Bio 2

Electron Distribution

Diagrams for the First 18 Elements in the Periodic table

7

Cl 1

Sodium + Cl

Electron Distribution and Chemical Properties

Chemical behavior of an atom depends mostly on the number of electrons in its outermost shell, or valence shell

Valence electrons are those that occupy the valence shell

The reactivity of an atom arises from the presence of one or more unpaired electrons in the valence shell

Atom with completed valence shells are unreactive or inert.

The Formation and Function of Molecules depend on chemical bonding atoms

Atoms within complete valence shells can share or transfer valence electrons with certain other atoms

This usually results in atoms staying close together held by attractions called chemical bonds

Covalent Bonds

A covalent bond is the sharing of a pair of valence electrons by two atoms

In a covalent bond, the shared electrons count as pair of each atom’s valence shell

Two or more atoms held together by covalent constitute as a molecule

Atomic mass of 1 He

4.003

The molecular formula, for example, H2 indicates that the molecule consists of

two hydrogen atoms

Electron sharing can be shown by an electron distribution diagram or structural formula'

for example in the structural formula H ,←> the line represents a pair or

shared electrons

a _ Hydrogen the sharing of one pair of electrons, molecule is indicated by a single line between atoms

For example H ←> H

single bond

A double bond, the sharing of two pairs of electrons, is indicated by a double line between atoms

For example O=O

Each atom that can share valence electrons has a bonding capacity, the number of

bonds that the atom can form

Bonding capacity, or _, usually corresponds to the number of electrons required to complete the outermost valence shell of the atom

valence

Covalent Bonds (5 of 7)

pure elements are comprised of molecules of one type of atom such as H2 and O2

Molecules composed of a combination of two or more types of atoms such as H2) or CH are called compounds

H

H C H => CH4

H

in a nonpolar covalent bond the atoms share the electrons equally

In a polar covalent bond, one atom is more electron negative, the atoms do not share the electrons equally

Unequal sharing of electrons causes a partial positive or negative charge to each atom or molecule

H2O water => polar covalent bound => CH4

O

H H

Atoms sometimes strip electrons from their less electro negative bonding partners

The two resulting oppositely charged atoms/ or molecules/ are called IONS

A cation is a positively changed ion

An anion is negatively charged ion

An ionic bond is an attraction between an anion and a cation

When one atom of sodium (11 N2) encounters one atom of chlorine (17Cl. the ions valence of the sodium is transferred to the chlorine atom

The sodium has a positive charge without the chlorine atom has a negative charge.

both atoms end up with their valence shells complete

Ionic Bonds (2 of 3)

When one atom of Sodium (‘NA) encounters one atom of chlorine (12 CL), the lone valence electron of the sodium is transferred to the chlorine atom

The sodium atom has a positive energy whereas the chlorine atom has a negative charge

Both atoms end up with their valence shell complete

Iconic Bonds (3 of 3)

Compounds formed by Iconic bonds are called ionic compounds or salts

salts such as sodium chlorine (table salt) are often found in nature as crystals

Weak Chemical Interactions

most of the strongest bonds in organisms are covalent bonds, that form a cell’s molecules

Many large biological molecules are held in their functional form by weak bonds

The ionic bond, as it exists between ions dissociation water, in one’s type of weak chemical interactions.

Others are hydrogen bonds, and van der waals interactions

Hydrogen Bonds

A Hydrogen bond forms when a hydrogen atom covalently bonded to one electron negative atom nearby

In living cells the electronegative partners are usually oxygen or nitrogen atoms

Van Der Waals Interactions

Electrons may be distributed asymmetrically in molecules or atoms

The resulting regions of positive or negative charge enable all atom’s and molecules to stick to one another

The weak vander waals interactions occur only when atoms and molecules are very close together

Cumulatively such interactions can be strong, as between molecules of a gecko’s foot and a wall surface

Molecular shape and Function (1 of 2)

A molecule’s size and shape are key to its function in the cell

Molecular shape determines how biological molecular shapes determine how biological molecular recognize and respond to each other

Molecular shape and function (2 of 2)

Biological molecules may end temporarily to each other through weak interactions their shapes can have similar biological effects

Concept 2.4 Chemical Reactions make and Break chemical bond (1 of 4)

Chemical reaction are making and breaking chemical bonds

the starting molecules of a chemical reaction are called reactants

The final molecules of a chemical reaction are called products

Concept 2.4 Chemical Reactants make and Break (3 of 4)

Photosynthesis an important reaction

Sunlight powers the conversion of carbon dioxide and water the glucose and oxygen

Concept 2.4: chemical reactions make and break chemical bonds (4 of 4)

All Chemical reactions are theoretically reversible: Product of the foward reaction become reactants for the reverse reaction

Chemical equilibrium is reached when the forward and reverse reaction rates are equal

Concept 2.5 Hydrogen Bonding Gives Water properties that help make life Possible On Earth (1 of 2)

All organisms are made mostly of water and live in an environment dominated by water

Water molecules are polar molecules with the oxygen region having a partial negative charge (-) and the hydrogens having a partial positive charge (8+)

Two water molecules are held together by a hydrogen bond

At any instant, most of the water molecules are hydrogen bonded to their neighbors

Concept 2.5 Hydrogen Bonds gives water properties that help make life possible on Earth (2 of 2)

Four emergent properties of water contribute to Earth’s suitability for life

Cohesive behavior

Ability to moderate temperature

Expansion upon freezing versatility as a solvent

Cohesion of water molecules (1 of 3)

water molecules stay close to each other due to hydrogen bonding. This is called cohesion

Cohesion of water molecules (2 of 3)

Cohesion due to hydrogen bonding contributes to the transport water and nutrients against gravity in plants

Adhesion, the clinging of one substance to another, also plays a role

Adhesion of water touch walls by hydrogen bonds helps counter the downward pull of gravity

Cohesion of water molecules (3 of 3)

surface tension is a measure of how hard it is to break the surface of a liquid

Surface tension related to cohesion

Moderation of temperature by water

water absorbs heat from warmer and releases stored heat cooler air

water can absorb or release a large amount of heat with only a slight change in its own temperature

Temperature and Heat (1 of 2)

kinetic energy is the energy of motion

Thermal energy is a measure of the total amount of kinetic energy due to molecular motion

Temperature represents the average kinetic energy of molecules

Thermal energy in transfer from one body of matter to another is defined as heat.

Temperature and Heat (2 of 2)

A calorie (cal) is the amount of heat required to raise the temperature of 1 gml of water by 7 C.

The Calories on food packages are actually kilocalories (Kcal), where 1 cal-= 1,000 cal

The joule (j) is another unit of energy, where 1 J = 0.1234 or 1 cal=4.184 J

Water’s specific Heat (1 of 2)

The specific heat of a substance is the amount of heat that must be absorbed or left for 1 J of that substance to change its temperature by 1 o C

The specific heat of water is 1 Cal (g o C)

Water resists changing its temperature because of it high specific heat

Water’s high specific heat (2 of 2)

Water’s high specific heat can be traced to hydrogen bonding

Heat is absorbed with hydrogen bonds break

Heat is released when hydrogen bonds form

The high specific heat of water keeps temperature fluctuates within limits that permit life.

Evaporation Cooling

Evaporation (vaporization) transformation of a is transformation of a structure liquid to gas

Heat of vaporization is the heat a liquid must absorb for it to converted to gas

As a liquid evaporates, its remaining surface cools, a process called evaporative cooling

Evaporation cooling of water helps stabilize temperature in bodies of water and organisms

Floating of Ice on Liquid Water (1 of 2)

ice floats in liquid water because hydrogen bonds in ice are more “ordered” making ice less dense than water

Water reaches its greatest density at o4C

is ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth

Floating of Ice on Liquid (2 of 2)

Floating Ice can insulate the water below, allowing life to exist under the froze surface

Ice also provides a solid habitat animals such as polar bears, seals, and walruses

Many scientists are worried that these bodies of ice are at risk disappearing due to global warming

Water: The solvent of Life (1 of 3)

A solution is a liquid that is a heterogeneous mixture of substances

A solvent of substances

A solvent is the solving agent of a solution

The solute is the substance that is dissolved

An aqueous solution if one in which water is the solvent

Water is the solvent (2 of 3)

Water is a versatile solvent due to its polarity due to its polarity