1-What are the fundamental building blocks of all matter?
Atoms are fundamental building block of all matter. they consist of even smaller subatomic particles. Positively charged protons (p+) and uncharged neutrons occur in an atom’s core, or nucleus. Negatively charged electrons (e–) move around the nucleus. Charge is an electrical property: Opposite charges attract, and like charges repel.
A typical atom has about the same number of electrons and protons. All atoms have protons. The number of protons in the nucleus is called the atomic number, and it determines the type of atom, or element. Elements are pure substances, each consisting only of atoms with the same number of protons in their so all atoms with six protons in their nucleus are carbon atoms, no matter how many electrons or neutrons they have. Carbon, the substance, consists only of carbon atoms, and all of those atoms have six protons.nucleus. For example, the atomic number of carbon is 6.
1.a-Isotopes and Radioisotopes
All atoms of an element have the same number of protons, but they can differ in isotope. Isotopes are forms of an element that differ in the number of neutrons their atoms carry. We define isotopes by their mass number, which is the total number of protons and neutrons in atomic nucleus. Isotope with an unstable nucleus is called radioisotope. Atoms of a radioisotope have an unstable nucleus that breaks up spontaneously. As a nucleus breaks up, it emits radiation—subatomic particles, energy, or both—a process called radioactive decay.
1.b-Tracers
Radioisotopes can be used as tracers to study biological processes.A tracer is a molecule or any substance with a detectable component. For example, a molecule in which an atom (such as 12C) has been replaced with a radioisotope (such as 14C) can be used as a radioactive tracer. When delivered into a biological system such as a cell, body, or ecosystem, this tracer may be followed as it moves through the system with instruments that detect radiation.
Remember!
-All matter consists of atoms, tiny particles that in turn consist of electrons moving around a nucleus of protons and neutrons.
-An element is a pure substance that consists only of atoms with the same number of protons. Isotopes are forms of an element that have different numbers of neutrons.
-Unstable nuclei of radioisotopes break down spontaneously (decay) at a predictable rate to form predictable products.
2-Why do atoms interact?
2.a-Electrons Matter
Electrons are really, really small. Electrons carry energy, but only in incremental amounts. An electron gains energy only by absorbing the exact amount needed to boost it to the next
energy level. Likewise, it loses energy only by emitting the exact difference between two energy levels.
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| Shell Models |
A lot of electrons may be zipping around in the same atom. Despite moving really fast(around 3 million meters per second), they never collide. Why not? For one reason, electrons in an atom occupy different orbitals, which are defined volumes of space around the atomic nucleus.
A shell model helps us visualize how electrons populate atoms. In this model, nested “shells” correspond to successively higher energy levels. Thus, each shell includes all of the rooms (orbitals) on one floor (energy level) of our atomic apartment building.
2.b-About Vacancies
When an atom’s outermost shell is filled with electrons, we say that it has no vacancies. When an atom has no vacancies, it is in its most stable state. Helium, neon, and argon are examples of elements with no vacancies. Atoms of these elements are chemically stable, which means they have no tendency to interact with other atoms. Thus, these elements occur most frequently in nature as solitary atoms.
By contrast, when an atom’s outermost shell has room for another electron, it has a vacancy. Atoms with vacancies tend to get rid of them by interacting with other atoms; in other words, they are chemically active.
Solitary atoms that have unpaired electrons are called free radicals. With some exceptions, free radicals are very unstable, easily forcing electrons upon other atoms or ripping electrons away from them. This property makes free radicals dangerous to life.
Atoms with an unequal number of protons and electrons are called ions. Ions carry a net (or overall) charge. Sodium ions (Na+) offer an example of how atoms gain a positive charge by losing an electron. Other atoms gain a negative charge by accepting an electron. For example, an uncharged chlorine atom has 17 protons and 17 electrons. The outermost shell of this atom can hold eight electrons, but it has only seven.
Remember!
-An atom’s electrons are the basis of its chemical behavior.
-Shells represent all electron orbitals at one energy level in an atom. When the outermost shell is not full of electrons, the atom has a vacancy.
-Atoms with vacancies tend to interact with other atoms.
3-How do atoms interact in chemical bonds?
A chemical bond is an attractive force that arises between two atoms when their electrons interact. Chemical bonds link atoms into molecules. For example, a water molecule consists of three atoms: two hydrogen atoms bonded to the same oxygen atom. A water molecule is also a compound, which means it has atoms of two or more elements.
3.a- Ionic bonds
Ionic bond is a type of chemical bond in which a strong mutual attraction links ions of opposite charge.
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| ionic bonds |
Any such separation of charge into distinct positive and negative regions is called polarity. An ionic bond is very polar because the atoms that are participating in it have a very large difference in electronegativity. An electronegativity is a measure of the ability of an atom to pull electrons away from other atoms. When atoms with a lower difference in electronegativity interact, they tend to form chemical bonds that are less polar than ionic bonds.
3.b-Covalent bonds
The covalent bonds do form between atoms with a small difference in electronegativity or none at all. Covalent bond is a chemical bond in which two atoms share a pair of electrons.
Atoms share electrons unequally in a polar covalent bond. Covalent bonds in compounds are usually polar. By contrast, atoms participating in a nonpolar covalent bond share electrons equally.
Remember!
-When the electrons of atoms interact, the chemical bonds formed. A chemical bond may be ionic or covalent depending on the atoms taking part in it.
-An ionic bond is a strong mutual attraction between two oppositely charged ions. Ionic bonds are very polar.
-Atoms share a pair of electrons in a covalent bond. The covalent bond is polar when the atoms share electrons unequally. A chemical bond is an attractive force that holds between two atoms.
4-What are life-sustaining properties of water?
4.a-Hydrogen Bonding in Water
Hydrogen bone is an attraction between a covalently bonded hydrogen atom and another atom taking part in a separate covalent bond.
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| Hydrogen bonds |
Hydrogen bonds are on the weaker end of the spectrum of atomic interactions, and they form and break much more easily than covalent or ionic bonds. Even so, many of them form, and collectively they are quite strong. As you will see, hydrogen bonds stabilize the characteristic structures of biological molecules such as DNA and proteins. They also form in tremendous numbers among water molecules.
4.b-Water’s Special Properties
Water Is an Excellent Solvent The polarity of the water molecule and its ability to form hydrogen bonds make water an excellent solvent, which means that many other substances easily dissolve in it. Substances that dissolve easily in water are hydrophilic (water-loving). Ionic solids such as sodium chloride (NaCl) dissolve in water because the slight positive charge on each hydrogen atom in a water molecule attracts negatively charged ions (Cl–), and the slight negative charge on the oxygen atom attracts positively charged ions (Na+).
Sodium chloride is called a salt. A salt is a compound that releases ions other than H+ and OH– when it dissolves in water. A dissolved substance is called solute. Uniform mixure of solute completely dissolved in solvent is solution. And the liquid that can dissolve other substance is solvent.
Water does not interact with hydrophobic (water-dreading) substances such as oils. Oils consist of nonpolar molecules, and hydrogen bonds do not form between nonpolar molecules and water.
Water Has Cohesion. Property of a substance that arises from the tendency of its molecules to resist separating from one another is called cohesion.
Cohesion plays a role in many processes that sustain multicelled bodies. As one example, water molecules constantly escape from the surface of liquid water as vapor, a process called evaporation. Evaporation is resisted by hydrogen bonding among water molecules. In other words, overcoming water’s cohesion takes energy. Thus, evaporation sucks energy (in the form of heat) from liquid water, and this lowers the water’s surface temperature.
Cohesion works inside organisms, too. Water molecules evaporate from leaves, and replacements are pulled upward from roots. Cohesion makes it possible for columns of liquid water to rise from roots to leaves inside narrow pipelines of vascular tissue. In some trees, these pipelines extend hundreds of feet above the soil.
Water Stabilizes Temperature All atoms jiggle nonstop, so the molecules they make up jiggle too. We measure the energy of this motion as degrees of temperature. Temperature is the measure of molecule motion.
Hydrogen bonding keeps water molecules from jiggling as much as they would otherwise, so it takes more heat to raise the temperature of water compared with other liquids. The stability of temperature is an important part of homeostasis, because function of most of the molecules of life can perform properly only within a certain range of temperature.
Remember!
-Extensive hydrogen bonding among water molecules, which
arises from the polarity of the individual molecules, ives water special properties.
-Liquid water is an excellent solvent. Hydrophilic substances such as salts and sugars dissolve easily in water to form solutions. Hydrophobic substances do not dissolve in water.
-Water also has cohesion, and it stabilizes temperature.
5-Why are hydrogen ions important in biological systems?
Concentration refers to the amount of a particular solute dissolved in a given volume of fluid. Hydrogen ion (H+) concentration is a special case. We measure the amount of hydrogen ions in a solution using a value called pH. When the number of H+ ions equals the number of OH– ions in the liquid, the pH is 7, or neutral. The higher the number of hydrogen ions, the lower the pH. A one-unit decrease in pH corresponds to a tenfold increase in the number of H+ ions, and a one-unit increase corresponds to a tenfold decrease in the number of H ions
Substances called bases accept hydrogen ions, so they can raise the pH of fluids and make them basic, or alkaline (above pH 7). Acids give up hydrogen ions when they dissolve in water, so they lower the pH of fluids and make them acidic (below pH 7).
Together, carbonic acid and bicarbonate constitute a buffer, a set of chemicals that can keep the pH of a solution stable by alternately donating and accepting ions that contribute to pH.
Remember!
-The number of hydrogen ions in a fluid determines its pH. Most biological systems function properly only within a narrow range of pH.
-Acids release hydrogen ions in water; bases accept them. Salts release ions other than H+ and OH–.
-Buffers help keep pH stable. Inside organisms, they are part of homeostasis.
Click here to read about: BIOLOGICAL MARCROMOLECULES
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