Autumn semester Flashcards
How is the periodic table arranged?
Elements are sequenced in increasing atomic number across the periods, and arranged so that elements with similar chemical properties fall in the same groups.
What are the sap,d,f blocks of the periodic table?
s-block = groups 1-2
p-block = 13- 18
d-block = 3-12
f block = bottom part
What is the composition of an atom?
The atom consists of negatively charged electrons which move around a central nucleus containing positively charged protons and neutrons.
What is an isotope?
An isotope are elements with the same atomic number but have different mass numbers (differ with amount of neutrons)
What is the octet rule in relation to the driving force for reactions?
The octet rule states that atoms will continue to share electrons until they have acquired an octet of valence electrons. The driving force of any reaction is the formation of the electronic structure of a noble gas.
Each orbital contains maximum 2 electrons where there is 1 s orbital, 3 p orbitals, five d orbitals and 7 f orbitals.
What is the concept and importance of the valence shell?
The valence shell is the highest energy set of orbitals that contain the outer (valence) electrons. It is important as it is the orbital that is gaining or losing electrons.
How do you identify valence electrons for an element and why are they important?
Valence electrons are the electrons listed after the last noble gas configuration (outer electrons)
Valence electrons are important is chemistry as in an ionic compound is tell us the oxidation state and number of bond formed.
How is the structure of the atoms found in experiment?
Rutherford > conducted an experiment in which alpha (HE2+)particles where shot through a gold foil. He found that he vast majority of the ions went straight through with minimal scattering (proving high amount of space between electrons) however some where reflected back at large angles suggesting there presence of a larger charged mass (the nucleus).
Additional he tried to use classical mechanics to explain atom behaviour, this failed as the electrons gradually decayed and collided with nucleus.
Bohr > suggested that energy of an electron in a particular orbital was quantised, he proved this by exciting hydrogen particles which showed shared bands of light emitted showing well defined gaps.
What is the concept of energy levels around an atom?
Electrons located around an atom are found in defined area called energy levels, they represent the 3d space surrounding an atom where electrons are most likely to be.
What is the definition of ionisation energy?
Ionisation energy is the amount of energy needed to remove 1 electron from an atom. The ionisation energies where found to agree exactly with those measured experimental in the atomic spectrum of hydrogen.
What is electromagnetic radiation?
A from of energy consisting of oscillating electric and magnetic fields which travel at the speed of light (c).
e.g visible light, microwaves, x-rays … ( they all have wavelengths)
What is the equation relating to frequency, wavelength and velocity of light?
λv = c
λ = wavelength (m)
v = frequency (Hz/s^-1)
c = speed of light (2.998 x 10^8 m/s)
How does flame test observations relate to atomic structure?
The flame test is used to visually determine the identity of an unknown metal or metalloid ion based on the characteristic color the salt turns the flame of a bunsen burner. The heat of the flame converts the metal ions into atoms which become excited and emit visible light. The colour emitted corresponds with the energy change of the gaps between energy levels.
What is a photon?
In some situation it was found that the behaviour of light/radiation cannot always be thought of as waves but instead particles. Max Planck proposed that electromagnetic radiation could only be emitted and absorbed in quanta of radiation called photons.
The energy of a photon is proportional too its frequency.
What is the equation relating the energy of a photon to frequency?
E = hv
E = energy (j)
h = planck’s constant (6.63 x 10^-34 js)
v = frequency (Hz or s^-1)
What is the photoelectric effect?
The ejection of electrons from a material pm irradiation by light (Light hits the material and electrons are thus displaced).
The electrons are only ejected from the surface is the frequency of the radiation is above a threshold value called the work function.
How is the photoelectric effect applicable in the phototube?
a vacuum tube containing a cathode made of a metal with a small work function so that electrons would be easily emitted. The current released by the plate would be gathered by an anode held at a large positive voltage relative to the cathode
What is the de Broglie relationship and how is it used?
De Broglie pointed out that the energies calculated for a wave and for a particle must be equal for anything behaving as both showing wave-particle duality.
λ = h/p
λ = Wave length (m) (limiting factor)
h = Planck’s constant (6.63 x 10^-34 js)
p = momentum of particle (mass x velocity)
What is the uncertainty principle?
As an electron has wave-like properties with wave length = order of magnitude of the size of an atom, you can determine the probability that an electron is in a certain place.
What is the Heisenberg uncertainty principle?
An electron has a position in space defined by x,y,z coordinates and has a momentum (p=mv) parallel to each axis (px,py,pz)
thus :
ΔxΔpx ≥ h / 4π
Where Δx, Δpx are the uncertainties in measuring x and px, we can know the position of an electron thus the less we know about its momentum (and vis versa).
Example of the Heisberg uncertainty: calculate the minimum uncertainty in the position of an electron (mass = 9 x 10^-31 kg) moving at 3 x 10^5 m/s with an uncertainty in the velocity of +/- 10^2 m/s?
Δx = h / 4π x Δpx
Δpx = mass x velocity
= (6.63 x 10^-34) / 4π x ((9x10^-31)x(2x10^2)) = 293 nm
What is the concept of a wave function and why is it used?
The wave function tell us how likely an electron is in a particular location at a given time its denoted by the symbol 𝚿 (psi). It is calculated using Schrodinger equation.
What is the physical significance of the wave function?
Max Born suggested that the probability of finding a particle in any region of space is proportional to 𝚿^2.
Thus in a region were 𝚿^2 is large, the probability of finding and electron is high and vis versa.
(𝚿 = 0 there is a node)
What is the Schrodinger equation?
Schtodinger used wave function to produce a mathematical equations to calculate the behaviour of an atom however it can only be solved for two body problems ( a system containing only a nucleus and an electron).
H𝚿 = E𝚿
𝚿 = R (r)Y (θ, Ф)
n,l l,ml
R =
What are the answers that can be obtained from the Schrondinger equation?
- the allowed wavefuntions for the particle
- solutions are only possible for certain energies
- the probability density for a particle at any point is proportional to the 𝚿^2 at that point.
- each allowed coltuion for the hydrogen atoms defines an allowed atomic orbital
What is the component symbols of the wave function?
R = radial wave function
Y = angular wave function
n = principle quantum number
l = angular momentum
ml = quantum number
What are the definition of all the quantum numbers and their allowed values?
n = the principle quantum number it determines the size of the orbital and for a hydrogen atom the energy. Its allowed values are (1,2,3,4,5 ….)
l = orbital angular momentum quantum number it determines the shape of the orbital (any whole number from 0 to (n-1).
ml = magnetic quantum number determines the orientation in space of the orbital (excluding in a magnetic field). Its values re any whole number between -l to +l.
How does the allowed value of the quantum numbers to the structure of the atomic orbitals?
Together the quantum numbers provides information of the properties of an atomic orbitals (the number and type of orbitals possible)
The type of orbital:
l = 0 (s orbital) l = 1 (p orbital) l = 2 (d orbital) and l = 3 (f orbital)
l = n - 1
e.g.
for n = 1, l = 0 and ml = 0 meaning that the orbital is a singular 1s orbital
for n = 2, l = 1 and ml = 0 so the orbital is one 2s but l can also be l = 1 and ml = -1,0,1 so the orbital is three distinct 2p orbitals.
- if the orbitals have the same value of n = same shell, if both n and l are the same = same sub shell
How can radial and angular wave functions be used to plot three dimensional shapes of the orbitals?
The wave function 𝚿 varies with position, this variation means that 𝚿 is a function of x, y and z. As atoms are spherical so 𝚿 can be represented as a function of one distance r and two angles θ, Ф (spherical coordinates) .
𝚿 = R (r)Y (θ, Ф)
n,l l,ml
What is radial wave function?
Shows the probability of finding an electron in a spherical shell at a distance r from the nucleus, this is found by plotting 4pir^2R^2
What is angular wave function?
Shows the shape and orientation of orbitals. (overall shape depends entirely on l and ml). Angular nodes are presents no of nodes = l.
How do you interpret useful information from radial probability function plots?
- As n increases from a given value of l the size increases
- there is a very small probability of finding the electron long way from the nucleus so accurate size definition is not easy
- there are areas called nodes where the probability of finding an electron in 0.
number of radial nodes = n - l - 1b e.g. 0 for 1s, 2p and 3d, 1 for 2s and 3p and 2 fro 3s.
What is the shapes and orientation of the s, p,d orbital?
s orbitals = these are spherical with the same phase of wave function across the whole boundary surface.
p orbitals = these are dungbell shaped with px,py and pz all have the same shape but different orientations and contains both negative and positive phases and one angular node where 𝚿 = 0 (2p) as n increases the number of nodal planes also increase.
d orbitals = have a clover like shapes with four lobes and two angular nodal planes.
What are the main types of chemical bonds?
- covalent bonds (electron sharing)
- ionic bonds (electron transfer)
- metallic bonding (positive metal ion held together by a sea of delocalised electrons)
What are the characteristics of the main chemical bonds?
- ionic bonds are formed between arose with low ionisation energies and high electron affinities
- covalent bonds are formed through the attraction between the shared pair of electrons and both positive nuclei
What is the definition of electronegativity?
The ability of an atom in a molecule to attract electron density to itself (ability to attracted the shared pair of electrons in a covalent bond)
- in a homonuclear bond the electrons are equally shared (H2)
- in a heteronuclear bond the sharing will be unequal (HCl)
The Pauline scale shows electronegativity increasing through a diagonally from left to right up the periodic table?
How does the periodic table aid in predicting types of chemical bonds?
The periodic table allows fro predictions of electronegativity to take place (Pauline scale)/
- no electronegativity creates a purse covlents bond (X-X)
- small to moderate electronegativity differences create polar covalent bonds (C - O with delta + and -)
- very large electronegativity differences make ionic bonds (K+Br-)
what are lewis structures and rules?
Lewis strutters allows us to determines the connectivity and formal charges of the atoms in a structure (not the shape).
- every atoms wants to achieve an octet of electrons
- each pair of shred electrons gives one bond
- often no more than 4 bonds to an atom
How do you draw lewis structures?
- Add up total number of valence electrons (determine how may pairs of electrons can be used)
- decide on the central atoms and draw it in the middle and surrounding with other bonding atoms
- join the atom to central atoms with single bonds
- keep adding remaining pair of electrons to form multiple bonds if appropriate
- add any remaining electrons as lone pairs
- check number of electrons in immediate surrounding and assign formal charges (circle around atom if e- inside circle = valence means no charge, if < valence e- = + and > valence e- = -)
Lewis structure example CO3^2-?
- C = 4 valence e-
O = 6 valence e-
+ 2e- (add for negative charge) = 24 e- thus 12 electron pairs
2.central atom = C (lowest electronegativity) - sings bonds
- 1 double bond between O and C as carbon is unlikely to have more than four bonds (can’t expand octet)
- remains e- pairs are lone pairs
- 2 oxygens have 7e- in circle so both have negative charges the C and Oxygen attached with double bonds are neutral.
What are the main exceptions in lewis structures?
- Some elements cannot achieve a full octet e.g. boron has 3 valence e- but can only form three bonds (need 5 to fit octet)
- Elements in the second and subsequent rows can accommodate more than 8 electrons by expanding their octet e.g. SF6
- Quite often their are several lewis structures for a given molecule or ion
What is the concept of resonance?
Resonance involves the blending of two or more lewis structures, where the atoms have the same relative positions but different electronic arrangement. the electrons are delocalised resulting in a resonance hybrid which is lower in energy.
e.g. CO^2 = there are three different positions of the double C-O bond (one shorter and two longer bonds) however experiments shows that all the C-O bonds are the same length showing resonance.
How do you use lewis structures to rationalise properties?
- bond strength
N2O4 the lewis structure shows tray repulsion between positive charges between the conjoined N atoms led to a very weak N-N - Bond lengths
O-O distance in H2O2 is longer than F2O2 as H-O-O-H but F is more electronegative (pulls electron density) and has resonance forms making the bond longer in H2O2 - explains stability
NCO^- = CNO-, NCO^- has one formal - charge and two resonance forms and CNO- haas three formal charges making it more unstable.
How do you use lewis structures to predict the most stable arrangements of atoms in a structure?
The most stable lewis structures keep
- formal changes to a minimum
- when formal charges are necessary they should be kept consistent with relative electronegativities
- like charges should be kept as far as possible for each other
What are coordinate bonds and their characteristics?
A coordination bond is where one atom (donor) gives both electrons to another (acceptor) to form a normal covalent bond. It is symbolised by an arrow.
What is meant by the term VSEPR?
VSEPR stands for Valence Shell Electron Pair Repulsion. It can be used to determine the 3D shape of a covalent molecule.
How can VSEPR theory be used to determine the appropriate system for molecules and ions?
- determine the lewis structure for the molecule
- count valence electrons
What are the names and shapes of the basic 3d shapes (ABn) of covalent molecules?
- AB2 = linear
- AB3 = trigonal planar
e.g BF4 e- from B = 3, e- from B-F = 3 resulting in 3 e- pairs. - AB4 = tetrahedral
e.g. CH4 e- from C = 4, e- form C-H bond = 4 so four electron pairs. - AB5 = trigonal bipyramid/square based pyramid
e.g. PF5 e- from P = 5 and P-F bonds = 5 so 5 electron pairs giving trigonal bipyramidal - AB6 = octahedral
e.g. SF6 electrons from S is 6 and S-F bond = 6 so 6 electron pairs.
How to ions impact the shape of a molecule?
For molecules that have a permeant charge, the electrons from the central atom and the electron from each bond are still counted however for every + charge 1 electron is subtracted and for every negative charge 1 electron is added.
e.g. electrons from N = 5 and electrons from N-H bond = 4, - 1e- for positive charge on central atom making it AB4 and tetrahedral.
How do lone pairs rationalised the shapes of molecules and ions?
If some of the central atoms electron do not participate in binding they are called lone pairs, they affect the shape as repulsion from lone pairs are greater that the bonded pairs as they are located closer to the central atom so must be counted as a region of electron density.
What are the main shapes of molecules/ions with lone pairs?
AB2L = v -shaped, based on trigonal planar
AB3L = pyramidal, based on tetrahedral
AB2L2 = v shaped, based on tetrahedral
AB4L = seesaw, based on trigonal bipyramidal, with lone pair in equatorial position.
AB3L2 = T shaped with lone pairs at equatorial positions
AB2L3 = linear, repulsion cancel each other out
AB5L = square based pyramidal
AB4L2 = square planar
lone pair of electrons with occupy the position with the least amount of repulsion.
What are the main bond angles with each shape?
Trigonal planar = 120 degrees
Tetrahedral = 109.5 degrees
Trigonal bipyramidal = ax and eq is 90 degrees and eq and eq is 120 degrees
Octahedral = 90 degrees
How do lone pairs modify bond angles?
Repulsion from lone pairs are greater that the bonded pairs as they are located closer to the central atom and thus take up more space this pushes the bonded pair of electrons closer together and lowers the angle between them.
BP - BP < BP-LP < LP-LP
How can you apply VSEPR theory with molecules containing coordination bonds?
VSPER theory is applied as normal and the shape of the molecule is not affected by the presence of coordination bonds.
How can you apply VSEPR theory with molecules containing multiple bonds?
When applying VSPER theory only the first bond between two atoms significantly affects the the structure, so the shape should be deduced assuming the molecule only contains single bonds. The electron count is then
1. valence electrons at central atom
2. valence electron for every single bond present
3. -1 electron for every double bond and -2 electrons for every triple bond.
however like lone pairs multiple bonds take up more space around the atom so can make the bond angles smaller than expected.
How can you apply VSEPR theory with molecules containing resonance?
VSPER theory should be applied to one of the resonance forms as normal, the double bond is delocalised due to the resonance meaning all bond lengths are the same.
e.g. CO3^2-
3 electrons from C
3 electrons from C-O bonds
-1 electron for double bond
6 total electrons
3 electron pairs
so trigonal planar shape with bond angles off 120 degrees.
What are the shapes of extended structures of carbon?
The extended strutters of carbon are diamond and graphite. Diamond contains tetrahedral carbons atoms in an extended network of covalent bonds (one of the hardest natural substance).
Graphite contains trigonal planar carbon atoms arranged in layers held together by weak intermolecular forces (very soft and electrical conductor)
