CHEAT SHEET - Campbell Biology Chapter 2 - The Chemical Context of Life

CHEAT SHEET - Campbell Biology Chapter 2 - The Chemical Context of Life 

There are three types of subatomic particles:

• positive protons inside the atom’s nucleus
• neutral neutrons also in the nucleus
• negative electrons buzzing around the nucleus

Protons and neutrons weigh 1 Dalton each; electrons weigh practically nothing

Each element has a unique atomic number:

• the atomic number equals the number of positive protons in each atom of the element; this number never changes • if the atom is neutral the atomic number also equals the number of negative electrons; atoms that have an imbalance of negative electrons are called ions • cations have lost negative electrons and are positively charged, while anions have gained negative electrons and are negatively charged

Each element also has a (larger) atomic mass

number:

• the atomic mass number equals the number of protons plus neutrons • the number of protons never changes, but the number of neutrons can vary, resulting in different isotopes with different atomic mass • the atomic mass number is an average among isotopes; you must round it to a whole number before doing calculations

Atomic mass number – atomic number = the number of neutrons

Living things are mostly carbon, oxygen, hydrogen

and nitrogen: COHN

The electronic configuration predicts the chemical

behavior of the atom:

• Electrons exist in energy levels or shells: up to 2

electrons fit in the first, up to 8 in the second, and up to 18 in the third shell • Shells are filled from first to last • The last-used shell is called the valence shell • Electrons already located in the valence shell are called valence electrons • The number of electrons needed (still missing) to fill the valence shell or reach 8 valence electrons is called the co-valence or simply the valence • If the (co-) valence is 4 or less it is also known as the bonding capacity • Atoms “prefer” to have full shells or have 8 valence electrons in the valence shell (octet rule) and will bond with other atoms to get there

Within each shell, electrons exist inside spaces

called orbitals:

• orbitals for the first 18 elements include s and p

orbitals: the first shell has s, the second and third

shell have both s and p • up to 2 electrons fit in each s orbital, up to 6 in each p; s is filled before p in the same shell • the electronic configuration of Ar (atomic number 18 with 18 electrons) is 1s22s22p63s23p6 Elements are placed in the periodic table based on

their electronic configuration:

• Row = period = the valence shell number • Column = group = the number of valence electrons already located in the valence shell (with the exception of the noble or inert gases in group 8 which have either 2 or 8 valence electrons; they do however share the fact that they have “full” valence shells, giving them a valence or bonding capacity of zero) • With the exception of H, elements in groups 1-3 are considered metals; elements in groups 6 and

• as well as H and C are considered non-metals

Atoms with incomplete valence shells (in groups 1- 7) will gain, lose or share valence electrons with other atoms, forming a strong chemical bond. All participating atoms must end up with a full valence shell or a valance shell with 8 valence electrons in it, or the deal is off. There are two types of strong

chemical bonds:

• ionic bonds are very strong when dry and are

formed between a metal (in groups 1-3, but not

• and a non-metal (C or H or in groups 6-7).

Ionic bonds involve a transfer of valence

electrons: the metal will lose valence electron(s)

to become a cation while the non-metal will gain the valence electron(s) to become an anion. For example Na and Cl combine to form the ionic

compound NaCl:

• covalent bonds are always strong (wet and dry)

and are formed by sharing valence electrons between 2 non-metals (C or H or in groups 6-7).Each atom must contribute one valence electron to the sharing arrangement; one shared pair of valence electrons forms one covalent bond. E.g.two H atoms may share their valence electrons to

form an H2 molecule:

Covalent bonds may be single, double or triple.

An example of a triple bond would be N2:

All atoms belonging to an element have a certain

electronegativity: a measure of how strongly

they attract and hold on to valence electrons.Elements near the top right of the periodic table (F, N, O, Cl) have the highest electronegativity, elements in the middle (C and also H) have medium electronegativity, and elements on the left (Na and Mg) have the lowest electronegativity.

There is a huge difference in electronegativity of Na (very low) and Cl (very high); this is why Cl can overpower and “steal” valence electrons from Na to form the ionic bond.

Relative electronegativity of the two atoms also determines which type of covalent bond will be

formed. There are two types:

• nonpolar covalent bonds are formed by equal

sharing of valence electrons between atoms of same or similar electronegativity. Neither atom is strong enough to hog the shared electrons that make up the bond. For example, two H atoms always have the same electronegativity, so an H- H bond is nonpolar covalent; C and H have similar electronegativity, so a C-H bond is also nonpolar covalent.

• Polar covalent bonds are formed by unequal

sharing of valence electrons between atoms with a moderate difference in electronegativity. One atom is moderately stronger than the other; although it continues sharing, it does hog the shared electrons. Because electrons are negatively charged, this makes the more strongly electronegative atom appear slightly more negative (delta negative) compared to the less electronegative atom, which now appears slightly more positive (delta positive). Because we see a difference, we speak of “poles”, hence the name polar covalent bond. For example, O (very high) is somewhat more electronegative than H (medium), so an O-H bond will be polar covalent; O will be delta negative and H will be delta positive.

Hydrogen or H-bonds are weak bonds that exist between polar molecules or between polar parts of very large molecules • H-bonds do NOT involve gaining, losing or sharing valence electrons; an O-H, H-H, or a C-H bond is NOT an H-bond because an they are formed by sharing valence electrons; these covalent bonds are usually represented by a solid line • H-bonds do require a delta positive H atom on one molecule to H-bond with a delta negative F, N, O, or Cl on a second molecule; H-bonds are usually represented by a dotted line • each H2O molecule has two polar covalent O-H bonds. Because of its V-shape, each H2O molecule has a delta negative side vs. a delta positive side and is said to be polar as well.• The delta negative O on one H2O molecule can weakly H-bond with a delta positive H atom on a

second H2O molecule:

van der Waals attractions are weak bonds between transiently positive and negative regions on molecules (also no transfer or sharing of electrons)

CHEAT SHEET - Campbell Biology Chapter 2: The Chemical Context of Life

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