Chapter 2

Atoms, Molecules and Life

Outline for Today

•       Basic Chemistry

–    Atoms

–    Isotopes

–    Molecules and Compounds

–    Chemical Reactions

•       Properties of Water

•       Acids and Bases

Basic Chemistry

•       There are 92 naturally-occurring elements.

–    Over 90% of human body is composed of four elements.

v Carbon.

v Nitrogen.

v Oxygen.

v Hydrogen.

Atoms

•       The smallest unit of matter that retains the element’s physical and chemical properties.

–    Can not break down an element by normal chemical means

•       Structure

–    Nucleus

v Protons

v Neutrons

–    Electrons

Electrons Orbit the Nucleus, Forming Electron Shells

Atoms Are Composed of Even Smaller Particles

Atoms

•       Atomic number = number of protons.

•       Atomic weight (mass number) = protons + neutrons.

Elements and Atoms

The Periodic Table

•       Elements are arranged into a chart called the periodic table.

•       The table groups elements by their chemical properties .

 

Isotopes

•       Isotopes - atoms of the same element that differ in the number neutrons.

–    Radioactive isotopes spontaneously decay, releasing rays & subatomic particles.

v Can be used as a tracer

v Radiation kills healthy cells, but can also be used to kill cancerous cells.

Use of Radiation to Study the Brain

 Thyroid Scan

•       Measures health of thyroid by detecting radioactive iodine taken up by thyroid gland

 

Molecules and Compounds

•        A molecule is a group of atoms bonded together.  May be the same element – i.e. oxygen, O2

•       A compound is a group of atoms of different elements bonded together – i.e. water H2O

How Do Atoms Form Molecules?

•       Atoms Interact When There Are Vacancies in Their Outermost Electron Shells

 

Ionic Reactions

•       During an ionic reaction, atoms give up or take on an electron to stabilize their outer shells.

•       Ions are particles that carry a positive (+) or negative (-) charge.

–    The attraction between oppositely charged sodium ions and chloride ions forms an ionic bond.

Ionic Reaction

Covalent Reactions

•       Atoms share electrons in covalent bonds

–    Single bond – one electron pair shared.

–    Double bond

–    Triple bonds

•       Most biological molecules use covalent bonding

Covalent Reactions

How Do Atoms Form Molecules?

•       Atoms sharing electrons to form covalent bonds are uncharged

–    Covalent Bonds Are Either

v Non-polar

v Polar

 

 

•       Hydrogen Bonds Form between Molecules with Polar Covalent Bonds

 

 

Water and Living Things

•       Water molecules are polar

–    Oxygen is more electronegative (electron-hungry) than hydrogen

v  Oxygen slightly negative

v Hydrogen slightly positive

–    Hydrogen bond – attraction between hydrogen of one water molecule to oxygen of another water molecule

Hydrogen Bonding between Water Molecules

Properties of Water

•       Water is liquid at room temperature.

•       Water is a solvent for polar molecules.

•       Water molecules are cohesive.

•       Water temperature rises and falls slowly.

•       Water has a high heat of vaporization.

•       Frozen water is less dense than liquid water.

 

Why Is Water So Important to Life?

•       Water Can Form Ions

Acids and Bases

•       Acids dissociate in water and release hydrogen ions (H+).

–     Acidic solutions have more H+ ions than pure water

•       Bases take up hydrogen ions (H+) or release hydroxide ions (OH-)

–     Basic or alkaline solutions have fewer H+ ions than pure water

–     Buffers help keep the pH within normal limits by taking up excess hydrogen ions or hydroxide ions.

pH Scale

•       The pH scale measures acidity or alkalinity of a solution.

–    Neutral = 7.

–    Acidic < 7.

–    Basic > 7.

v Logarithmic Scale

The pH Scale

 

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