Chapter 2 Atoms And Molecules

Front 1

Wood ants live in coloines of up to?

Back 1

thousands of ants in a single colony

Front 2

Colonies of wood ants have a mechanism to deal with threats, what is it?

Back 2

To fire folic acid from their abodmens which rains down on their attackers

Front 3

Some species of ants don't use it as a firing mechanism, what do they use it for instead?

Back 3

Spray on themselves as a defense against microbes

Front 4

Where did folic acid gets it name from?

Back 4

The latin word for ant, formica

Front 5

Scientists have long known chemicals play a role in?

Back 5

Serveal different aspects of ant behaviour insect communication, defence and attraction of mates

Front 6

Organisms are composed of?

Back 6

Matter which is anything that takes up space and has mass

Front 7

What is matter made up of?

Back 7

Elements

Front 8

What is an element?

Back 8

An element is a substance that cannot be chemically broken down into a simpler form

Front 9

Chemists recognize how many natural elements?

Back 9

92

Front 10

Each element has a symbol derived from?

Back 10

Usally from the first letter or two of its name, some symbols are derived from latin or german, natrium for sodium for example

Front 11

What is a compound?

Back 11

A compound is a substance consisting of 2 of more different elements in a fixed ratio

Front 12

Table salt for example is?

Back 12

Sodium Chloride

Front 13

What is pure sodium and pure chlroine?

Back 13

Pure sodium is a metal and pure chlorine is a toxic gas

Front 14

A compound has characteristics different from?

Back 14

Those of the original elements that made it up

Front 15

Define mass

Back 15

Mass is the amount of matter in something where as the weight of an object is how strong gravity pulls on matter in the object

Front 16

Out of the 92 elements as a % barrier, what is the boundary?

Back 16

Out of the 92 elements, between 20 and 25 per cent are essential elements

Front 17

There is some variation in elements?

Back 17

Between organims, for example some organisms require more than another, like a human needs 25 elements and plants need 17

Front 18

What 4 elements make up 96.3% of living matter?

Back 18

Oxygen 65%, Carbon 18.5%, Hydrogen, 9.5% and Nitrogen 3.3%

Front 19

Trace elements are reuqired in?

Back 19

Small doses as they are not required in large doses, like iodine which keeps the thyroid gland producing a hormone

Front 20

What happens if you don't get enough iodine?

Back 20

The human thyroid will grow to abnormal size and wont produce the hormone, this is called goiter

Front 21

How much iodine is enough and where can you find iodine in foods?

Back 21

15 milligrams and seafood and iodized salt

Front 22

What is an example of a trace element that is required by all organisms?

Back 22

Iron is needed, iodine is only needed inverterabrates

Front 23

Some naturally occurring elements are?

Back 23

Toxic to humans and other organisms, like asrenic

Front 24

How can arsenic get in water?

Back 24

Arsenic occurs naturally and can makes it way into underground water sources which contaminates it and is drilled up and when drunk can cause a varitey of diseases and can be fatal, this is a promblem in south asia

Front 25

What are the trace elements required by humans?

Back 25

Boron, chromium, cobalt, copper, fluorine, iodine, iron, manganese, molybdnum, selenium, silicon, tin, vanadium and zinc

Front 26

Some species have become adpated to?

Back 26

environments that contain elements that are toxic, an example of this is serpetine communities

Front 27

What is serpentine?

Back 27

A jade like mineral which contains high amounts of element like nickel, cobalt and chromium

Front 28

Most species of plants can't survive in serpentine conditions but some?

Back 28

Have adaptions that allow them to surive

Front 29

Presumably, variants of ancestral, nonserpentine speicies arose?

Back 29

that could surive in serpentine soils and through natural selection a diverse range off species was produced

Front 30

Reseachers are studying whether serpentine commuinites could be used to?

Back 30

Take up toxic heavy metals in contaminated areas, concentrating them for safer disposal

Front 31

What is this plant that is serpentine resisant?

Back 31

Tiburon mariposa lily

Front 32

Each element consists of atoms that are?

Back 32

specific to that atom and cant be found in other atoms

Front 33

What is an atom?

Back 33

The smallest unit of matter that still retains properties of an element

Front 34

What are atoms composed of?

Back 34

Subatomic particles, one here are protons neutrons and electrons

Front 35

Reseachers have discovered over a hundred different type of particles by?

Back 35

using high energy coillsions to generate new particles

Front 36

Protons and electrons are?

Back 36

electrically charged, with one unit of charge in each

Front 37

What is a charge on a proton, neutron and electron?

Back 37

A proton is positively charged, electron negatively charged and a neutron is neutral

Front 38

Protons and neutrons are packed tighly in the?

Back 38

nucleus of an atom, with the protons giving it a positive charge

Front 39

How are the electrons attracted to it?

Back 39

The electrons that are rapidly moving around are attracted to the + charge of the nucleus which keeps them close

Front 40

Neutrons and protons weigh about what in grams?

Back 40

1.7 to the 10 to the power of negative 24

Front 41

Why are grams not useful in weighing atoms?

Back 41

Because they weigh so little, grams are to big

Front 42

What do we use instead to weigh atoms and molecules?

Back 42

Daltons named after john dalton, british scientist who helped develop atomic theory around 1800, a proton and eletron are about 1 dalton each

Front 43

The dalton is the same as?

Back 43

The atomic mass unit of amu

Front 44

Why can we ingore the weight of electrons in calcultaing masses in atoms?

Back 44

Because they weight about 1/2000 when compared to a proton or neutron

Front 45

Atoms of the various elements have different number of?

Back 45

Subatomic particles in the atoms

Front 46

All elements of a particular element have the same bumber of?

Back 46

protons in their nuclei

Front 47

The number of protons is known as?

Back 47

The atomic number

Front 48

An atom is normally?

Back 48

neutral, meaning it has the same number of protons and electrons

Front 49

What number can be used to decude the number of neutrons in an atom?

Back 49

The mass number

Front 50

How can we find the neutron number?

Back 50

Mass number take away the atomic number

Front 51

What is the simplest atom?

Back 51

Hydrogen which has no neutrons

Front 52

Why is most of the atoms mass from the nucleus?

Back 52

Because the electrons weight compared to the nucleus is tiny

Front 53

In daltons how much does sodium weigh?

Back 53

29.9898 daltons

Front 54

What is an isotope?

Back 54

A isotope of a element is an atom which has the same number of protons but a different number of neutrons which give it a greater mass

Front 55

In nature an element occurs as?

Back 55

a mixture of isotopes

Front 56

How many isotopes does carbon have?

Back 56

carbon 12, carbon 13 and carbon 14

Front 57

What is the most common isotope of the element carbon?

Back 57

carbon 12, which makes up 99% of the natural carbon found in nature

Front 58

Isotopes of a element may have greater mass, but they behave the same in a?

Back 58

Chemical reaction

Front 59

Both carbon 12 and 13 are?

Back 59

stable isotopes meaning that they dont give off random particles from their nuclei, this is called decay

Front 60

What is a radioactive isotope?

Back 60

A nucleus decays randomly, giving off particles and energy, if a proton is given off then the element converts to the new number of protons element

Front 61

Radioactive isotopes have many useful applicatios in?

Back 61

Biology

Front 62

Cells can use radioactive atoms just the way they would use?

Back 62

a non radioactive variant of the element

Front 63

How are the radioactive isotopes used as diagnostic tools in medicine?

Back 63

Incorperated into biologically active molecules, which are then used as tracers to track atoms during metabolism

Front 64

For example how are kidney disorders diagnosed?

Back 64

By injecting small doses of radioactive labeled sunstances into the blood and then analysing the tracers moleucles excreted in the urine

Front 65

Radioactive tracers are also used in?

Back 65

sophisticated imaging devices, like PET scanners that can moniter growth and metabolsim of cancers in the body

Front 66

What from the isotopes can be damging to cells?

Back 66

The cells in the body are effected by the radiation, the damage depends on the amount and type of the radiation

Front 67

What is an example of one of the most serious enviromental threats from radioactive fallout?

Back 67

A nuclear meltdown like three mile island

Front 68

The doses of most isotopes used in medical diagnosis are?

Back 68

relatively safe

Front 69

What does PET stand for?

Back 69

Positron-emission tomography

Front 70

Researchers measure radioactive decay in fossils to?

Back 70

Date them to see how old they are

Front 71

Fossils provide a large body of evidence for?

Back 71

evolution, documenting differences between organisms from the past and those living at present and gives us insight into the species that have dissapered over time

Front 72

While the layering of fossil beds establishes that?

Back 72

the deeper the fossil, the older the fossil, we cannot determine the actually age based on position alone

Front 73

A parent isotope decays into a?

Back 73

Daughter isotope at a fixed rate, known as r=the half life of an atom

Front 74

What is halflife?

Back 74

The amount of time it takes for 50 % of the parent isotope to decay

Front 75

Each radioactive isotope has a half life that is not effected by?

Back 75

temperature, pressure or any other enviromental vairable

Front 76

Using radiometric dating scientists measure the ratio of?

Back 76

different isotopes and calculate how many half lives have passed since an organisms was fossilized or a rock was formed

Front 77

Half life ranges vary from?

Back 77

a few seconds to a billion years

Front 78

Uranium 238 has a half life of?

Back 78

4.5 billion years

Front 79

Uranium 238 was used to determine?

Back 79

that rocks on the moon are roughly 4.5 billions years old

Front 80

When two atoms approach each other during a chemical reaction, their nuclei?

Back 80

don't react as they can't reach each other

Front 81

Of the three subatomic particles we have looked at, what one is directly involved in chemical reactions?

Back 81

electrons

Front 82

An atoms electrons vary in the amount of?

Back 82

energy the possess

Front 83

Energy is defined as?

Back 83

the capacity to cause change

Front 84

Potential energy is the energy that?

Back 84

matter possess because of its location or structure

Front 85

For example, water in a resivour on a hill has?

Back 85

potential energy because of its altitude

Front 86

When the gates are opened?

Back 86

the water runs downhill, the energy can be used to do work such as moving a turbine to generate electricty, becuase the energy has expended, the water has less energy it had when it was at the top of the hill

Front 87

Matter has a natural tendency to move towards?

Back 87

the lowest possible state of potential energy, in our example the water flowing down the hill

Front 88

To restore pe of a reservoir?

Back 88

work must be done to elevate the water against gravity

Front 89

The electrons of an atom have potential energy due to?

Back 89

their distance from the nucleus

Front 90

The negatively charged electrons are attracted to?

Back 90

the positively charged nucleus, so it takes more work to move a given eletron farther away from the nucleus

Front 91

So the more distant an electron from the nucleus?

Back 91

the higher its potenital energy

Front 92

Changes in potential energy in electrons can only occur in?

Back 92

steps of fixed amounts

Front 93

Similarly, an electrons pe is also determined by?

Back 93

its energy level

Front 94

An electron can only exist?

Back 94

at certain energy levels, not between them

Front 95

An electrons energy level is correlated with its?

Back 95

average distance from the nucleus

Front 96

Electrons can be found?

Back 96

in different elctron shells, each with its own average distance and energy level

Front 97

An electron can move from one shell to another, but only by?

Back 97

absorbing or losing an amount of energy equal to the difference in pe bewteen ist position in the old shell and that in the new shell

Front 98

When an electron absorbs energy it?

Back 98

moves to a shell further from the nucleus

Front 99

When an electron loses energy it?

Back 99

falls back to a shell closer to the nucleus, and the lost energy is usally given back to the enviroment as heat

Front 100

For example, sunlight excites electrons in?

Back 100

the surface of a car to a higher energy levels, when the electrons move back to their original levels, the cars surface heats up, this thermal energy can be transferred to the air or to your hand if you touch the car

Front 101

The chemical behavior of an atom is determined by?

Back 101

the distribution of electrons in the atoms shells

Front 102

The first shell can obly hold

Back 102

2 electrons

Front 103

In an atom with more than 2 electrons then extra electrons must?

Back 103

occupy the next shell as the first one is full

Front 104

The second shell holds a max of?

Back 104

8 electrons

Front 105

The chemical behaviour of an atom depends mostly on the?

Back 105

outermost shell of the atom

Front 106

We call the outer shell and outer elctrons?

Back 106

Valence shell and Valence electrons

Front 107

Atoms with similiar number of valence elctrons exhibit?

Back 107

similiar behaviour duirng chemical reactions

Front 108

Sodium fluriode is used?

Back 108

commonly in tooth paste to prevent tooth decay

Front 109

An atom with a full outer shell is?

Back 109

unable to react with atoms

Front 110

These elements with full outer shells are said to?

Back 110

be inert, chemically unreactive

Front 111

In the early 1900s, the electron shells of an atom were visualized as?

Back 111

concentric paths of electrons orbiting the nucleus, somewhat like planets orbiting the sun

Front 112

In reality we can never know?

Back 112

the true location on an electron

Front 113

What can we do instead?

Back 113

describe the space in which an electron spends most of its time

Front 114

What is an orbital?

Back 114

The three dimensional space where an electron is found 90%

Front 115

Each electron shell contains?

Back 115

electrons at a particular energy level, distributed among a specific number of orbitals of distinctive shapes and orientations

Front 116

The first shell has only?

Back 116

one spherical s orbital called 1s

Front 117

The second shell has?

Back 117

4 orbitals, one large spherical orbital called 2s and 3 dumbbell-shaped p orbitals called 2p orbitals

Front 118

No more than?

Back 118

2 electron can occupy 1 orbital

Front 119

Electrons in each of the four orbitals have?

Back 119

nearly the same energy, but they move in different volumes of space

Front 120

The reactivity of an atom arises?

Back 120

from the presence of unpaired electrons in one or more orbitals in the valence shells

Front 121

What is a covalent bond?

Back 121

Is the sharing of valence electrons by two atoms

Front 122

Two or more atoms held together by covalent bonds consititue a?

Back 122

molecule, in this case hydrogen, when 2 hydrogen molecules overlap

Front 123

Hydrogens molecular formula indicates?

Back 123

that the molecule consists of two atoms of hydrogen

Front 124

Electron sharing can be depicted by?

Back 124

an electron distribution diagram or by a lewis dot structure

Front 125

What is a lewis dot structure?

Back 125

in which element symbols are surrounded by dots that represent the valence electrons

Front 126

We can also use a?

Back 126

structural formula h-h, where the line represents a single bond, a pair of shared electrons

Front 127

A space filling model comes closest to?

Back 127

representing the actual shape of molecules

Front 128

Oxygen has 6 electrons in the valence shell and therefore?

Back 128

needs 2 more electrons to complete the valence shell

Front 129

Two oxygen atoms form a molecule by?

Back 129

sharing 2 pairs of valence electrons

Front 130

What are the electrons in the oxygen held together by?

Back 130

A double bond

Front 131

Each atom that can share valence electrons has a bonding?

Back 131

capacity corresponding to the number of covalent bonds that the atom can form

Front 132

When the atom forms, the bonds give the atom a?

Back 132

full shell of outer eletcrons

Front 133

This bonding capacity is called the atom's?

Back 133

valence and usally equals the number of unpaired electrons required to complete the valence shell of the atom

Front 134

The molecules h2 and o2 are pure elemenets because?

Back 134

they are made up of one element, where as a compound is composed of 2 or more different elements

Front 135

Methane is the main component of?

Back 135

natural gas with the formula CH4

Front 136

How is a covalent bond formed?

Back 136

when some atoms approach each other and need to fill a shell, the nucleus from each atoms attracts the electrons from the other atom and share electrons with each other to statify the atoms valence

Front 137

Atoms in a molecule attract shared electrons to?

Back 137

different degrees, depending on the element

Front 138

What is the electronegativity of a covalent bond?

Back 138

The attraction of a particular atom for the electrons in a covalent bond

Front 139

The more electronegative an atom is?

Back 139

the more strongly it pulls shared electrons towards its self

Front 140

In a covalent bond between 2 atoms of the same element, the electrons are?

Back 140

shared equally because the 2 atoms have the same electronegativity, the tug of war is at a stand off, this is known as a nonpolar covalent bond

Front 141

However, when an atom is bonded to a more electronegative atom, the electrons are?

Back 141

not shared evenly between the bond, this type of bond is called a polar covalent bond

Front 142

Polar covalent bonds vary in their?

Back 142

polarity, depending on the relative eletronegavitivty of the 2 atoms

Front 143

Oxygen is one of the most?

Back 143

electronegative elements, attracting shared electrons much strongly than hydrogen

Front 144

In a covalent bond between oxygen and hydrogen, the electrons spend more time at?

Back 144

the oxygen nucleus then at the hydrogen nucleus

Front 145

Because electrons are negativily charged then?

Back 145

they are pulled towards the oxugen nucleus giving the nucleus a parital negative charge and each hydrogen has a parital positive charge

Front 146

In some cases, 2 atoms are so unequal in their attraction for valence electrons that the more?

Back 146

electronegative atom strips an electron completely away from its partner

Front 147

The 2 resultinf oppositely charged atoms or moleucles are known as?

Back 147

ions

Front 148

A positive ion is called a?

Back 148

cation

Front 149

A negative ion is called ?

Back 149

anion

Front 150

Because of their opposite charges, cations and anions attract?

Back 150

each other, this attraction is called a ionic bond

Front 151

Note that the transfer of an electron is not?

Back 151

by itself, the formation of a bond: rather, it allows a bond to form because it results in 2 ions of opposite charge.

Front 152

Any two ions of opposite charge can form?

Back 152

an ionic bond, the ions do not need to have acquired their charge by an electron transfer with each other

Front 153

Compounds formed by ionic bonds are known as?

Back 153

ionic compounds or salts

Front 154

Salts are often found in nature as?

Back 154

crystals of various shapes and sizes.

Front 155

Each salt crystal is an aggregate of?

Back 155

vast number of cations and anions bonded by their electrical attraction and arranged in a 3 dimensional lattice

Front 156

Unlike a covalent compound, which consits of?

Back 156

molecules having a definite size and number of atoms , an ionic compound does not consist of molecules

Front 157

The formula for an ionic compound, such as NaCI indicates only?

Back 157

the ratio of elements in a crystal of a salt, Sodium chloride in not a molecule

Front 158

Not all salts have a equal number of?

Back 158

cations and anions

Front 159

For example, the ionic compound magnesium chloride has 2?

Back 159

chloride ions for each magnesium ion

Front 160

The term ion also applies to?

Back 160

entire molcules that are electrically charged

Front 161

In the salt ammonium chloride, the anion is a?

Back 161

single chloride ion but the cation is ammonium, a nitrogen atom covalently bonded to four hydrogen atoms. The whole ammonium ion has an electrical charge of 1+ becuase it has given up 1 electron and thus is 1 short

Front 162

Enviroment affects?

Back 162

the strength of ionic bonds

Front 163

In a dry salt crystal, the bonds are so strong that?

Back 163

it takes a hammer and chisel to break enough of them to crack they crystal in two

Front 164

Most drugs are manufactured as salts because?

Back 164

they are quite stable when dry but can come apart easily in water

Front 165

When a hydrogen atom is covalently bonded to an?

Back 165

electronegative atom, the hydrogen atom has a partial positive charge that allows it to be attracted to a different electronegative atom nearby

Front 166

This attraction between a hyrdogen and an electronegative atom is called a?

Back 166

hydrogen bond

Front 167

In living cells, the electronegative partners are usally?

Back 167

oxygen or nitrogen atoms

Front 168

Even a molecule with nonpolar covalent bonds may?

Back 168

have positively and negatively charged regions.

Front 169

Electrons are not always evenly,

Back 169

distributed at any instant, they may accumulate by chance in one part of a molecule or another

Front 170

The results are ever changing?

Back 170

regions of positive and negative charge that enables all atoms and molecules to stick to one another

Front 171

These van der waals interactions are?

Back 171

individually weak and occur only when atoms and molecules are very close together

Front 172

When many such interactions occur simultaneously, however?

Back 172

they can be very powerful: Van der waals interactions allow a gecko lizard to walk straight up a wall

Front 173

The anatomy of a gecko's foot including?

Back 173

many minuscule hair like projections from the toes and strong tendons underlying the skin - strikes a balance between maximum surface contact with the wall and necessary stiffness of the foot.

Front 174

The vans der waals interactions between the foot molecules and the?

Back 174

molecules of the wall's surface are so numerous that despite their individual weakness, together they can support the gecko's body weight

Front 175

This discovery has inspired development of an?

Back 175

artificial adhesive called Geckskin

Front 176

A patch of geckskin the size of an index card can?

Back 176

hold a 700 pound weight to a wall

Front 177

Van der waals interactions, hydrogen bonds, ionic bonds in water and other weak bonds may form not only between molecules but also?

Back 177

between parts of a large molecule, such as a protein

Front 178

The cumulative effect of weak bonds is to?

Back 178

reinforce the three dimensional shape of the molecule

Front 179

A molecule has a characteristic size and?

Back 179

precise shape, which are crucial to its function in the living cell

Front 180

A molecule consisting of 2 atoms such O2, is always?

Back 180

linear, but most molecules with more than 2 atoms have more complicated shapes

Front 181

These shapes are determined by?

Back 181

the position of the atoms orbitals

Front 182

When an atom forms covalent bonds, the orbitals in its valence shell undergo?

Back 182

rearrangement

Front 183

For atoms with valence electrons in both s and p orbitals form?

Back 183

the single s and three p orbitals form 4 new hybrid orbitals shaped like identical teardrops extending from the region of the atomic nucleus

Front 184

If we connect the larger ends of teardrops with lines we form a?

Back 184

tetrahedron, a pyramid with a triangle base

Front 185

For water molecules, two of the hybrid orbitals in?

Back 185

the oxygen's valence shell are shared with hydrogens

Front 186

The result is a?

Back 186

molecule shaped roughly like a V, with its two covalent bonds at an angle of 104.5 degrees

Front 187

The methane molecule has the shape of?

Back 187

a completed tetrahedron because all 4 hybrid orbitals of the carbon atoms are shared with the hydrogen atoms

Front 188

The carbon nucleus is at the?

Back 188

centre, with its four covalent bonds radiating to the hydrogen nuclei at the corner of the tetrahedron

Front 189

Larger molecules containing multipule carbon atoms, including many of the molecules that make up living matter have?

Back 189

more complex overall shapes

Front 190

However, the tetrahedral shape of a carbon atom bonded to four other atoms is often a?

Back 190

motif within such molecules

Front 191

Molecular shape is crucial, it determines?

Back 191

how biological molecules recognize and respond to one another with specificity

Front 192

Biological molecules often bind?

Back 192

temporarily to each other by forming weak chemical bonds

Front 193

But only if?

Back 193

their shapes are complementary

Front 194

Consider the effects of opaites, such as morphine and heroin dervied from?

Back 194

opium

Front 195

Opiates releive?

Back 195

pain and alter mood by weakly binding to specific receptor molecules on the surface of the brain

Front 196

In 1975 what was discovered?

Back 196

endorphines

Front 197

What are endophins?

Back 197

are signaling molecules made by the puiltary gland that bind to the receptors, reliving pain and producing euphoria during times of stress, such as intense excercise

Front 198

Opiates have shapes similar to?

Back 198

endorphines and mimic them by binding to endorphin receptors in the brain

Front 199

This is why opiates and endorphins have?

Back 199

similiar effects

Front 200

The making of chemical bonds, leading to?

Back 200

changes in composition of matter, called chemical bonds

Front 201

The starting materails are called?

Back 201

reactants to the products

Front 202

Matter is?

Back 202

consevered in a chemical reaction

Front 203

Reactions can't

Back 203

create or destroy electrons, but can rearange the electrons among the atoms

Front 204

What is the chemical reaction for photosynthesis?

Back 204

6C02 + 6H20 - C6H12O6 + 6O2

Front 205

The raw materials of photosynthesis are?

Back 205

carbon dioxide, which is taken from the air and water which is abosbed from the soil

Front 206

within plant cells sunlight powers the?

Back 206

conversion of these ingredients into glucose and oxygen molecules

Front 207

All chemical reactions are?

Back 207

reversible, with the prodcuts of the forward reaction becoming the reactants for the reverse reaction

Front 208

One of the factors effecting rate of reaction is?

Back 208

the concentration of reactants. The greater the concentration of reactant molecules, the more collisions with one another, so a higher chance of reacting, so more product is formed. Can be reversed for products

Front 209

Eventually, the forward and revrse reactions occur at?

Back 209

the same rate, and the relative concentrations of products and reactants stop changing

Front 210

The point at which the reactions offset one another excatly is called the?

Back 210

chemical equilibrium

Front 211

This is a dynamic equilibrium, reactions are?

Back 211

still going on, but with no net effect on the concentrations of reactants and products

Front 212

Equilibrium does not mean?

Back 212

that the reactants and products are equal in concentration, but only that their concentrations have stabilized at a particular ratio

Front 213

In some chemical reactions, the equilibrium point may lie so?

Back 213

far to the right that these reactions go essentially to completion, where almost of all the reactants have been converted to products

Front 214

What plant is this?

Back 214

Elodea, a fresh water plant.