Biology Chapter 2: Chemical Bonds and Reactions

Chapter 2: The Chemical Context of Life

• This chapter explores the chemical aspects of life, focusing on the role of atoms, elements, compounds, and chemical bonding in living organisms.

Matter and Elements

• Matter is anything that occupies space and has mass.
• Organisms are composed of matter.
• Matter is made up of elements.
• An element is a substance that cannot be broken down into other substances by chemical reactions.
• About 20-25% of the 92 natural elements are required for life, called essential elements.
• Carbon, hydrogen, oxygen, and nitrogen make up 96% of living matter.
• Most of the remaining 4% consists of calcium, phosphorus, potassium, and sulfur.
• Trace elements are required by organisms in only minute quantities.

Atoms

• Atoms of different elements differ in their number of subatomic particles.
• Each element consists of unique atoms.
• An atom is the smallest unit of matter that retains the properties of an element.
• Atoms are composed of subatomic particles including protons, neutrons, and electrons.
• Protons have a positive charge.
• Neutrons have no charge.
• Electrons have a negative charge.
• Protons and neutrons are located in the atomic nucleus; electrons surround the nucleus.
• Neutron and proton masses are almost identical, measured in daltons.
• Electrons are much smaller and are often ignored when calculating the total mass of an atom.

Atomic Number and Mass Number

• An element's atomic number is the number of protons in its nucleus.
• An element's mass number is the sum of protons and neutrons in the nucleus.
• Atomic mass is the atom's total mass and can be approximated by the mass number.

Isotopes

• All atoms of an element have the same number of protons but may differ in the number of neutrons.
• Isotopes are two atoms of an element that differ in the number of neutrons.
• Radioactive isotopes decay spontaneously, giving off particles and energy.

Radioactive Tracers

• Radioactive isotopes are often used as diagnostic tools in medicine.
• Radioactive tracers can be used to track atoms through metabolism.
• They can be combined with sophisticated imaging instruments.
• PET scanners can monitor the growth and metabolism of cancers in the body.

Radiometric Dating

• A "parent" isotope decays into its "daughter" isotope at a fixed rate, expressed as the half-life of the isotope.
• In radiometric dating, scientists measure the ratio of different isotopes to calculate how many half-lives have passed since the fossil or rock was formed.
• Half-life values vary from seconds or days to billions of years for different isotopes.

The Energy Levels of Electrons

• Energy is the capacity to cause change.
• Potential energy is the energy matter possesses because of its location or structure.
• Matter has a natural tendency to move toward the lowest possible state of potential energy.
• Electrons differ in the amount of potential energy based on their distance from the nucleus.
• Changes in electron energy can only occur in steps of fixed amounts.
• Electrons are found in different electron shells, each with a characteristic average distance and energy level.

Electron Distribution and Chemical Properties

• The chemical behavior of an atom is determined by the distribution of electrons in the electron shells.
• The periodic table displays electron distribution for each element.
• The left-to-right sequence in rows corresponds to adding electrons and protons sequentially.

Valence Electrons

• Valence electrons are those in the outermost shell, or valence shell.
• The chemical behavior of an atom is mostly determined by the number of valence electrons.
• Elements with a full valence shell are chemically inert.

Electron Orbitals

• An orbital is a three-dimensional space where an electron is found 90% of the time.
• Each electron shell consists of a specific number of orbitals.
• No more than 2 electrons can occupy a single orbital.
• Atoms interact to complete their valence shells.

Chemical Bonding

• Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms.
• These interactions often result 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 are considered part of each atom's valence shell.
• Molecules consist of two or more atoms held together by covalent bonds.
• Single, double, and triple covalent bonds exist based on the number of electron pairs shared.
• Structural and molecular formulas are used to represent atoms and bonding in molecules.
• Covalent bonds are common between atoms of the same element or different elements.

Electronegativity

• Atoms in molecules attract electrons to varying degrees; this attraction is called electronegativity.
• The more electronegative an atom, the more strongly it pulls shared electrons toward itself.
• Nonpolar covalent bonds have equal sharing of electrons.
• Polar covalent bonds have unequal sharing of electrons; this can result in partial charges (δ+ or δ−) for the atoms.

Ionic Bonds

• Atoms sometimes strip electrons from their bonding partners, forming ions.
• Positive ions (cations) and negative ions (anions) attract each other, forming ionic bonds.
• Compounds formed by ionic bonds are called ionic compounds or salts.
• Salts are often found as crystals and the formula for an ionic compound indicates the ratio of elements in the crystal.

Weak Chemical Interactions

• Most strong bonds in organisms are covalent bonds forming a cell's molecules.
• Many large biological molecules are held in their functional form by weak bonds.
• The reversibility of weak bonds is an advantage.
• Several types of weak chemical interactions are important in organisms such as hydrogen bonds and Van der Waals interactions.

Molecular Shape and Function

• A molecule's size and shape are key to its function.
• Hybridization of orbitals during covalent bonds can create predictable molecular shapes that influences molecular function and molecular recognition.
• Molecular shape determines how biological molecules recognize and respond to each other.

Chemical Reactions

• Chemical reactions are the making and breaking of chemical bonds.
• The starting molecules of a reaction are called reactants, and the resulting molecules are called products.
• Chemical reactions are reversible.
• Chemical equilibrium is reached when the rates of forward and reverse reactions are equal, at which point the relative concentrations of reactants and products do not change.

Photosynthesis

• Photosynthesis is a crucial chemical reaction using sunlight to convert carbon dioxide and water into glucose and oxygen, represented by the equation 6CO2 + 6H2O → C6H12O6 + 6O2.

Test your understanding of chemical bonds and reactions in Biology Chapter 2. This quiz covers topics such as hydrogen bonding, electronegativity, and the properties that influence molecular shapes and functions. Get ready to explore the intricate connections between atoms and molecules in living cells.