Biology Chapter 2

Lecture 2 overview

Matter

Any physical substance that has mass and occupies space (volume)

Elements

A pure substance made of only one kind of atom that cannot be broken down into simpler substances by normal chemical reactions

• Have specific chemical properties

• Have specific physical properties

Molecules

A group of two or more atoms held together by chemical bonds

CHON

The four most common elements in living organisms

Atoms

The smallest basic unit of matter that keeps the physical and chemical properties of an element

• Composed of protons, neutrons, and electrons

Nucleus

The dense, central core of an atom. It contains positively charged protons and neutral neutrons, holding nearly all of the atom’s mass

Electrons

A negatively charged subatomic particle that orbits the atomic nucleus in an electron cloud

• Located in orbitals surrounding the nucleus

• Mass = 0 amu

Protons

A stable, positively charged subatomic particle located in the dense core (nucleus) of an atom

• Mass = approximately 1 amu

• Charge = +1

Neutrons

An uncharged subatomic particle located in the dense core, or nucleus, of an atom

• Mass = 1

• Charge = 0

• atomic mass - atomic number = # of neutrons

Bohr model

An early simplified depiction of how sub-atomic particles are arranged within an atom

• Protons are located in nucleus and electrons in circular orbits around the nucleus

Atomic number

Is the number of protons

Atomic mass

The mass of the atom, equal to number of protons and neutrons

• The number of neutrons can vary in an element

• Expressed in amu

Isotopes

Alternate forms of a chemical element that have the same number of protons (and electrons) but a different number of neutrons, which changes the atomic mass

Radioisotopes

Isotopes that emit radiation and decay to more stable form

• Unstable and emit radiation as the nucleus breaks up

• Radiometric dating takes advantage of this phenomenon to determine fossil age

Half-life

The time needed for a radioactive isotope or a specific substance to drop to half of its starting amount through decay or breakdown

• Time it takes for one-half of the atoms in a sample to decay

Orbits (Bohr Model)

Electron shells or energy levels

• An electron found in the lowest available energy level (closest to the nucleus)

• Electrons fill orbitals closest to nucleus first, then sequentially further away in order

• Fixed, circular paths where electrons travel around the atom's nucleus

• However, electrons are not in planet-like orbits (Bohr models are early/incomplete)

Electron orbitals

Complex shapes that describe how electrons are spatially distributed around the nucleus

• Quantum mechanics predicts where a electron might be at any given time

• The predicted area is known as the orbital

Electron orbitals vs. orbits

• An electron orbital is a 3D space around an atom's nucleus where an electron has a high probability (about 90%) of being found

• An orbit is an outdated, fixed circular path (like a planet around the sun) from the old Bohr model, which modern quantum mechanics proves is inaccurate

Orbital

A three-dimensional region around an atomic nucleus where there is a high probability (about 90%) of finding an electron

Valence shell

An atom's outermost electron energy level

• The electrons residing in this shell are called valence electrons

• They determine how an atom reacts and forms chemical bonds (covalent or ionic) with other atoms

• The most stable configuration occurs when the valence shell is filled

Octet rule

A chemical rule stating that atoms of main-group elements tend to gain, lose, or share electrons so that they have eight valence electrons in their outer shell

• This arrangement gives them a stable, low-energy state like a noble gas

• Only the outermost shell needs to have 8 valence electrons to achieve stability

Chemical reactions

Changes in the distribution of electrons between atoms

• Electrons determine how atoms interact

• A process that breaks and forms chemical bonds. It changes reactants into products without creating or destroying atoms

Reactants

Substances at the beginning of a chemical reaction

• During the reaction, their chemical bonds break, and the atoms rearrange to form new substances (products)

Products

The end-substances or molecules left after a chemical or metabolic reaction takes place

Irreversible reaction

Moves in one direction until all the reactants are used

• Chemical process that proceeds forward to completion.

• The reactants turn into products, but the products cannot change back into reactants under normal cellular conditions

Reversible reaction

Reactants are converted to products but some product can be converted back to reactant

• A chemical reaction that can go in both directions. Reactants turn into products, and the products can also turn back into reactants

Chemical bond

An attractive force that links atoms or ions together to form molecules or compounds

Covalent bonds

A strong chemical link formed when two atoms share one or more pairs of valence electrons

• Ex; when two Hydrogens and an Oxygen share electrons via covalent bonds, a water molecule is formed

• More than one set of electrons can be shared (double/triple covalent bonds)

Ionic Bonds

Atoms lose or gain electrons

• A chemical bond formed when one atom completely transfers one or more valence electrons to another atom.

• This electron transfer creates oppositely charged ions—a positive cation and a negative anion—which stick together through a strong electrostatic attraction

Ionic compounds

Metals lose electrons and nonmetals gain electrons to achieve an octet (full shell)

• A neutral substance made of positive and negative ions held together by electrostatic forces called ionic bonds

Polar Bonds

Covalent bonds with an unequal sharing of electrons

Polar Covalent Bonds

A type of chemical bond where two atoms unequally share valence electrons because they have different electronegativities.

• The atom with the higher electronegativity pulls the shared electrons closer to itself. This creates partial positive and partial negative charges within the molecule

Polar Covalent Bonds - Water

• Oxygen has higher electronegativity than hydrogen

• This means that it wants the electrons more than hydrogen does

Non-polar covalent bonds

Electrons are equally shared by the atoms

• A chemical link where two atoms share their outer electrons equally. This equal sharing happens because the bonded atoms have the same electronegativity

Non-polar covalent bonds: Methane

Carbon shares electrons equally with four hydrogens

• Both bond type and molecular shape determine whether a molecule is polar or nonpolar