Cơ sở hóa học

ELECTRON SHELLS Electrons vary in the amount of energy they possess  Each electron exists within a discrete energy level  These energy levels are represented by electron shells  Mos

NỘI DUNG BÀI GIẢNG

VẬT CHẤT

 Vật chất là bất cứ vật gì có khối lượng và thể tích

NGUYÊN TỐ

 Một nguyên tố không thể phân chia thành các chất đơn giản bằng các phản ứng hóa học bình thường

 Có khoảng 100 nguyên tố khác nhau

 Mỗi nguyên tố có một ký hiệu nhất định

 Ví dụ: Carbon (C), hydrogen (H), v.v.

 Nước (H2O), glucose, v.v không phải là nguyên tố.

 Có khoảng 25 nguyên tố cần thiết cho sự sống

 Gần 96% khối lượng vật chất sống là từ 4 nguyên

nguyên tử

 An atom is the smallest unit into which an

element can be subdivided while retaining its properties

 Comprised of smaller “subatomic” particles

ATOMIC STRUCTURE

 Protons and neutrons form an atom’s nucleus

 Electrons are present outside of the nucleus

ATOMIC MASS

 Protons and neutrons have significant mass

 The number of protons plus neutrons in an atom is termed its atomic mass

 Electrons have negligible mass

 This number is slightly variable for many elements

 Isotopes are atoms of the same element

possessing different atomic masses

 Due to different numbers of neutrons

 Identical chemical behavior

 Radioactive isotopes

 Decay at a constant rate into

more stable forms

 May decay into another element

 Various uses

 Important research tools

 Monitor biological processes

 Diagnostic tools in medicine

 Determine age of fossils

 Sometimes produce superheroes

 Each radioactive isotope has a fixed rate of decay

 Unaffected by temperature, pressure etc

 The time required for half of a sample to decay is

termed the radioisotope’s half-life

 e.g., Half-life of 14C is ~5,730 years

 Fossils contain isotopes of elements that

accumulated while they were alive

 Accumulation stops upon

 Ages of fossils can be determined using

radiometric dating

 Compares accumulating “daughter” isotope to

remaining “parent” isotope

 e.g., Carbon dating is useful for dating fossils up to 75,000 years old

 13 half-lives

 Radioisotopes with

longer half-lives can

be used to date older

fossils

ELECTRON SHELLS

 Electrons vary in the amount of energy they possess

 Each electron exists within a discrete energy level

 These energy levels are

represented by electron

shells

 Most atoms possess multiple electron shells

ELECTRON SHELLS

 Electrons exist within electron shells

 The first shell must be completely filled before electrons are placed in the second shell, etc

 The first shell can hold 2 electrons

 The next few electrons can each hold 8 electrons

 A sodium atom possesses a single electron in its outermost electron shell

 Tends to lose this electron

 Loss of the electron

produces a sodium ion

 A charged form of a sodium atom

 A chlorine atom possesses seven electrons in its outermost electron shell

 Tends to gain a single electron

 Gain of this electron produces a chloride ion

 A charged form of

a chlorine atom

 Indefinite size and number of ions

 Ions present in a fixed ratio

 e.g., 1:1 ratio in NaCl

COVALENT BONDS

 Electrons are not always gained or lost

 Sharing is always in pair(s) of electrons

 Shared electrons contribute to electron shells of both atoms

 This sharing of electrons is termed a covalent bond

 A triple covalent bond

involves the sharing of

three pairs of electrons

COVALENT BONDS

 Electron sharing can be equal or unequal

 Equal sharing results in no separation of charges

COVALENT BONDS

 Electron sharing can be equal or unequal

 Unequal sharing results in a separation of charges

tend to attract electrons

more strongly than

carbon or hydrogen

 They thereby possess partial negative charges

COVALENT BONDS

 Water is a polar molecule

 The hydrogen atoms possess partial positive charges

 The oxygen atom possesses a partial negative charge

HYDROGEN BONDS

 Attraction between a hydrogen atom bearing a

partial positive charge and another atom bearing a partial negative charge

 Much weaker than covalent or ionic bonds

CHEMISTRY OF WATER

 Water is a critically important molecule

 Most of the Earth’s surface is submerged in water

 Life on Earth began

in water

 Life evolved in water

for 3 billion years

before spreading onto

CHEMISTRY OF WATER

 Water is the most prevalent molecule within living organisms

 Cells are about 70 – 95% water

 Most cells are themselves surrounded by water

CHEMISTRY OF WATER

 Water molecules possess polar covalent bonds

 Able to participate in H-bonds

 Water molecules

interact with each other

 Water is held together

by these H-bonds

 Water attaches to other

ions and molecules

CHEMISTRY OF WATER

 Surface tension results from cohesion

 Difficult to break the surface of water

 e.g., Water strider, skipping rocks, etc.

WATER AS A SOLVENT

 Water is a powerful and versatile solvent

 A wide variety of molecules and ions dissolve in

water

 e.g., Sugars, salts, some proteins, etc.

 Interacts well with polar or charged molecules and ions

 Most chemical reactions within organisms occur in a water medium

WATER AS A SOLVENT

 Polar or charged molecules and ions are

hydrophilic

 Interact favorably with water

 Similar structures surround polar molecules

WATER AS A SOLVENT

 Many molecules and ions dissolve in water

 Water is the solvent

 The salt, sugar, dye, etc is the solute

 A solution in which water is the

solvent is termed an aqueous solution

WATER AS A SOLVENT

 Not all hydrophilic molecules dissolve in water

 Typical of very large hydrophilic molecules

 e.g., Cotton consists of very large molecules of cellulose

DISSOCIATION OF WATER

 Most water molecules exist as H2O (H-O-H)

 A small fraction of water molecules exist in a

dissociated state

 ~1 in 554 million molecules in pure water

 A hydrogen atom shifts from one water molecule to another

 H-O-H  H + & OH - (hydrogen ion & hydroxide ion)

 2H2O  H3O + & OH - (hydronium ion & hydroxide ion)

 Concentrations are not always equal

 Can be altered by the addition of acids or bases

 As [H + ] ↑, [OH - ] ↓, and vice versa

ACIDS, BASES, AND pH

 An acid is a substance that increases the H+concentration of a solution

 Generally donates additional H+

-ACIDS, BASES, AND pH

ACIDS, BASES, AND pH

 pH is a quantitative measure of [H+]

 Ranges from 0 (most

acidic) to 14 (most basic)

 Each pH unit represents

a tenfold change in the

concentration of H+

 e.g., pH 4 has 100 times more H + than pH 6

ACIDS, BASES, AND pH

 The interior pH of most cells is close to 7

 Even slight changes in pH can be harmful to cells and organisms

 Chemical processes of the cell are very sensitive to concentrations of H + and OH -

 The shapes of biological molecules can be altered by changes in H + and OH - concentrations

 Biological fluids contain buffers

 Substances or systems that minimize changes in pH by accepting or donating H +

ACID PRECIPITATION

 Acid precipitation represents a serious assault on water quality

 Uncontaminated rain has a pH of about 5.6

 Slightly acidic due to carbonic acid formed from dissolved

CO2

- Acid precipitation is more acidic than this

 pH 4.3 rain has been measured in the U.S.

ACID PRECIPITATION

 Acid precipitation

 Caused primarily by the presence in the atmosphere of sulfur oxides and nitrogen oxides

 React with water to form strong acids

 Fall to earth with rain or snow

ACID PRECIPITATION

 Acid precipitation

 The burning of fossil fuels in factories and

automobiles is a major source of acid precipitation

 Electrical power plants burning coal produce more of these pollutants than any other source

 Winds carry these pollutants away

 Acid rain may fall far from industrial centers

ACID PRECIPITATION

 Acid precipitation

 Can damage life in lakes and streams

 Can remove mineral ions from soil

 e.g., Calcium and magnesium ions

 Can increase the

solubility of certain ions

 e.g., Aluminum can reach toxic concentrations

 Campbell, Neil A and Reese, Jane B Biology, 7th

edition Pearson Education, Inc 2005

 Campbell, Neil A., Reese, Jane B., Taylor, Martha R., and Simon, Eric J Biology, Concepts and Connections,

5th edition Pearson Education, Inc 2006

 Nester, Eugene W., Anderson, Denise G., Roberts, C Evans Jr., and Nester, Martha T Microbiology, A

Companies, Inc 2007

 Limson, Janice 2002

http://www.scienceinafrica.co.za/2002/june/lactose.htm