Chemistry Review

Essential Elements of Life What do you think? About 25 of the 92 elements are essential to life 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 those required by an organism in minute quantities

Chemistry Review Atoms the smallest unit of matter that still retains the properties of an element Subatomic Particles Atoms are composed of subatomic particles Relevant subatomic particles include:

Neutrons (no electrical charge) Protons (positive charge) Electrons (negative charge) Neutrons and protons form the atomic nucleus Electrons form a cloud around the nucleus Atomic Number and Atomic Mass Atoms of the various elements differ in number of subatomic particles An elements atomic number is the number

of protons An elements mass number is the sum of protons plus neutrons in the nucleus Atomic mass, the atoms total mass, can be approximated by the mass number Chemical Bonding The formation and function of molecules depend on chemical bonding between atoms

Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms These interactions usually result in atoms staying close together, held by attractions called chemical bonds Covalent Bonding A covalent bond is the sharing of a pair of valence electrons by two atoms In a covalent bond, the shared electrons count as part of each atoms valence shell

LE 2-10 Hydrogen atoms (2 H) Hydrogen molecule (H2) A molecule consists of two or more atoms held together by covalent bonds A single covalent bond, or single bond, is

the sharing of one pair of valence electrons A double covalent bond, or double bond, is the sharing of two pairs of valence electrons Covalent bonds can form between atoms of the same element or atoms of different elements LE 2-11b Name (molecular

formula) Oxygen (O2) Electronshell diagram Structural formula Spacefilling model

LE 2-11c Name (molecular formula) Water (H2O) Electronshell diagram

Structural formula Spacefilling model Electronegativity Electronegativity is an atoms attraction for the electrons in a covalent bond The more electronegative an atom, the more strongly it pulls shared electrons toward itself

In a nonpolar covalent bond, the atoms share the electron equally In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally LE 2-12 O

H + H H2O + Ionic Bonds

Atoms sometimes strip electrons from their bonding partners An example is the transfer of an electron from

sodium to chlorine After the transfer of an electron, both atoms have charges A charged atom (or molecule) is called an ion An anion is a negatively charged ion A cation is a positively charged ion An ionic bond is an attraction between an anion and a cation LE 2-13 Na

Sodium atom (an uncharged atom) Cl Chlorine atom (an uncharged atom) Na+

Cl Sodium ion (a cation) Chlorine ion (an anion) Sodium chloride (NaCl) Ionic Bonds

Compounds formed by ionic bonds are called ionic compounds, or salts Salts, such as sodium chloride (table salt), are often found in nature as crystals Na+ Cl Weak Chemical Bonds Most of the strongest bonds in organisms are covalent bonds that form a cells

molecules Weak chemical bonds, such as ionic bonds and hydrogen bonds, are also important Weak chemical bonds reinforce shapes of large molecules and help molecules adhere to each other Hydrogen Bonds A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to

another electronegative atom In living cells, the electronegative partners are usually oxygen or nitrogen atoms LE 2-15 + Water

(H2O) + Hydrogen bond Ammonia (NH3) +

+ + Van der Waals Interactions Molecules or atoms that are very close together can be attracted by fleeting charge differences These weak attractions are called van der Waals interactions Collectively, such interactions can be strong, as between molecules of a geckos

toe hairs and a wall surface

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