3.1.1- Atomic Structure Flashcards
What letter is used to represent the atomic number of an atom?
Z
What does the atomic number tell us about an element?
atomic number = number of protons in an atom
What letter represents mass number?
A
How is the mass number calculated?
number of protons + number of neutrons
Define relative atomic mass
mean average mass of all atoms of an element (taking isotopes into account) relative to 1/12 mass of an atom of carbon12
What are isotopes of an element?
- different forms of the same element
- have the same number of protons (and electrons) but a different number of neutrons
- they still have the same chemical properties
What is the mass of a proton?
1
What is the charge of a proton?
+1
What is the mass of a neutron?
1
What is the charge of a neutron?
0 (no charge)
What is the mass of an electron?
0 (negligible)
What is the charge of an electron?
-1
define ionisation
process in which atoms lose or gain electrons and become ions
- cations (+)
- anions (-)
Define 1st ionisation energy
the energy required to remove one electron from one mole of gaseous atoms to form one mole of gaseous 1+ ions
1st ionisation energy general formula
M(g) = M+(g) + e-(g)
2nd ionisation energy general formula
M+(g) = M2+(g) + e-(g)
Trends in ionisation energies:
- ionisation energy as more electrons are removed
- more electrons removed = successive ionisation energy increases
-more significant jumps in ionisation energy where electrons are being removed from the next principle energy level ( as electrons are closer to the nucleus on nearer electron shells so are held more tightly)
Trends in ionisation energies:
group 2 elements and 1st ionisation energy
- nuclear charge increases down group 2 but the 1st ionisation energy decreases
- this means electrons are being removed from successively higher energy levels, which lie further from the nucleus and so are less attracted to the nucleus
- outer electrons also ‘shielded’ from nuclear pull by inner shells
Trends in ionisation energies
going along a period and ionisation energy (overall trend)
- as you go along a period (L-R), the 1st ionisation energy increases
- this is because as you go along a period, the number of protons in the nucleus increases so the attraction between the nucleus and outer electrons increases
- therefore amount of energy needed to remove an electron increases
Why are all ionisation energies endothermic?
- endothermic = positive energy changes and require energy
- endothermic as energy is needed to overcome the electrostatic attraction between electron + nucleus
what is an electron shell known as?
principle energy level
Max number of electrons for sublevel S
2
Max number of electrons for sublevel P
6
Max number of electrons for sublevel D
10
Max number of electrons for sublevel F
14
1st principle energy level:
- sub levels
-max number of electrons
- sublevels: 1S
- max number of electrons: 2
2nd principle energy level:
- sub levels
-max number of electrons
- sublevels: 2S, 2P
- max number of electrons: 8
3rd principle energy level:
- sub levels
-max number of electrons
- sublevels: 3S, 3P, 3D
- max number of electrons: 18
4th principle energy level:
- sub levels
-max number of electrons
- sublevels: 4S, 4P, 4D, 4F
- max number of electrons: 32
What does the Aufbau principle state?
lowest energy sublevels are occupied 1st
Sub levels (increasing energy)
1S, 2S, 2P, 3S, 3P, 4S, 3D, 4P
- positive ions lose electrons from 4S before 3D
What is an orbital?
a region around the nucleus than can hold up to two electrons
P sublevel
-number of orbits
-max number of electrons
-number of orbits: 3
- max number of electrons: 6
S sublevel
-number of orbits
-max number of electrons
-number of orbits: 1
- max number of electrons: 2
D sublevel
-number of orbits
-max number of electrons
-number of orbits: 5
- max number of electrons: 10
F sublevel
-number of orbits
-max number of electrons
-number of orbits: 7
- max number of electrons: 14
How many electrons can an orbital contain?
no more than 2 electrons (Pauli exclusion principle)
What would the relationship between 2 electrons in the same orbital in terms of their spin be?
have opposite spin as they repel each other as they are both negative
Hund’s rule
single electrons must occupy all empty orbitals within a sublevel before they start to form pairs in orbitals
Exceptions: Chromium
- Only has one electron in its 4S orbital before filling 3D
1S(2)…3P(6), 4S(1), 3D(5)
Exceptions: Copper
- only has one electron in its 4S orbital before filling 3D
-1S(2)…3P(6), 4S(1), 3D(10)
Time of flight mass spectrometer
- order
- ionisation area
- acceleration area
- flight path (ion drift)
- ion detector
Time of flight mass spectrometer
- ionisation area method 1
Electrospray:
sample dissolved in volatile solvent e.g. water/methanol and injected through a fine hypodermic needle to give an aerosol. needle attached to positive terminal of a high voltage power supply and particles gain a proton from the solvent as they leave the needle, producing XH+ ions (+1 charge and mass of Mr +1) (ions rarely fragment)
X(g) + solvent -H = XH+(g) + solvent-
(solvent- evaporates)
Time of flight mass spectrometer
- ionisation area method 2
Electron bombardment/impact:
normally sample is initially vaporised
electron gun (hot wire filament with current through it emitting high speed electrons) knocks of one electron from each particle to form 1+ molecular ions (these ions fragment)
X(g) + e- = X+(g) +2e-
When would you use electron impact/bombardment for ionisation in a mass spectrometer?
for organic or inorganic molecules with a low formula mass
When would you use electrospray for ionisation in a mass spectrometer?
for substances with a higher molecular mass including biological molecules e.g. proteins
Time of flight mass spectrometer
-acceleration
positive ions attracted towards a negatively charged plate. electric field is applied to the ions. each ion receives the same amount of kinetic energy. ions with a lower mass/charge ratio accelerate more quickly so ions with a lower mass experience greater acceleration
Time of flight mass spectrometer
- ion drift
ions pass through hole in the plate to the flight chamber, they drift through at constant velocity. ions form a beam with constant kinetic energy, travel along tube to detector. time of flight is therefore directly proportional to the square root of mass. ions with less mass (lower mas/charge ratio) pass through most quickly
Time of flight mass spectrometer
-detection
positive ions pick up electrons, current flows (size of current depends on number of ions arriving), m/z value and time of flight recorded. largest current from most abundant ions
How to find relative atomic mass
(abundance *relative mass of isotope) /100
