Chemistry

6 Chemistry

Chemistry is a part of our everyday lives. Almost three quarters of the objective information in a client’s record consists of laboratory data derived from chemical analytical testing. Laboratory tests and chemical analysis play an important role in the detection, identification, and management of most diseases. The client’s evaluation, diagnosis, treatment, care, and prognosis are, at least in part, based on the chemical information from laboratory tests that involve traditional technologies of chemistry. A sound, basic knowledge of chemistry enables the health care professional to reduce the risk of mishandled biologic samples and misdiagnosis and thereby deliver safer and higher quality care.

Chemistry is the study of matter and its properties. Everything in the universe is made or composed of different kinds of matter in one of its three states: solid, liquid, or gas. Matter is defined by its properties. Chemistry is a study of those properties and how those properties relate to one another. Chemistry is a very broad field of study and can be divided into areas of specialization such as physical or general chemistry, biochemistry, and organic and inorganic chemistry. This chapter reviews chemistry from the most basic of substances to complex compounds.

Scientific Notation, the Metric System, and Temperature Scales

Scientific Notation

Scientific notation is the scientific system of writing numbers. Scientific notation is a method to write very big or very small numbers easily. Scientific notation is composed of three parts: a mathematical sign (+ or −), the significand, and the exponential, sometimes called the logarithm.

1. The mathematical sign designates whether the number is positive or negative.

There is an understood (+) before a positive significand as there is in all positive numbers.

2. The significand is the base value of the number or the value of the number when all the values of ten are removed.

3. The exponential is a multiplier of the significand in powers of ten (Table 6-1). A positive exponential multiplies by factors of ten. A negative exponential multiplies by one tenth (0.1).

Table 6-1 Exponentials *

10⁹	1,000,000,000
10⁶	1,000,000
10³	1,000
10²	100
10¹	10
10⁰	1
10^-2	0.01
10^-6	0.000001
10^-9	0.000000001

* 1.0 is understood to be the significand with each of the above exponentials.

Some calculators or other devices may write the exponent as an “e” or “E” as in 3.2 e5 or 3.2 E5 instead of 3.2 × 105, which is called E notation.

Example

Consider −9.0462 × 10⁵, where the minus (−) sign means negative, 9.0462 is the significand or base value, and 10⁵ is the exponential or multiplier of the significand in the power of ten. In our example above −9.0462 × 10⁵ equals −9.0462 × 10 × 10 × 10 × 10 × 10 or −904620.

Example

Consider 4.7 × 10^-3, where the absence of the (+) sign is understood as positive, 4.7 is the significand or base value, and 10^-3 is the exponential or multiplier of the significand in the negative power of ten (as tenths). In our example above, 4.7 × 10^-3 equals (4.7 × 0.1 × 0.1 × 0.1) or 0.0047.

Move the decimal in the significand the number of places equal to the exponent of 10. When the exponent is positive, the decimal is moved to the right and when the exponent is negative, the decimal is moved to the left.

When writing a number between −1 and +1 always place a zero (0) to the left of the decimal. Write 0.62 and −0.39 (do not write .62 or −.39); this will avoid mistakes when reading the number and locating the decimal.

The Metric System of Measurement

The metric system is a method to measure weight, length, and volume. It is a simple, logical, and efficient measurement system. The basic measurements of the metric system are grams, liters, and meters. A gram (g) is the basic measure of weight, a liter (L) is the basic measure of volume, and a meter (m) is the basic measure of distance.

Each metric measurement is composed of a metric prefix and a basic unit of measure. An example is “kilogram” where “kilo” is the prefix and “gram” is the basic unit of measure.

The prefixes are the same and have the same meaning or value, regardless of which basic unit of measurement (grams, liters, or meters) is used. Prefixes are the quantifiers of the measurement units. All of the prefixes are based on multiples of ten. Any one of the prefixes can be combined with one of the basic units of measurement. Some examples are deciliter (dL), kilogram (kg), and millimeter (mm) (Table 6-2).

Table 6-2 The Prefixes

In medicine, the prefixes in black are the most frequently used (see Table 6-2).

Some comparisons may give more insight to sizes: A meter is a little more than 3 inches longer than a yard. A dime is a little less than 2 cm in diameter. A kilogram is about 2.2 lb. A liter is a little more than a quart.

Temperature Scales

The three most common temperature systems are Fahrenheit, Celsius, and Kelvin.

Fahrenheit (F) is a temperature measuring system used only in the United States, its territories, Belize, and Jamaica. It is rarely used for any scientific measurements except for body temperature. It has the following characteristics:

a. Zero degrees (0° F) is the freezing point of sea water or heavy brine at sea level.

b. 32° F is the freezing point of pure water at sea level.

c. 212° F is the boiling point of pure water at sea level.

d. Most people have a body temperature of 98.6° F.

Celsius (C; sometimes called Centigrade) is a temperature system used in the rest of the world and by the scientific community. It has the following characteristics:

a. Zero degrees (0° C) is the freezing point of pure water at sea level.

b. 100° C is the boiling point of pure water at sea level.

c. Most people have a body temperature of 37° C.

Kelvin (K) is used only in the scientific community. Kelvin has the following characteristics:

a. Zero degrees (0 K) is −273° C and is thought to be the lowest temperature achievable or absolute zero (0).

b. The freezing point of water is 273 K.

c. The boiling point of water is 373 K.

d. Most people have a body temperature of 310 K, but this is never used.

Table 6-3 Important Temperatures in Fahrenheit and Celsius

Condition	Examples of Fahrenheit (F) and Celsius (C) Temperatures
Melting ice	21° C	32° F
Normal body temperature	37° C	98.6° F
Boiling water	100° C	212° F

Atomic Structure and the Periodic Table

Atomic Structure

The basic building block of all molecules is the atom. An atom’s physical structure is that of a nucleus and orbits, sometimes called electron clouds. The nucleus is at the center of the atom and is composed of protons and neutrons. At the outermost part of the atom are the orbits of the electrons, which spin around the nucleus at fantastic speeds, forming electron clouds. The speed of the electrons is so great that, in essence, they occupy the space around the nucleus as a cloud rather than as discrete individual locations. The electrons orbit the nucleus at various energy levels called shells or orbits, almost like the layers of an onion. As each orbital is filled to capacity, atoms begin adding electrons to the next orbit. Atoms are most stable when an orbital is full. However, most of the volume of an atom is empty space. See Figure 6-1 for examples of atoms.

Figure 6-1 Models of the atom. The nucleus consists of protons (+) and neutrons at the core. Electrons inhabit outer regions called electron shells or energy levels (A) or (B) clouds.

(From Patton KT, Thibodeau GA: Anatomy and physiology, ed 7, St. Louis, 2010, Mosby.)

Protons have a positive electrical charge, electrons have a negative charge, and neutrons have no charge at all. Ground state atoms tend to have equal numbers of protons and electrons, making them electrically neutral. When an atom is electrically charged, it is called an ion or it is said to be in an ionic state. This usually occurs when it is in a solution or in the form of a chemical compound. An atom in an ionic state will have lost electrons, resulting in a net positive charge or will have gained electrons, resulting in a net negative charge. The atom is called a cation if it has a positive charge and an anion if it has a negative charge.

The Periodic Table

Matter is defined by its properties. It can also be stated that the properties of matter come from the properties of their composite elements, and the periodic table organizes the elements based on their structure and thus helps predict the properties of each of the elements (Figure 6-2).

Figure 6-2 Periodic table of elements.

(From Patton KT, Thibodeau GA: Anatomy and physiology, ed 7, St. Louis, 2010, Mosby.)

The periodic table is made up of a series of rows called periods (hence the name periodic table) and columns called groups. It is, at its simplest, a table of the known elements arranged according to their properties. It is usually possible to predict, for example, the charge of a main group (the A Group) atom or element, when it exists as an ion, by its location in the table. Group IA has a plus one (+1) charge, group IIA has a positive two (+2) charge, and group IIIA has a positive 3 (+3) charge. Group IVA can have either a positive four (+4) or a negative four (−4) charge. The negative charges are as follows: group VA has a negative three (−3) charge, group VIA has a negative two (−2) charge, and group VIIA has a negative one (−1) charge. Group VIIIA, called the noble gases, have no charge when in solution; they remain neutral in nearly all situations. This is because their outer orbits are complete. These gases are also very stable and chemically inert for the same reason. Another property that can be generally deduced by the periodic chart is the number of electrons in the outer electron shell or cloud. Group IA will have one (1) electron in its outer shell. Group IIA will have two (2) electrons in its outer shell. Group IIIA will have three (3), Group IVA will have four (4), and on through all of the A groups. The Groups 3 IIIB through 12 IIB are called transition metals and are not as straightforward to predict because of some exceptions to the rules.

An important principle to remember is that the properties of each element can be predicted based on its location in the periodic chart.

Atomic Number and Atomic Mass

Two important numbers or properties of atoms that can be obtained from the periodic table are the atomic number and the atomic mass.

The atomic number is the number of protons in the nucleus, and it defines an atom of a particular element. For instance, any atom that has eleven (11) protons, no matter how many neutrons or electrons, is sodium (Na). If an atom has six (6) protons, it is carbon (C). The atomic number is located at the top of each of the squares in a periodic table. It is always a whole number.

The atomic mass of an atom is the average mass of each of that element’s isotopes. Isotopes are different kinds of the same atom that vary in weight. Protons and neutrons each have approximately the same mass or weight, which makes up nearly all of the atom’s total mass. The atomic mass is the number at the bottom of each of the squares in the periodic table, and it is usually a decimal number. For a given element, the number of protons remains the same, whereas the number of neutrons varies to make the different isotopes. The most common isotope of an atom, generally, has the same number of protons and neutrons in its nucleus. The element Carbon 12 (¹²C), the most common carbon, has (6) protons and six (6) neutrons. The isotope used for “carbon dating” is Carbon 14 (¹⁴C), which has 6 protons and 8 neutrons.

Chemical Equations

An element or atom is the simplest form of matter that can naturally exist in nature. It can exist as pure substance or in combination with other elements. When they exist in combination with other elements, the combination is called a compound, and they combine in whole number ratios. A part of an element does not naturally exist; at least one atom of the element is present in a chemical reaction. For instance, the elements sodium (Na) and chlorine (Cl) will combine perfectly as whole elements or atoms in a one-to-one ratio to make the compound table salt (NaCl).

Chemical equations are simply recipes. Ingredients, called reactants, react to produce a desired end result or compound, called products. Equations are written in the following manner:

In any chemical reaction, an arrow between the reactants and the products is drawn out. This arrow symbolizes the direction of the reaction. Some reactions move toward the product side as seen above, and some reactions will move toward the reactant side with an arrow pointing toward the reactants instead of the products.

There are also reactions that will create both reactants and products at the same time.

An example is the reaction of aqueous silver nitrate (AgNO₃) and aqueous potassium chloride (KCl) to produce solid silver chloride (AgCl) and potassium nitrate (KNO₃).

The law of conservation of mass states that mass cannot be created or destroyed during a chemical reaction. Therefore, once the reactants have been written and the products predicted, the equation must be balanced. The same number of each element must be represented on both sides of the equation. The above example has one silver atom, one nitrogen atom, three oxygen atoms, one potassium atom, and one chloride atom on each side of the equation. Therefore nothing in the way of matter was created or destroyed; it was simply rearranged.

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Apr 10, 2017 | Posted by admin in NURSING | Comments Off

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