Chapter 2 Anatomy Atoms and Molecules

Two major criteria differentiate subatomic particles

1)mass 2)charge

Atoms are composed of three subatomic particles

1)neutrons 2)protons 3)electrons

Atoms

All matter is composed of atoms. It is the smallest particle that exhibits the chemical properties of an elements.

Cation

Any ion that has a positive charge. Loses one electron.

Anion

Any ion that has negative charges. Gaining of electrons.

Octet Rule

Atoms obtain an outer shell with eight electrons and gain chemical stability through the loss, gain, or sharing of electrons. Not all elements follow the octet rule, but it is accurate for our purposes here. Ions are formed from the loss or gain of electrons, and covalently bonded molecules are formed by the sharing of electrons. We describe the formation of ions and covalently bonded molecules in the next two sections. (Page 33).

Chemical Compounds

Chemical compounds are stable associations between two or more elements combined in a fixed ratio. These associations are classified as either ionic compounds or molecular compounds. (Page 34).

Periodic Table

Elements differ in the number of subatomic particles. The periodic table may be used to obtain the number of subatomic particles in an atom of a specific element. Several important features for each element appear in the periodic table, including the element's symbol, atomic number, and average atomic mass. (Page 30).

Isotopes

Elements in nature usually occur as a mixture of isotopes. Isotopes (ˉı′sˉo-tˉop; iso = same) are different atoms of the same element that have the same number of protons and electrons but differ in the number of neutrons. Isotopes exhibit essentially identical chemical characteristics but have different atomic masses. For example, carbon exists in three isotopes: carbon-12, carbon-13, and carbon-14 (figure 2.3). All isotopes of carbon have six protons in their nuclei—however, carbon-12 has six neutrons, carbon-13 has seven neutrons, and carbon-14 has eight neutrons. Generally, one isotope is usually more common than the others. Carbon-12 is the most common isotope for carbon. ISOTOPES HAVE DIFFERENT NUMBER OF NEUTRONS (Page 32).

Biological Half Life

In comparison, the time required for half of the radioactive material (e.g., from a medical test using radioactive contrast material) to be eliminated from the body is the biological half-life. Biological half-life is also applied to substances that are nonradioactive, such as hormones or drugs (see section 17.4b). (Page 32).

Ions

Ions are atoms or groups of atoms with either a positive charge or a negative charge. They are produced from the loss or gain of one or more electrons, respectively. Ions also function as electrolytes (which can form cations or anions) and they help maintain electrolyte balance, and there may also be an imbalance if we are getting too much or too little of it. (Page 34).

Major Elements in the Human Body

Oxygen Carbon Hydrogen Nitrogen Calcium Phosphorus

Atomic Nucleus

Neutrons and protons compose almost the entire mass or weight of an atom and are located at the center, or core, of the atom, called the atomic nucleus. The atomic nucleus contains mass because it has both neutrons and protons, and it is positively charged because of the protons. (Page 30).

Radioisotopes

Some isotopes are referred to as these. They are unstable because they contain an excess amount of neutrons.

Minor Elements in the Human Body

Sulfur Potassium Sodium Chlorine Magnesium Iron

Electron

The electron is the third component of an atom. An electron has a very small mass—only about 1/1800th of the mass of a proton or neutron—and makes a negligible (very small) contribution to total mass. Each electron has a negative charge of one (-1) Electrons are located at varying distances from the nucleus in regions called orbitals, often depicted as either an electron cloud or as discrete energy shells. Both the cloud model and shell model indicate where the electrons are most likely found as depicted in figure 2.2. (Page 30). (Page 30).

Diagramming Atomic Structures

The innermost shell may hold up to two electrons and the second shell up to eight electrons. All subsequent shells have a capacity of eight electrons but may also house more than eight electrons (the conditions for when the third and higher shells hold more than eight electrons are beyond our purposes here). (Page 32).

Mass

The mass of an atom is expressed as the atomic mass unit (amu), or dalton. (The mass of a subatomic particle is very small: consider that 1 amu is equal to 1.66 × 10−27 kilograms.) Neutrons and protons each have a mass of 1 amu. (Page 30). (Page 30).

Number of Electrons

The number of electrons in an atom is determined indirectly by the atomic number. This is possible because all atoms are neutral. The number of negatively charged electrons must equal the number of positively charged protons for an atom to be neutral. Each electron has a charge of -1 and counters the +1 charge of a proton in the nucleus—therefore, the atom has zero charge, meaning it has no net charge. Atoms that have lost or gained electron(s) do not have the same number of protons and electrons and are referred to as ions (described later). (Page 32).

Valence Shell

The number of electrons in the outer shell. Valence Electrons found by columns of periodic table. 1-8.

Number of Neutrons

The number of neutrons can be determined by subtracting the atomic number (number of protons) from the atomic mass (protons and neutrons). For example, to calculate the number of neutrons in sodium (23 11Na), you would take the total of 23 protons and neutrons, and subtract the number of protons = 11; thus, sodium has 12 neutrons (23 - 11 = 12). (Page 32).

Number of Protons

The number of protons is the atomic number—thus, carbon has six protons and oxygen has eight protons. (Page 32).

Physical Half Life

The time it takes for 50% of the radioisotope to become stable is its physical half-life.

Element

There are 92 naturally occuring elements. Hydrogen is the smallest and lightest elements Uranium is the largest and heaviest element. Grouped into major, minor and trace elements based on percentage each composes by weight in the human body. Major elements make up over 98% and minor elements less than 1% (figure 2.1b). In comparison, trace elements appear in the body in only limited amounts (less than 0.01%). Only 12 elements occur in living organisms in greater than trace amounts: oxygen, carbon, hydrogen, nitrogen, calcium, phosphorus, sulfur, potassium, sodium, chlorine, magnesium, and iron. All of these elements except iron have atomic numbers (described shortly) between 1 and 20 in the periodic table. Consequently, our emphasis in this chapter will be on only the first 20 elements. (Page 30).

Matter

This is what the human body is composed of. Matter takes up mass and space. There are three types of matter. 1)solid 2)liquid 3)gas

Polyatomic ions

Two of the common anions listed in table 2.1, bicarbonate ion (HCO3 - ) and phosphate ion (PO4 3- ) are composed of more than one atom and are referred to as polyatomic ions. These form when one or more atoms in a structure composed of many atoms has lost or gained electrons. (Page 35).

Proton

has a positive charge of one (+1)

Atomic Mass (Atomic Weight)

indicates the mass of both protons and neutrons Electrons not included because of their relative small mass The average atomic mass is rounded to the nearest whole number and it is designated by a superscript to the left of the chemical symbol when it is written. For example, a sodium atom, with an atomic number of 11 and an average atomic mass of 22.99, is designated as 23 Na. 11 (Page 31).

Atomic number

indicates the number of protons in an atom of that element and is located above its symbol in the periodic table. H For example, 1 shows that the nucleus of a hydrogen atom has one proton, and C indicates that the carbon nucleus has six 6 protons. (Page 31).

Neutron

is uncharged. It is neutral.

TO DIAGRAM AN ATOM

• Proton number (p) = Atomic number • Neutron number (n) = Atomic mass (p + n) - atomic number (p) • Electron number (e) = Proton number (p) (Page 32).