AP Biology 1.1-1.2: Water and Elements of Life

Element

An element is a form of matter with a unique set of chemical and physical properties that cannot be decomposed into simpler substances by ordinary chemical reactions.

Matter

Matter is anything that occupies space and has mass.

Which four elements are especially abundant in living organisms?

Oxygen, carbon, hydrogen, and nitrogen—represented by O, C, H, and N—are the four elements common to all living organisms.

Atom

An atom is the smallest unit of an element that retains that element’s chemical properties.

What are the two major regions of an atom?

The nucleus contains protons and neutrons, while the surrounding electron cloud contains electrons in orbitals. Most of an atom’s volume is empty space.

What are the charges, approximate masses, and locations of protons, neutrons, and electrons?

A proton has charge $+1$, mass about $1$ amu, and is located in the nucleus. A neutron has charge $0$, mass about $1$ amu, and is in the nucleus; an electron has charge $-1$, negligible mass on the atomic-mass scale, and occupies orbitals around the nucleus.

Why do neutral atoms have no net charge?

A neutral atom contains equal numbers of positively charged protons and negatively charged electrons, so their charges cancel.

Atomic number

An element’s atomic number, $Z$, is the number of protons in each atom of that element. It uniquely identifies the element.

Mass number

The mass number, $A$, is the total number of protons and neutrons in an atom’s nucleus: $A=Z+N$, where $N$ is the number of neutrons.

How can the number of neutrons in an isotope be calculated?

The number of neutrons is $N=A-Z$, or mass number minus atomic number. Electron mass is ignored in this approximation.

Isotope

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.

How do carbon-12 and carbon-14 differ?

Both contain six protons, but carbon-12 has six neutrons whereas carbon-14 has eight. If neutral, each isotope also has six electrons.

Why is an element’s atomic mass on the periodic table often not a whole number?

The listed atomic mass is a weighted average of the masses of the element’s naturally occurring isotopes, so it may be fractional.

How is an element’s weighted average atomic mass calculated from its isotopes?

Multiply each isotope’s mass by its fractional abundance and add the products: $\text{average atomic mass}=\sum(\text{isotope mass})(\text{fractional abundance})$.

Radioisotope

A radioisotope is an isotope with an unstable nucleus that undergoes radioactive decay, emitting radiation and forming a more stable, lower-energy configuration.

Half-life

Half-life is the time required for half of the nuclei in a radioactive sample, or half of the original amount of an isotope, to decay.

What fraction of a radioactive isotope remains after $n$ half-lives?

The remaining fraction is $\left(\frac{1}{2}\right)^n$. Equivalently, $N=N_0\left(\frac{1}{2}\right)^{t/t_{1/2}}$.

How does carbon-14 dating estimate the age of formerly living material?

Living organisms continually exchange carbon with the atmosphere, but exchange stops at death. The declining carbon-14-to-carbon-12 ratio is compared with the initial ratio and interpreted using carbon-14’s approximately $5{,}730$-year half-life.

Why is carbon-14 dating most useful for relatively recent biological remains?

Carbon-14 has a half-life of about $5{,}730$ years, so after roughly $50{,}000$ years too little remains for this method to be reliably useful.

How is the periodic table organized?

Elements are arranged by increasing atomic number. Groups contain elements with similar chemical properties, largely because their valence electrons have similar arrangements.

What determines an element’s chemical reactivity?

Reactivity depends primarily on the number and arrangement of an atom’s electrons, especially its valence electrons.

Valence electrons

Valence electrons are the electrons in an atom’s outermost occupied energy level. They strongly influence bonding, stability, and chemical reactivity.

Octet rule

The octet rule is the tendency of many main-group atoms to achieve eight valence electrons by gaining, losing, or sharing electrons. Hydrogen and helium are stable with two electrons in their first shell.

Why are noble gases generally unreactive?

Group 18 elements have filled valence shells—two electrons for helium and generally eight for the others—so they have little tendency to gain, lose, or share electrons.

What ions are commonly formed by elements in groups 1 and 17?

Group 1 atoms commonly lose one electron to form $+1$ cations. Group 17 atoms commonly gain one electron to form $-1$ anions.

What is the usual ion-forming tendency of a group 14 element such as carbon?

Because it has four valence electrons, carbon commonly achieves stability by sharing electrons and forming several covalent bonds rather than simply gaining or losing four electrons.

Bohr model

The Bohr model represents electrons as occupying quantized principal energy levels or shells around the nucleus. It is useful for basic energy-level ideas but is less accurate than the orbital model.

What happens when an electron absorbs or releases energy?

An electron can absorb a photon and move to a higher energy level, producing an excited state. When it returns to a lower level, it releases a photon whose energy equals the energy difference between the levels.

Orbital

An orbital is a region of space where an electron has a high probability of being found; it is not a fixed circular path around the nucleus.

How many orbitals and maximum electrons are in the $s$, $p$, $d$, and $f$ subshells?

An $s$ subshell has one orbital and holds up to two electrons; $p$ has three orbitals and holds six; $d$ has five orbitals and holds ten; and $f$ has seven orbitals and holds fourteen.

What is the maximum number of electrons in the first four principal energy levels?

The maximum capacities are $2$, $8$, $18$, and $32$ electrons for principal levels $n=1$, $n=2$, $n=3$, and $n=4$, respectively.

What is the correct general order for filling low-energy orbitals?

Electrons fill lower-energy orbitals before higher-energy orbitals. For the orbitals emphasized here, the sequence begins $1s$, $2s$, $2p$, $3s$, $3p$, and then higher-energy orbitals.

How are electrons distributed among orbitals of equal energy?

Electrons occupy degenerate orbitals singly before pairing, consistent with Hund’s rule. Each orbital can contain a maximum of two electrons.

What is the electron configuration of neutral lithium and neutral neon?

Lithium has three electrons and the configuration $1s^2 2s^1$. Neon has ten electrons and the configuration $1s^2 2s^2 2p^6$.

Molecule

A molecule is a discrete particle made of two or more atoms joined by chemical bonds.

Compound

A compound is a substance whose particles contain atoms of two or more different elements chemically bonded together. Water, $H_2O$, is a compound, whereas $O_2$ is not.

What is the difference between a molecule and a homonuclear molecule?

A molecule contains two or more chemically bonded atoms. A homonuclear molecule, such as $O_2$, contains atoms of only one element and therefore is not a compound.

Covalent bond

A covalent bond forms when atoms share one or more pairs of electrons. Covalent bonding is especially common in molecular substances and carbon-based compounds.

Ionic bond

An ionic bond is the electrostatic attraction between oppositely charged ions, typically formed after electron transfer from a metal to a nonmetal.

Cation and anion

A cation is a positively charged ion formed by losing electrons. An anion is a negatively charged ion formed by gaining electrons.

What happens when a neutral sodium atom becomes a sodium ion?

Sodium loses one electron: $Na\rightarrow Na^+ + e^-$. It still has 11 protons but now has 10 electrons, giving it a net charge of $+1$ and a filled outer shell.

What happens when a neutral chlorine atom becomes a chloride ion?

Chlorine gains one electron: $Cl+e^-\rightarrow Cl^-$. It has 17 protons and 18 electrons, producing a net charge of $-1$ and a filled valence shell.

Why does sodium chloride have an overall charge of zero?

The $+1$ charge of each $Na^+$ ion balances the $-1$ charge of each $Cl^-$ ion. The resulting ionic solid has the formula $NaCl$ and electrical neutrality overall.

Electrolyte

An electrolyte is an ion, or a substance that produces ions in solution, that supports electrical conduction. Sodium, potassium, and calcium ions are biologically important electrolytes involved in processes such as nerve signaling, muscle contraction, and water balance.

Chemical reaction

A chemical reaction rearranges atoms by breaking and forming chemical bonds. Reactants are the starting substances, products are the substances formed, and an arrow indicates the direction represented by the equation.

What does it mean for a chemical equation to be balanced?

A balanced equation has the same number of atoms of each element on both sides, in accordance with conservation of matter. Coefficients are changed to balance an equation, not subscripts within chemical formulas.

Balance the decomposition of hydrogen peroxide into water and oxygen.

The balanced equation is $2H_2O_2\rightarrow 2H_2O+O_2$. Two hydrogen peroxide molecules produce two water molecules and one oxygen molecule.

Reversible reaction and chemical equilibrium

A reversible reaction can proceed in both directions, represented by $\rightleftharpoons$. At equilibrium, the forward and reverse reactions continue but occur at equal rates, so the macroscopic concentrations remain constant.

How can removing a product drive a reversible reaction toward more product?

Removing a product lowers its concentration, causing the system to shift toward products to counter the disturbance, consistent with Le Châtelier’s principle and the law of mass action.

What equilibrium system helps regulate hydrogen-ion concentration in blood?

The bicarbonate buffer system can be represented as $HCO_3^-+H^+\rightleftharpoons H_2CO_3\rightleftharpoons CO_2+H_2O$. Exhalation removes $CO_2$, helping pull the overall process toward additional $CO_2$ formation and consume excess $H^+$.