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Samenvatting molecules & reactivity
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MOLECULES & REACTIVITYINTRODUCTION
Conjugated bonds – alternated single and double bonds, like a benzene ring
Stereochemistry – the arrangement of an asymmetric carbon atom. There are left
and right turned molecules, which have different properties
Bacteria become resistant to antibiotics, for example penicillin. The bacteria produce
penicillinase, an enzyme that breaks down penicillin. So penicillin has no therapeutic
effect anymore.
Organic molecules: based on carbon atoms
CHAPTER 1: ELECTRONIC STRUCTURE AND BONDING
Protons – positively charged
Neutrons – no charge
Electrons – negatively charged
The nucleus consist of protons and neutrons and the electron cloud consist of
electrons. So mass is concentrated in the nucleus, but size is determined by the
electron cloud. Electrons have kinetic energy, and this energy counteracts the
attractive force of the positively charged protons that pull the negatively charged
electrons toward the nucleus. Otherwise they will lose kinetic energy.
Atomic number: number of protons
Mass number: number of protons + neutrons
Isotopes have the same atomic number, but a different mass number, so they have
the same amount of protons, but a different amount of neutrons.
Atomic weight: average weighted mass of its atoms
Molecular weight: the sum of the atomic weights of all the atoms in the molecule
EXAMPLE:
The isotopes of carbon are 12C occurs 98.9%, 13C occurs
1.1% and 14C occurs 0%.
Atomic weight = 0.989 x 12 + 0.011 x 13 + 0 x 14 = 12.011 Da
The number in the periodic system is the atomic number, so the number of protons,
and thus also the number of electrons in that element.
In quantum mechanics, a particle is treated as a wave to characterize the motion of
an electron around the nucleus. This gives probability of locations around the
nucleus, but it is not exact. Wave functions or orbitals tell us the energy of the
electron and the volume of space around the nucleus where an electron is most likely
to be found.
1
,Orbital: space around the nucleus where up to 2 electrons can be found. Degenerate
orbitals have the same energy.
The first shell (s) is the closest to the nucleus and has
the lowest energy. The closer the atomic orbital is to the
nucleus, the lower its energy. So, energy of s < p.
Each shell contains 1 s atomic orbital. Each second, in addition to its s atomic orbital,
contains 3 degenerate p atomic orbitals. In each orbital can 2 electrons be found,
thus in the first shell 2, but in the second shell 8 (2 x (1+3)).
H & He – first shell (2 electrons)
Li t/m Ne – first & second shell (2 + 8 = 10 electrons)
From low to high energy:
1s < 2s < 2p < 3s < 3p < 3d
Aufbau principle: an electron goes into the atomic
orbital with the lowest energy
Pauli exclusion principle: no more than 2 electrons
can be in an atomic orbital
Hund’s rule: an electron goes into an empty
degenerate orbital rather than pairing up (so 1 in
2px and 1 in 2py, instead of 2 in 2Px for example)
Every arrow represents 1 electron and the direction of the arrow represents their spin
(magnetic moment). The directions of the magnetic field should be opposites in the
orbital, so opposite arrows.
Valence electrons: electrons in an atom's outermost shell
Core electrons: electrons in inner shells (below the outermost shell)
Valence electrons participate in chemical bonding; core electrons do not. Elements in
the same column of the periodic table have similar chemical properties because they
have the same number of valence electrons. Lithium and sodium, which have similar
chemical properties, are in the same column because each has one valence electron.
EXAMPLES:
Boron – atom number 5, so 3 valence electrons
Nitrogen – atom number 7, so 5 valence electrons
Fluorine – atom number 9, so 7 valence electron
Natrium – atom number 11, so 1 valence electron (2 in first shell, 8 in second shell, 1
in third shell)
Lewis’s theory: an atom will give up, accept, or share electrons in order to achieve a
filled outer shell
2
,Octet rule: atoms will achieve an outer shell that contains 8 electrons (stable)
H and He don’t follow the octet rule, but instead have 2 electrons in the valence shell
to be stable.
A lithium3 atom normally has 1 electron in the outer shell, it
cannot get 7 more electrons to get the outer shell filled so it gets
rid of 1 electron, now the outer shell is filled. The lithium ion is
now positive. A fluorine9 atom normally has 7 electrons in the
outer shell, so it gains 1 electron, now the outer shell is filled. The
fluoride ion is now negative.
H – giving up electron → H+
H – picking up electron → H-/hydride ion
This is the reason why hydrogen has 2 places in the periodic system. Hydrogen
usually has 1 electron, so when it gives up or picks up an electron, it achieves an
empty or filled outer shell.
IA – giving up 1 electron
IIA – giving up 2 electrons
VIIA – picking up 1 electron
VIA – picking up 2 electrons
Inert gases don’t react because they got their outer shell full.
Ionic bonds are formed by the transfer of electrons. An ionic bond is the attraction
between ions of opposite charges. Attractive forces between opposite charges are
called electrostatic attractions.
Covalent bonds are formed by
sharing electrons, so that both atoms
experience a full shell.
- Nonpolar covalent bonds
- Polar covalent bonds
Nonpolar covalent bonds: equal sharing of electrons, because the bonded atoms
have the same or similar electronegativities
H-H or H-C or F-F
Polar covalent bonds: unequal sharing of electrons between
bonded atoms with different electronegativities
The arrow points to the end with the highest
electronegativity, so the negative end of the bond.
Electronegativity: the ability of an element to attract electrons.
3
, The element with the highest electronegativity is fluorine
because it only needs 1 electron to achieve a full shell.
On the other hand, K has a low electronegativity so it
wants to give away an electron in order to achieve a full
shell. Carbon has an average number and is forced to
share its electron. C4+ or C4- is very unstable, so that’s the
reason why carbon shares its electrons and why it is
used in so many compounds.
The greater the difference in electronegativity, the more polar the bond and the
greater the dipole moment.
Difference < 0.5: nonpolar covalent bond
Difference 0.5 – 1.9: polar covalent bond
Difference > 1.9: ionic bond
Dipole moment = size of the charge x distance between the charges
Electrostatic potential maps:
Hydrogen has a higher electronegativity than
Lithium, so the electrons are more around the
hydrogen nucleus. The size of the atom is
determined by the electron cloud and that’s the
reason why hydrogen is larger than Lithium.
Formal charge = the number of valence electrons – (the number of lone-pair
electrons + the number of bonding electrons)
Oxygen has 6 valence electrons, 2 lone-pair
electrons and 3 bonding electrons, so the formal
charge = 6 – (2 + 3) = 1, thus +1 charge
If carbon does not form 4 bonds, it has a charge or
a radical (extremely reactive).
Positive charged carbon atom: carbo-cation
Negative charged carbon atom: carb-anion
- Nitrogen (N) forms 3 bonds, otherwise charged.
- Oxygen (O) forms 2 bonds, otherwise charged.
- If hydrogen or halogen forms 1 bond, otherwise charged (or
radical)
Halogens: F, Cl, Br, I, At, Ts
The number of bonds plus the
number of lone pairs equals 4.
4
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