3.1.1 Atomic Structure Flashcards
What are atoms made up of?
Protons
Neutrons
Electrons
What is the relative mass and relative charge of a proton?
Relative mass = 1
Relative Charge = +1
What is the relative mass and relative charge of a neutron
Relative mass = 1
Relative Charge = 0
What is the relative mass and relative charge of an electron?
Relative Mass = 1/2000
Relative Charge = -1
What is the mass number?
Total number of protons and neutrons in the nucleus
What is the Atomic Number?
Number of protons in the nucleus - it identifies the element
All atoms of the same element have the same number of protons
What is the number of electrons in a neutral atom?
Number of electrons is the same as the number of protons (Atomic Number)
How do you find out the number of neutrons?
Mass number minus Atomic Number
What are Isotopes?
Atoms of the Same Element with Different Numbers of Neutrons
Define Isotopes
Isotopes of an element are atoms with the same number of protons but different number of neutrons
What decides the chemical properties of an element?
Number and arrangement of electrons
Why do Isotopes of an element have different physical properties?
Physical properties tend to depend on the mass of the atom
What is the Relative Atomic Mass (Ar)?
Average mass of an atom of an element on a scale where Carbon-12 is 12
What is Relative Isotopic Mass?
Mass of an atom, of an isotope, of an element on a scale where an atom of Carbon-12 is 12
What is Relative Molecular Mass (Mr)?
The average mass of a molecule on a scale where an atom of Carbon-12 is 12
How do you find out the Relative Molecular Mass (Mr)?
Add up the relative atomic mass values of all the atoms in the molecule
e.g.
Mr of C2H6O = (2x12) + (6x1) + 16 = 46
Why is Relative Atomic Mass not usually a whole number?
It is an average.
What is the Relative Isotopic Mass of Chlorine (e.g.)?
35
What is the Relative Atomic Mass of Chlorine (e.g.)?
35.5
What can a Mass Spectrometer tell you?
Relative Atomic Mass (Ar), Relative Molecular Mass (Mr) and Relative Isotopic Abundance
What are the 4 things that happen during Time of Flight (TOF) Mass Spectrometry?
1) Ionisation - Ionising sample by Electrospray Ionisation or Electron Impact Ionisation
2) Acceleration - positively charged ions are accelerated by an electric field so that they all have the same kinetic energy (lighter ions move faster than heavier ones)
3) Ion Drift - Ions enter region with no electric field, so they drift through. Lighter ions drift faster than heavier ones
4) Detection - as lighter ions travel @ higher speeds in drift region, reach detector in less time than heavier ions. Detector detects charged particles and a mass spectrum is produced
What is Electrospray Ionisation?
Sample is dissolved and pushed through small nozzle at high pressure.
High voltage is applied to it, causing each particle to gain an H+ ion (proton)
Sample is turned into a gas made up of +ve ions
What is Electron Impact Ionisation?
Sample is vaporised and an ‘electron gun’ is used to fire high energy electrons at it. This knocks one electron off each particle, so they become +1 ions
What is a Mass Spectrum?
Mass/Charge plotted against Abundance
How do you work out Relative Atomic Mass from a Mass Spectrum in 3 Steps?
1) For each peak, read the % abundance from y-axis and relative atomic mass from x-axis. Multiply them together to get the total mass for each isotope
2) Add up these totals
3) Divide by 100
How can Mass Spectrometry be used to identify Elements?
Elements with different isotopes produce more than one line in a mass spectrum because the isotopes have different masses.
This produces characteristic patterns which can be used as ‘fingerprints’ to identify certain elements
How can Molecules be identified by Mass Spectrometry?
1) Molecular ion, M+, is formed in the mass spectrometer when one electron is removed from the molecule
2) This gives a peak in the spectrum with a mass/charge ratio equal to the Relative molecular mass of the molecule
3) This can be used to help identify an unknown compound
What are electron shells made up of?
Sub-shells and Orbitals
What is the term used for the number of a shell?
Principal Quantum Number
Further away shell is from nucleus, higher its energy and the larger its Principle Quantum Number
What is the electron structure for Chromium (Cr)?
[Ar], 4s1, 3d5
What is the electron structure for Copper (Cu)?
[Ar], 4s1, 3d10
How does number of outer shell electrons decide the chemical properties of an element?
1) s block elements (Group 1 & 2) have 1 or 2 outer shell electrons. These are easily lost to form positive ions with an inert gas configuration
2) Elements in Groups 5, 6 and 7 (in the p block) can gain 1, 2 or 3 electrons to form negative ions with an inert gas configuration. Groups 4 to 7 can also share electrons when they form covalent bonds
3) Group 0 (inert gases) have completely filled s and p sub-shells and don’t need to bother gaining, losing or sharing electrons - their full sub-shells make them inert
4) The d block elements (transition metals) tend to lose s and d electrons to form positive ions
What is ionisation?
Removal of one or more electrons
What is the First Ionisation Energy?
Energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
What are 3 important points about ionisation energies?
1) You MUST use the gas state symbol, (g), because ionisation energies are measured for gaseous atoms
2) Always refer to 1 mole of atoms, as stated in the definition, rather than to a single atom
3) The Lower the ionisation energy, the Easier it is to form an ion
What factors affect Ionisation Energy
1) Nuclear Charge
2) Distance from Nucleus
3) Shielding
How does Nuclear Charge affect Ionisation Energy?
The More Protons there are in the Nucleus, the more positively charged the nucleus is and the Stronger The Attraction for Electrons
How does Distance from Nucleus affect Ionisation Energy?
Attraction falls rapidly w/ distance. An electron close to the nucleus will be much more strongly attached than one further away
How does Shielding affect Ionisation Energy?
As # of electrons b/w outer electrons and the nucleus increases, outer electrons feel less attraction towards the nuclear charge. Lessening of pull of the nucleus by inner shells is called Shielding (or Screening)
What does a high ionisation energy mean?
There is a high attraction b/w the electron and the nucleus so more energy is needed to remove the electron
What is Second Ionisation Energy?
Energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions
Why do successive ionisation energies increase within each shell?
Because electrons are being removed from an increasingly positive ion - there’s less repulsion amongst the remaining electrons, so they’re held more strongly by the nucleus
Why are there big jumps in ionisation energy?
These occur when a new shell is broken into - an electron is being removed from a shell Closer to the nucleus
What are the trends in First Ionisation Energies?
1) The 1st Ionisation Energies of elements down a group of the periodic table decrease
2) The 1st Ionisation Energies of elements across a period generally increase
Why does Ionisation Energy Decrease down Group 2?
1) Each element down has an extra electron shell compared to the one above, extra inner shells will shield the outer electrons from the attraction of the nucleus
2) Also, extra shell means that the outer electrons are further away from the nucleus, so the nucleus’s attraction will be greatly reduced
Why does Ionisation Energy Increase across a Period?
1) General trend is for the ionisation energies to increase
2) Number of protons is increasing = stronger nuclear attraction
3) Extra electrons are roughly @ same energy level, even if the outer electrons are in different orbital types
4) Generally little extra shielding effect or extra distance to lessen attraction from nucleus
5) Small drops between Groups 2 and 3, and 5 and 6.
Whys is there a drop in Ionisation Energy between Group 2 and 3?
Drop Shows Sub-Shell Structure
1) e.g. Al’s outer electron is in a 3p orbital rather than a 3s. 3p orbital has slightly higher energy than the 3s orbital
2) 3p orbital has additional shielding provided by 3s2 electrons
3) Both factors strong enough to override effect of increased nuclear charge, resulting in drop of ionisation energy
4) Pattern provides evidence for the theory of electron sub-shells
Why is there a drop in Ionisation Energy between Group 5 and 6?
Electron Repulsion
1) Shielding is Identical in the Phosphorus and Sulphur atoms, and the electron is being removed from an identical orbital
2) In Phosphorus’ case, electron is being removed from a singly-occupied orbital. But in Sulphur, the electron is being removed from an Orbital containing two electrons. Repulsion means that electrons are easier to remove from shared orbitals
3) More evidence for the electronic structure model
