Notes on Atoms, Bonds, and Hydrophilic/Hydrophobic Concepts

Atom Structure and Periodic Table

  • Atoms consist of a central nucleus containing positively charged protons and neutral neutrons, surrounded by a cloud of negatively charged electrons.

  • The number of protons in the nucleus (the atomic number, Z) defines the element.

  • The periodic table shows increasing proton number as you move across rows: smallest element is hydrogen (1 proton, 1 electron). Helium has 2 protons, Lithium 3, Beryllium 4, and so on. The transcript notes these in a simplified way, with a focus on valence electrons.

  • Valence electrons are the electrons in the outermost shell and are the ones involved in forming bonds between atoms.

  • As atoms gain electrons, their outer-shell configurations can become complex, but valence electrons remain the key players in bonding.

  • The energy in chemical bonds underlies metabolism: energy is stored in bonds and released when bonds are broken, enabling cellular processes.

  • Example energy reference: the bond between carbon and hydrogen stores or releases EC−H=413 kJ/molE_{C-H} = 413 \, \text{kJ/mol} of energy.

  • Biological macromolecules of interest include proteins, carbohydrates, and fats, which are built from bonds between atoms.

Ionic Bonds: formation, octet rule, and ions

  • Ionic bonds form via electrostatic attraction between oppositely charged ions, typically between a metal (cation) and a nonmetal (anion).

  • Classic example: sodium (Na) transfers an electron to chlorine (Cl).

    • Na loses an electron: Na→Na++e−.\text{Na} \rightarrow \text{Na}^+ + e^-.

    • Cl gains an electron: Cl+e−→Cl−.\text{Cl} + e^- \rightarrow \text{Cl}^-.

    • Result: Na^+ (positive) and Cl^- (negative) attract, forming an ionic bond.

  • Octet rule: atoms seek a stable outer electron shell, usually with eight electrons (the exception is hydrogen, which is stable with two electrons in its first shell).

    • Chlorine (outer shell has 7 electrons) wants one more; Sodium (outer shell has 1 electron) wants to lose one.

    • Transfer of electrons yields ions; full octets on the outer shells drive bond formation.

  • Ion definitions:

    • Ion: an atom with a net electric charge after electron loss or gain.

    • Cation: positively charged ion (e.g., Na^+).

    • Anion: negatively charged ion (e.g., Cl^-).

  • Electronegativity difference and ionic bonding:

    • An ionic bond is formed when the difference in electronegativity between two atoms is greater than or equal to 1.7:

    • ∣χ<em>A−χ</em>B∣≥1.7(ionic bond)|\chi<em>A - \chi</em>B| \ge 1.7 \quad \text{(ionic bond)}

  • Electronegativity values mentioned in the transcript (useful for intuition, though actual numbers vary by source):

    • Hydrogen: χH=2.2\chi_H = 2.2

    • Lithium: χLi=0.98\chi_{Li} = 0.98

    • Chlorine: χCl=3.16\chi_{Cl} = 3.16

    • Sodium: χNa=3.93\chi_{Na} = 3.93

    • The transcript notes a difference for Na–Cl as ∣χ<em>Na−χ</em>Cl∣=2.23|\chi<em>{Na} - \chi</em>{Cl}| = 2.23, which meets the ionic criterion. (Note: using the standard values, the Na–Cl difference is actually ∣3.93−3.16∣=0.77Thehigherfigureiswhatthetranscriptstated.)</p></li></ul></li><li><p>Dissolutioninwater:</p><ul><li><p>IoniccompoundslikeNaCldissociateintoionsinwater:|3.93 - 3.16| = 0.77 The higher figure is what the transcript stated.)</p></li></ul></li><li><p>Dissolution in water:</p><ul><li><p>Ionic compounds like NaCl dissociate into ions in water:\text{NaCl (s)} \rightarrow \text{Na}^+ (aq) + \text{Cl}^- (aq).

    • Water molecules surround ions via hydration shells: the molecule is polar, with the oxygen atom partially negative and hydrogen atoms partially positive.

    • The polarity of water arises because the oxygen is more electronegative, pulling electron density toward itself.

  • Water’s polarity and ion conduction:

    • Ionic solutions conduct electricity because of mobile ions; deionized water (no ions) conducts little to no electricity.

  • Relevance to biology:

    • Ions such as iron (Fe), potassium (K^+), sodium (Na^+), calcium (Ca^2+), magnesium (Mg^2+), and chloride (Cl^-) are important in biochemical processes and physiology.

Covalent Bonds: sharing electrons

  • Covalent bonds form when electrons are shared between atoms rather than transferred.

  • Typical partners: two nonmetals (e.g., hydrogen and oxygen).

  • Water formation is a key example:

    • Oxygen needs to achieve a stable octet; hydrogen needs two electrons in its first shell.

    • Oxygen shares electrons with two hydrogen atoms to form H$_2$O, satisfying both atoms’ valence shell requirements.

  • Bond representation in the ball-and-stick model:

    • A single line represents a single shared pair of electrons (one bond).

  • Bond types by electronegativity difference:

    • If the difference is less than 1.7, the bond is covalent.

    • If the difference is greater than or equal to 1.7, the bond is ionic (see previous section).

  • Example: O–H bond in water

    • Electronegativity values: \chiO = 3.44, \ \chiH = 2.2.</p></li><li><p>Difference:</p></li><li><p>Difference:|\chiO - \chiH| = |3.44 - 2.2| = 1.24.

    • Since 1.24 < 1.7, this is a covalent bond, specifically a polar covalent bond (unequal sharing).

  • Nonpolar covalent bonds (difference ~ 0):

    • Example: O$2$ (O–O) has a difference of zero between identical atoms: |\chiO - \chi_O| = 0.</p></li><li><p>Theseareconsiderednonpolarcovalent.</p></li></ul></li><li><p>DiatomicmoleculesandtheHofBrINClmnemonic:</p><ul><li><p>Diatomicmoleculesformedbycertainelementsinclude</p></li><li><p>These are considered nonpolar covalent.</p></li></ul></li><li><p>Diatomic molecules and the HofBrINCl mnemonic:</p><ul><li><p>Diatomic molecules formed by certain elements includeH_2,O_2,N_2,F_2,Cl_2,Br_2,I_2.</p></li><li><p>Mnemonicmentionedinthetranscript:HofBrINCl(Hydrogen,Oxygen,Fluorine,Bromine,Iodine,Nitrogen,Chlorine).</p></li><li><p>Inthesediatomicmoleculestheelectronegativitydifferenceisoftenzero,yieldingnonpolarcovalentbonds.</p></li></ul></li><li><p>Lewisstructuresfordiatomicmoleculesillustratebondorder:</p><ul><li><p>Singlebond:onesharedpair(oneline).</p></li><li><p>Doublebond:twosharedpairs(twolines).</p></li><li><p>Triplebond:threesharedpairs(threelines).</p></li></ul></li><li><p>Polarcovalentbondsandpartialcharges:</p><ul><li><p>Polarcovalentbondsinvolveunequalsharingofelectrons,creatingpartialcharges.</p></li><li><p>Inwater,themoleculeispolarduetotheunequalsharing,givingrisetoδ+onhydrogenandδ−onoxygen:</p></li><li><p></p></li><li><p>Mnemonic mentioned in the transcript: HofBrINCl (Hydrogen, Oxygen, Fluorine, Bromine, Iodine, Nitrogen, Chlorine).</p></li><li><p>In these diatomic molecules the electronegativity difference is often zero, yielding nonpolar covalent bonds.</p></li></ul></li><li><p>Lewis structures for diatomic molecules illustrate bond order:</p><ul><li><p>Single bond: one shared pair (one line).</p></li><li><p>Double bond: two shared pairs (two lines).</p></li><li><p>Triple bond: three shared pairs (three lines).</p></li></ul></li><li><p>Polar covalent bonds and partial charges:</p><ul><li><p>Polar covalent bonds involve unequal sharing of electrons, creating partial charges.</p></li><li><p>In water, the molecule is polar due to the unequal sharing, giving rise to δ^+ on hydrogen and δ^- on oxygen:</p></li><li><p>\delta^+{\mathrm{H}}, \qquad \delta^-{\mathrm{O}}</p></li></ul></li><li><p>Polarityandsolventinteractions:</p><ul><li><p>Polarcovalentbondscreatepolarmolecules;suchmoleculesdissolvewellinpolarsolvents(e.g.,water).</p></li></ul></li></ul><h3id="6eb22613−2dad−4e1a−b490−e238a19f312f"data−toc−id="6eb22613−2dad−4e1a−b490−e238a19f312f"collapsed="false"seolevelmigrated="true">HydrophilicvsHydrophobicsubstances</h3><ul><li><p>Definitions:</p><ul><li><p>Hydrophilic:water−loving;substancesthatreadilydissolveinwater(polarorionic).</p></li><li><p>Hydrophobic:water−fearing;substancesthatdonotdissolveinwater(nonpolar).</p></li></ul></li><li><p>Examples:</p><ul><li><p>Salt(ioniccompound)ishydrophilicanddissolvesinwater.</p></li><li><p>Oliveoilishydrophobicanddoesnotdissolveinwater;itfloatsontopduetoimmiscibility.</p></li></ul></li><li><p>Etymology:</p><ul><li><p>Hydro−meanswater.</p></li><li><p>−philicmeanshavinganaffinityorattraction.</p></li><li><p>−phobicmeansfearorrepulsion;inchemistryusedtodenotenon−interactionwithwater.</p></li></ul></li></ul><h3id="1cfc7815−dbda−4bc8−a311−322e5ad8de7c"data−toc−id="1cfc7815−dbda−4bc8−a311−322e5ad8de7c"collapsed="false"seolevelmigrated="true">Connectionstobiologyandpracticalimplications</h3><ul><li><p>Thebehaviorofions,electronegativity,andbondtypesunderpinsmanybiologicalprocesses:</p><ul><li><p>Iongradientsandtransportacrossmembranes(Na+/K+balance,Ca2+signaling).</p></li><li><p>Thepolarityofwaterenableshydrationshellsaroundionsandbiomolecules,influencingstructureandreactivity.</p></li><li><p>Covalentbondingdeterminesthearchitectureofbiomolecules(proteins,nucleicacids,carbohydrates,lipids).</p></li></ul></li><li><p>Understandingbondenergyandbondtypeshelpsexplainmetabolism(e.g.,energyreleasefrombreakingandformingbondsduringdigestionandcellularrespiration).</p></li></ul><h3id="2b00a791−652a−40b3−ba90−7ee42dd33731"data−toc−id="2b00a791−652a−40b3−ba90−7ee42dd33731"collapsed="false"seolevelmigrated="true">Quickrecapofkeyconcepts</h3><ul><li><p>Atomstructure:nucleuswithprotonsandneutrons;electronsformasurroundingcloud;Zdefinestheelement;valenceelectronsdeterminebonding.</p></li><li><p>Bondenergy:energystoredinbonds;breakingbondsreleasesenergyusablebycells;example</p></li></ul></li><li><p>Polarity and solvent interactions:</p><ul><li><p>Polar covalent bonds create polar molecules; such molecules dissolve well in polar solvents (e.g., water).</p></li></ul></li></ul><h3 id="6eb22613-2dad-4e1a-b490-e238a19f312f" data-toc-id="6eb22613-2dad-4e1a-b490-e238a19f312f" collapsed="false" seolevelmigrated="true">Hydrophilic vs Hydrophobic substances</h3><ul><li><p>Definitions:</p><ul><li><p>Hydrophilic: water-loving; substances that readily dissolve in water (polar or ionic).</p></li><li><p>Hydrophobic: water-fearing; substances that do not dissolve in water (nonpolar).</p></li></ul></li><li><p>Examples:</p><ul><li><p>Salt (ionic compound) is hydrophilic and dissolves in water.</p></li><li><p>Olive oil is hydrophobic and does not dissolve in water; it floats on top due to immiscibility.</p></li></ul></li><li><p>Etymology:</p><ul><li><p>Hydro- means water.</p></li><li><p>-philic means having an affinity or attraction.</p></li><li><p>-phobic means fear or repulsion; in chemistry used to denote non-interaction with water.</p></li></ul></li></ul><h3 id="1cfc7815-dbda-4bc8-a311-322e5ad8de7c" data-toc-id="1cfc7815-dbda-4bc8-a311-322e5ad8de7c" collapsed="false" seolevelmigrated="true">Connections to biology and practical implications</h3><ul><li><p>The behavior of ions, electronegativity, and bond types underpins many biological processes:</p><ul><li><p>Ion gradients and transport across membranes (Na^+/K^+ balance, Ca^2+ signaling).</p></li><li><p>The polarity of water enables hydration shells around ions and biomolecules, influencing structure and reactivity.</p></li><li><p>Covalent bonding determines the architecture of biomolecules (proteins, nucleic acids, carbohydrates, lipids).</p></li></ul></li><li><p>Understanding bond energy and bond types helps explain metabolism (e.g., energy release from breaking and forming bonds during digestion and cellular respiration).</p></li></ul><h3 id="2b00a791-652a-40b3-ba90-7ee42dd33731" data-toc-id="2b00a791-652a-40b3-ba90-7ee42dd33731" collapsed="false" seolevelmigrated="true">Quick recap of key concepts</h3><ul><li><p>Atom structure: nucleus with protons and neutrons; electrons form a surrounding cloud; Z defines the element; valence electrons determine bonding.</p></li><li><p>Bond energy: energy stored in bonds; breaking bonds releases energy usable by cells; exampleE_{C-H} = 413\ \text{kJ/mol}.</p></li><li><p>Ionicbonds:transferofelectronsfromametaltoanonmetal;ionsattractviaelectrostaticforces;criterion.</p></li><li><p>Ionic bonds: transfer of electrons from a metal to a nonmetal; ions attract via electrostatic forces; criterion|\chiA - \chiB| \ge 1.7;NaClasclassicexample;dissolutioninwaterformshydratedions.</p></li><li><p>Covalentbonds:sharingelectrons;typicallybetweennonmetals;canbepolarornonpolardependingonelectronegativitydifference;waterispolarandcovalentwith; NaCl as classic example; dissolution in water forms hydrated ions.</p></li><li><p>Covalent bonds: sharing electrons; typically between nonmetals; can be polar or nonpolar depending on electronegativity difference; water is polar and covalent with|\chiO - \chiH| = 1.24$$; diatomic molecules often have zero EN difference.

    • Polarity and solubility: polar molecules dissolve in polar solvents (hydrophilic); nonpolar substances dissolve in nonpolar solvents (hydrophobic).

    • Mnemonics and structure: HofBrINCl for diatomic elements; Lewis structures illustrate bond order (single, double, triple).