):

All atoms involved in chemical reactions retain their identity. That is, in a chemical equation a hydrogen atom may be a part of the chemical A and remains a hydrogen atom when it winds up in the chemical D.

An Example of a Nuclear Reaction in the Big Bang Plasma

It is suggested in Chapter 2 that an important nuclear reaction in the Big Bang plasma was the interaction of a neutron and a proton as reactants to form a deuteron and a gamma ray as products. A gamma ray (γ) is an energetic radiation with zero mass and zero charge originating in many nuclear reactions and is given off during the radioactive decay of many unstable nuclei. One can think of a gamma ray as a highly energetic x-ray. Gamma rays carry away large quantities of excess energy and are absorbed in surrounding matter, resulting in the generation of heat and chemical damage to that matter. The above word description of this nuclear reaction is represented in diagram form in Fig. 1-2:

The same nuclear reaction depicted in Fig. 1-2 is represented below in equation form in (1-2):

¹₁H⁺ + ¹₀n → ²₁ H⁺ + gamma ray (γ)      (1-2)

In all nuclear reactions, the sum of the mass numbers of the reactants must equal the sum of the mass numbers of the products (the numbers found in the upper left hand superscripts of reactants and products). This same relationship applies to the sums of atomic numbers of reactants and products (found in the lower left subscripts of reactants and products). Furthermore, the algebraic sum of the electrical charges of the reactants must equal the (algebraic) sum of the electrical charges of the products. Check to see that all of these relationships are valid in equation (1-2). The gamma ray has no mass or charge.

Chemical equations and ionization

A chemical equation is a symbolic expression that represents in an abbreviated form the laboratory observations of a chemical change. There are a number of general types of chemical equations. We first introduce the type of chemical equation representing ionization, which is needed to understand processes occurring during and after the proposed Big Bang.

Ionization of an atom is the removal of one or more negatively charged
electrons from the neutral atom to form a positive ion. When the hydrogen atom
absorbs a very large amount of energy, like that available through collisions in the Big
Bang plasma, it ionizes and dissociates, i.e., it comes apart into its component parts.
In order to be ionized, the neutral hydrogen atom must absorb enough energy to
dissociate it into a free electron and a proton. This ionization of the hydrogen atom
can be represented in diagram form (Fig. 1-3):

Figure 1-3 Ionization of a hydrogen atom to form a proton (a positive ion) and an electron (a negative ion)

The process in Fig. 1-3 also can be represented by the following chemical equation:

H + energy → H* (excess energy) → H⁺ + e^–              (1-3)

Volume and density

Volume is the measure of the bounded space occupied by an object. Volume can be calculated from the dimensions of an object and can be measured in units such as quarts, gallons, or liters. One liter (1 L) is approximately the same volume as one quart. One milliliter (1 mL) is one thousandth of a liter and is equal to the volume of a cube one centimeter on an edge
(1 cm³ ), where 2.54 cm = 1 inch .

2 liter soda bottle 1quart ~ 1 liter milk carton ~ 1000mL

Density (d) is the amount of mass (m) contained in a certain volume (v) and can be defined as density = d = mass/volume = m/v (mass divided by volume). Densities of solids and liquids are generally given in terms of grams per cubic centimeter (g/cm³). The density of water at 3.98°C is exactly 1.0000 g/cm³. “°C ” stands for degrees on the Celsius temperature scale, where water freezes at 0°C and boils at 100°C.