Atoms Element and Compounds

A knowledge of chemistry is essential for understanding organisms

Important to biology are inorganic compounds, including water, simple acids and bases, and simple salts

Elements Substances that cannot be

broken down into simpler substances by ordinary chemical reactions

Each has a chemical symbol

Four elements comprise the mass of most organisms Oxygen, carbon, hydrogen, and

nitrogen In addition, other elements,

such as calcium, and trace elements are present

Functions of Elements

Atom The smallest portion of an

element that retains its chemical properties

Subatomic particles include Electron—carries a negative

charge Proton—carries a positive

charge Neutron—uncharged particle

Every element has A fixed number of protons in

the atomic nucleus, known as the atomic number

The periodic table is a chart of the elements arranged by atomic number

Periodic chart (including Bohr models)

The atomic mass of an atom Is a number that indicates how

much matter it contains Is expressed by the atomic mass

unit (amu), also known as the dalton

Is indicated by a superscript to the left of the chemical symbol

Characteristics of protons, neutrons, and electrons

Isotopes Are two or more forms of atoms

of the same element Contain the same number of

protons and electrons, but the number of neutrons varies

Radioisotopes break down and emit radiation

Carbon Isotopes

Electrons move through orbitals

Electrons at the same principal energy level make up an electron shell

Electrons in a shell distant from the nucleus have greater energy

Valence electrons occupy the valence shell

Changes in electron energy levels are important in energy conversions in organisms

Atomic orbitals

The chemical behavior of an atom is determined by the number and arrangement of its valence electrons

When the valence shell is not full, the atom tends to lose, gain, or share electrons

A chemical compound consists of atoms of two or more elements

Atoms combine in a fixed ratio

Atoms may join to form a molecule

A chemical formula describes the chemical composition of a substance Simplest formula Molecular formula Structural formula

Molecular mass Sum of the atomic masses of

the component atoms of a single molecule

One mole is the amount of an element or compound whose mass in grams is equivalent to its atomic or molecular mass

The mole allows for comparison of atoms and molecules of very different mass

Chemical reactions in an organism Described by chemical equations Reactants are written on the left Products are written on the right Reactions can proceed

simultaneously in both directions At dynamic equilibrium, forward

and reverse rates of reaction are equal

Chemical bonds Forces of attraction that hold

atoms of a compound together The two principal types are

Covalent bonds Ionic bonds

Bond energy Energy necessary to break a

chemical bond

Covalent bonds Share electrons between atoms Each atom has a filled valence shell

Covalent compound Compound consisting mainly of

covalent bonds Example is hydrogen gas molecule Bond can be single, double, or triple

Covalent bonds

Number of Covalent bonds

Covalent bonds can be nonpolar or polar

Ion Particle with one or more units

of electrical charge Results when an atom gains or

loses electrons Cations—positively charged ions Anions—negatively charged ions

Cations and anions are involved in biological processes, such as muscle contraction

Sodium, potassium, and chloride ions are essential for this nerve cell to stimulate these muscle fibers

Ion bonds Formed due to attraction

between a cation and an anion An ionic compound is a

substance consisting of cations and anions bonded together

An example of ionic bond is the attraction between sodium ions and chloride ions

Ionic bonding

Hydrogen bonds Tend to form between an atom

with partial negative charge and a hydrogen atom covalently bonded to oxygen or nitrogen

Readily formed and broken While individually weak,

hydrogen bonds are strong when present in large numbers

Hydrogen bonding

Many energy conversions in a cell involve an electron transfer from one substance to another

Known as oxidation-reduction, or redox reaction

Large part of the mass of most organisms is water

Water is important as internal constituent and environmental factor

Water facilitates chemical reactions Hydrophilic substances—

interact readily with water, such as table salt

Hydrophobic substances—not disrupted or dissolved by water, such as fats

Water exists as gas, liquid, or solid

Hydrogen bonds are formed or broken as water changes state

Acid Substance that dissociates in

solution to yield hydrogen ions and an anion

Base Substance that dissociates to

yield a hydroxide ion and a cation when dissolved in water

The degree of a solution’s acidity is expressed in pH

Definition of pH The negative logarithm of the

hydrogen ion concentration Expressed in moles per liter

Neutral solution pH of 7

Acidic solution pH value of less than 7

Base solution pH greater than 7

An acid and a base react to form a salt plus water

pH valuesof commonsolutions