C1 - Atomic structure Flashcards
Define the term relative isotopic mass based 12c
the mass of the isotope on a scale on which the mass of a carbon 12 atom is exactly 12 units
Describe the structure of an atom in terms of electrons, protons and neutrons
Protons and neutrons are located in the nucleus. Electrons are located in energy levels surrounding the nucleus.
Describe the relative mass and relative charge of protons, neutrons and electrons
Protons have a relative mass of 1 and a relative charge of +1
Neutrons have a relative mass of 1 and a relative charge of 1
Electrons have a relative mass of 1/1836 and a relative charge of -1
Describe what is meant by the terms ‘atomic (proton) number’ and ‘mass number’
Atomic (proton) number is the number of protons in the nucleus of an atom. Mass (nucleon) number is the total number of protons and neutrons in the nucleus of an atom
Describe how to determine the number of each type of sub-atomic particle in an atom, molecule or ion from the atomic (proton) number and mass number
number of protons = atomic number.
number of electrons = atomic number.
number of neutrons = mass number - atomic number.
Define the term ‘isotopes’
Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
Define the term ‘relative isotopic mass’ based on the 12c scale
The mass of the isotope on a scale on which the mass of a carbon-12 atom is exactly 12 units.
Define the term ‘relative atomic mass’, based on the 12c scale
The weighted average of the masses of the isotopes on a scale on which the mass of a carbon-12 atom is exactly 12 units.
Define the term ‘relative molecular mass’
The ratio of the average mass of one molecule of an element or compound to one twelfth of the mass of an atom of carbon-12.
Define the term ‘relative formula mass’,
The relative formula mass of a substance made up of molecules is the sum of the relative atomic masses of the atoms in the numbers shown in the formula . Relative formula mass has the symbol, M r.
Describe how to calculate relative formula mass from relative atomic masses
Add up all of the relative atomic masses () of all elements in the formula.
What is produced by a mass spectrometer?
A mass spectrometer produces a mass spectrum which shows lines at m/z where ions of that mass are present.
What does m/z represent on a mass spectrum?
mass / charge ratio
Describe how to identify the molecular ion on a mass spectrum
In the mass spectrum, the heaviest ion (the one with the greatest m/z value) is likely to be the molecular ion.
What is shown by the relative heights of the peaks on a mass spectrum?
The relative heights of the peaks on the mass spectrum show the relative abundance of the different ions present.
Describe how to calculate the Ar of an element from a mass spectrum
∑ (isotope mass x isotope abundance) of all isotopes / 100
Predict the mass spectra, including relative peak heights, for a diatomic chlorine molecule
Chlorine has two isotopes 35 and 37. The mass spectrum for a chlorine molecule will have peaks at m/z 35 and 37 for the two Cl+ ions.There will be three peaks for the possible Cl2+ ions - 35 + 35 = 70, 35 + 37 = 72, 37 + 37 = 74. The relative heights of the 70, 72 and 74 lines are in the ratio 9:6:1 due to the ratio of the Cl35 and Cl37 isotopes
Describe how mass spectrometry can be used to determine the relative molecular mass of a molecule
The m/z value for the heaviest ion, the molecular ion (m+), is the relative molecular mass of the molecule
Define the term ‘first ionisation energy’
The energy involved in removing one mole of electrons from one mole of atoms in the gaseous state.
Define the term ‘successive ionisation energies’
The energy that is required to remove the electron one after the other. Successive ionization energy will depend upon the number of electrons present in the outermost shell.
Describe how ionisation energies are influenced by the number of protons, the electron shielding and the electron sub-shell from which the electron is removed
Nuclear charge increases with increasing atomic number, which means that there are greater attractive forces between the nucleus and outer electrons, so more energy is required to overcome these attractive forces when removing an electron.
Electrons in shells that are further away from the nucleus are less attracted to the nucleus so the further the outer electron shell is from the nucleus, the lower the ionisation energy.
The shielding effect is when the electrons in full inner shells repel electrons in outer shells preventing them to feel the full nuclear charge so the greater the shielding of outer electrons by inner electron shells, the lower the ionisation energy.
Spin-pair repulsion: paired electrons in the same atomic orbital in a subshell repel each other more than electrons in different atomic orbitals; this makes it easier to remove an electron (which is why the first ionization energy is always the lowest).
Explain why there is a general increase in first ionisation energy across a period
Across a period the nuclear charge increases, the distance between the nucleus and outer electron remains reasonably constant and the shielding by inner shell electrons remains the same.
There is a slight decrease in 1st I.E. between beryllium and boron as the fifth electron in boron is in the 2p subshell which is further away from the nucleus than the 2s subshell of beryllium. There is a slight decrease in 1st I.E. between nitrogen and oxygen due to spin-pair repulsion in the 2p subshell of oxygen.
Explain why first ionisation energy decrease down a group
Although going down a group the nuclear charge increases, the ionisation energy down a group decreases and it is due to the following factors:
The distance between the nucleus and outer electron increases, the shielding by inner shell electrons increases and so the effective nuclear charge is decreasing as shielding increases.
Describe how atomic emission spectra provide evidence for the existence of quantum shells
Spectral lines give evidence of electrons moving from one energy level to another within the atom.