بسم الله الرحمن الرحيم
(وَمَا يَعْزُبُ عَنْ رَبِّكَ مِنْ مِثْقَالِ ذَرَّةٍ فِي الأرض وَلا فِي السَّمَاءِ وَلا أَصْغَرَ مِنْ ذَلِكَ ولا اكبر) صدق الله العظيم يونس 61
Al - Iraqia university Prof. Dr. Samia –Alshahwani
College of medicine Y1 L4 - 2015
Human Biology
P1 Human organization
ch2 Chemistry of life
2.1 From Atoms to Molecules
Objectives: 1-Distinguish between atoms and element.
2-Illustrate atom structure.
3-Define isotope & list its application in medicine.
4-Distinguish between ionic and covalent bonds.
Concepts:
Matter: Anything that takes up space & has a mass, composed of element, it exist as (solid, liquid or gas).
Element: Basic building blocks of matter made of one type of atom.
Atom: Smallest unit of element, it retains its chemical physical properties.
Atomic number: Number of (protons) found in the nucleus of an atom and therefore identical to the charge number of the nucleus
Atomic mass: Average mass of all atoms of a particular element.
Is sub atomic particles mass, total mass of (protons, neutron, & electrons) in a single atom.
Mass number: The sum of (protons & neutrons) in atoms nucleus.
There are 92 natural elements, most of human body is composed of 4 elements: Carbon, nitrogen, oxygen, & hydrogen, other elements, such as iron, is important, iron-deficiency anemia results when the diet doesnt contain enough iron for making hemoglobin, which have important function, it transports oxygen to cells.
Elements have a name & a symbol e.g. carbon C, iron Fe., they are arranged in a periodic table named so because elements in a column show periodicity or behave similarly during a chemical reaction in Fig. 2.1 the first 36 elements, atoms symbols are in the center. The number above the symbol is the atomic number, the elements in each row are arranged in order, according to increasing atomic number, the number below each symbol is elements atomic mass ( average mass of all elements atoms),
Most of an atom is empty space. Same name is given to the element and its atoms.
Fig. 2.1 The number on top of each square is the atomic number, increases from left to right. The letter symbols represent each element & are sometimes abbreviations of Latin names. Below the symbol is the value for atomic mass
Normal chemical reactions do not produce elements. The majority of heavier elements, as iron, are produced by chemical reactions within stars, when they explode after their life ends, producing a supernova, which scatter heavier elements in space, & involved in planets formation, It is possible to split an atom, an atom is the smallest unit enter a chemical reactions, each atom has a central nucleus and pathways about (orbital).
* An atom is very small, it contains smaller subatomic particles.
*The subatomic particles, protons (+) charge & neutrons, are located in the nucleus and electrons (-) charge in orbital (Fig. 2.2).
Fig. 2.2 a The Atomic Structure of selected elements, showing protons (p) and neutrons (n) located in the nucleus and electrons (blue dots) in energy orbital around the nucleus.
To be electrically neutral, an atom must have same number of protons and electrons, so that the positive and negative charges are balanced. The atomic number tells how many protons and therefore how many electrons, an atom has, when it is neutral e.g. carbon atomic number is 6; therefore, a neutral atom of carbon has 6 protons and 6 electrons. Any given orbital hold two electrons. Each orbital is filled with electrons according to its energy level, the lowest energy orbital filled first.
The inner most orbital has the lowest energy level & holds the first two electrons, orbital are arranged in groups (shells). The first shell has one orbital; each shell for the first 18 atoms Fig. 2.1 can hold up to eight electrons. Using this information, we can determine how many electrons are in the outer shells of the atoms Fig. 2.2.a Hydrogen (H) has only one orbital that contains one electron. Helium fills the first orbital, the first shell, with its two electrons. Carbon (atomic number of 6) has two shells, & the outer shell has four electrons. Nitrogen (atomic number of 7) has two shells, & the outer shell has 5 electrons. Oxygen (atomic number of 8) has 2 shells, & the outer shell has 6 electrons. Elements with greater than 18 electrons possess additional shells & accommodate additional electrons.
Atom mass represents its matter quantity. The subatomic particles are so light that their mass called atomic mass units (amu). Protons and neutrons are each assigned one atomic mass unit, whereas electrons have an exceedingly small mass unit (0.00055 amu or 1/1,836th the mass of a proton). Protons and neutrons sum in the atoms nucleus is called the mass number. (Dont confuse an elements mass number with its atomic mass, the number below each symbol in Fig. 2.1. the mass number for one form of carbon is 6 protons + 6 neutrons = 12. The mass number is written before the atomic symbol, so the symbol for carbon with a mass number of 12 is 12C.
Fig 2.1.b Explanation of subscripts seen in atomic number notation
Atomic number is the number of protons in the nucleus of atom therefore identical to the charge number of nucleus, represented by the symbol Z.
The atomic number identifies a chemical element.
The atomic number, Z, should not be confused with the mass number, A, which is the number of nucleons, the total number of protons and neutrons in the nucleus of an atom. The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N. Since protons and neutrons have approximately the same mass ( & the mass of the electrons is negligible for many purposes), & the mass defect is usually very small compared to mass, the atomic mass of an atom is roughly equal to A
*Uses of radiation in medicine
Radiation takes different forms in environment. Light and heat radiation are produced by the Sun, ionizing radiation is produced by some minerals in the Earth. Despite these different forms, most people are concerned about one type that produced by nuclear energy and artificial radioactive materials.
Ionizing radiation has two uses in medicine for diagnosis and therapy. Intended to benefit patients, the benefit must outweigh the risk.
Most people have X ray to help physician diagnose disease. A much less common diagnostic procedure involves the administration of radionuclide to patients so that detectors outside the body can be used to observe how organs are functioning. Physicians use either of procedures if they cannot make a diagnosis without them. Radiation doses are generally low, although they can be appreciable in certain procedures. Higher doses are required to treat malignant diseases or malfunctioning organs sometimes in combination with other treatment. A beam of radiation may be used to irradiate the affected part of the body or a fairly high activity of a radionuclide may be administered to the patient.
The use of X rays for examining patients is called diagnostic radiology and the use of pharmaceuticals labeled with radionuclide for diagnosis or therapy is called nuclear medicine. When radiation beams are used to treat patients, the procedure is called radiotherapy
Isotopes
Isotopes of the same type of atom have the same number of protons (thus, the same atomic number) but different numbers of neutrons. Therefore, their mass numbers are different. e.g. Carbon 12 (12C) has 6 neutrons, carbon 13 (13C) has 7 neutrons, and carbon 14 (14C) has 8 neutrons. You can determine the number of neutrons for an isotope by subtracting the atomic number (carbons atomic number is 6; from the mass number Fig 2.1. Unlike the other two isotopes of carbon, carbon 14 is unstable and breaks down over time. As carbon 14 decays, it releases various types of energy in the form of rays and subatomic particles; therefore, it is a radioisotope. The radiation given off by radioisotopes can be detected in various ways.
Geiger counter is used to detect radiation.
Most naturally occurring elements exist as a mixture of isotopes, and average atomic mass of this mixture determines the element's atomic weight
Low Levels of Radiation
Radioisotopes are used in biology and medicine. A radioisotope behaves the same as the stable isotopes. This means that you can put a small amount of radioisotope in a sample and it becomes a tracer by which to detect molecular changes e.g., after a patient drinks a solution containing a minute amount of iodine 131, it becomes concentrated in the thyroid, the only organ to take up iodine to make the hormone thyroxin. A subsequent image of the thyroid indicates whether it is healthy in structure and function (Fig. 2.3a). The use of positron-emission tomography (PET) is a way to determine the comparative activity of tissues. Radioactively labeled glucose, which emits a subatomic particle known as a positron, can be injected into the body. The radiation given off is detected by sensors and analyzed by a computer. The result is a color image shows which tissues took up glucose and metabolically active (Fig. 2.3b). A PET scan of the brain can help diagnose a brain tumor, Alzheimer disease, epilepsy, and can determine whether a stroke has occurredHigh Levels of Radiation
Effects of radioactive substances in the environment:Cancer.
DNA damage.
Harm cells.
Far-reaching.
Long-lasting.
Fig. 2.3 Medical uses for low-level radiation.
a-The missing area (indicated by the arrow) in this thyroid scan indicates tumor presence, that does not take up radioactive iodine.b- PET (positron-emission tomography) scan reveals which brain portions are most active (red surrounded by light green).
Good use of radiation
Kill bacteria and viruses, food supply safety, (Fig 2, 4a)
Sterilize medical & dental products.
Cancer therapy (Fig. 2.4b).
Medical diagnosis (X-rays).
Fig. 2.4 High-level radiation uses
a. Radiation can be used to sterilize food by killing bacteria and fungi.
b. Radiation therapy to kill cancer cells.
Molecules and Compounds
Atoms bond with one another to form a molecule. A molecule can contain atoms of the same type, as when an oxygen atom joins with another oxygen atom to form oxygen gas, or, the atoms can be different, as when an oxygen atom joins with two hydrogen atoms to form water. When the atoms are different, a compound is formed. Two bond types join atoms: ionic bond & covalent bond.
Ionic Bonding
Atoms with more than one shell are most stable when the outer shell contains eight electrons. During an ionic reaction, atoms give up or take on an electron or electrons to achieve a stable outer shell. Fig. 2.5, reaction between sodium (Na) atom & chlorine (Cl) atom. Sodium, with one electron in the outer shell, reacts with a single chlorine atom. Once the reaction is finished & sodium loses one electron to chlorine, sodiums outer shell will have eight electrons. Similarly, a chlorine atom, which has seven electrons already, needs to acquire only one more electron to have a stable outer shell. Ions are particles that carry either a positive (+) or negative (−) charge. When the reaction between sodium & chlorine is finished, the sodium ion carries a positive charge because it now has one less electron
than protons, & the chloride ion carries negative charge because it has one more electron than proton. The attraction between oppositely charged sodium ions &chloride ions forms an ionic bond. The resulting compound, sodium chloride, table salt, we use to enliven food taste. Whereas calcium needs to lose two electrons, each chlorine, with seven electrons already, requires only one more electron to have a stable outer shell. The resulting salt (CaCl2) is called calcium chloride, used as a deicer in northern climates. The balance of ions is important to health. Too much sodium in blood contributes to high blood pressure. Calcium deficiency leads to rickets (a bowing of the legs) in children. Too much or too little potassium results in heartbeat irregularities can be fatal.
Fig. 2.5 Formation of an ionic bond.
a. During the formation of sodium chloride, an electron is transferred from the sodium atom to the chlorine atom. At the completion of the reaction, each atom has eight electrons in the outer shell, but each also carries a charge as shownb. In a sodium chloride crystal, ionic bonding between Na+ and Cl− causes the ions to form a three-dimensional lattice configuration in which each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions
Bicarbonate, hydrogen, and hydroxide ions are involved in body acidbase balance maintenance.
Covalent Bonding
Atoms share electrons in covalent bonds. The overlapping, outermost shells in Fig. 2.6indicate that the atoms are sharing electrons; each atom contributes one electron to the shared pair. These electrons spend part of their time in the outer shell of each atom; therefore, they are counted as belonging to both bonded atoms. Double and Triple Bonds Besides a single bond, in which atoms share only a pair of electrons, a double or a triple bond can form. In a double bond, atoms share two pairs of electrons; in a triple bond, atoms share three pairs of electrons, in Fig. 2.6b, each oxygen atom (O) requires two more electrons to achieve a total of eight electrons in the outer shell. 4 electrons are placed in the outer, overlapping shells.
Structural and Molecular Formulas
Covalent bond is presented in a number of ways. Structural formulas use straight lines to show covalent bonds between. Each line represents a pair of shared electrons. Molecular formulas indicate number of each atom type.A comparison follows:
Structural formula: HOH, O=O
Molecular formula: H2O, O2
Structural & molecular formulas for carbon dioxide. Carbon, with 4 electrons in the outer shell, requires 4 more electrons to complete its outer shell. Each oxygen, with 6 electrons in the outer shell, needs two electrons to complete its outer shell. Therefore, carbon shares two pairs of electrons with each oxygen atom, & the formulas are as follows:
Structural formula: O=C=O
Molecular formula: CO2
Thanks Samia 2015