3.1.1 Atomic structure! Flashcards
(22 cards)
What did Dalton propose about atoms in the early 19th century?
Atoms were invisible, solid spheres.
Different elemernts had different types of spheres.
How did Thomson’s discovery of the electron change the atomic model?
He proposed the plum pudding model
A sphere of positive charge with embedded electrons.
Briefly describe Rutherford’s Gold Foil Experiment (1909)
Involved firing alpha particles (positively charged) at a sheet of thin gold inside a vacuum.
What did Rutherford discover through the gold foil experiment?
Atoms are mostly empty space with a tiny, dense, positively charged nucleus.
Rutherfor’s nuclear model = A central nucleus surrounded by orbiting electrons.
How were protons and neutrons discovered?
- Protons: via Moseley’s research on atomic number
- Neutrons: discovered by Chadwick to explain extra mass in nuclei.
How did Bohr improve Rutherford’s model?
Electrons orbit the nucleus in fixed energy levels (shells) and absorb/emit specific amount of energy.
Bohr’s model = Introduction of subshells to explain that electrons in the same shell can have different energies.
What are Isotopes?
Atoms of the same element that have the same number of protons but different number of neutrons.
Most elements are made up of a mixture of isotopes.
Do isotopes have the same chemical and physical properties?
- Same chemical properties as isotopes have the same electronic configuration.
- Slightly different physical properties (e.g. mass & density) as isotopes have different atomic mass.
Define relative isotopic mass (Ar)
The average mass of an atom of an element compared 1/12 of the mass of an atom of carbon-12.
Define relative molecular mass (Mr)
The average mass of a molecule of an element of compound compared to 1/12 of the mass of an atom of carbon-12.
How do you calculate relative atomic mass from isotopic abundances?
Ar = (∑ (isotopic mass × % abundance))
÷ 100
What does a mass spectrometer measure?
Measures the mass-to-charge ratio (m/z) of ions.
Name the four stages of a TOF mass spectrometer.
- Ionisation
- Acceleration
- Ion drift
- Detection
What are the two ionisation methods used in TOF mass spectrometry?
- Electron Impact: High energy electrons are fired at the sample from an electron gun. This knocks off 1 electron from each atom/element to 1+ ions (leads to fragmentation)
- Electrospray Ionisation: The sample is dissolved in a volatile solvent and injected through a fine hypodermic needle at a very high voltage. The particles gains a proton and become ions.
What happens during the acceleration stage?
Ions are accelerated by an electric field to the same kinetic energy.
What happens in Ion Drift?
Ions enter flight tube, particles travel with constant speed and kinetic energy.
Lighter ions & ions with lower m/z travel faster than heavier ones.
How are ions detected in a TOF spectrometer?
The detector is a negatively charged plate.
Ions gain an electron at the detector, creating a current. The current size reflects isotope abundance.
Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.
- Ions will interact and be accelerated by an electric field/negative plate.
- Ions create a currect when hitting detector.
What does a mass spectrum show?
- A graph of relative abundance vs m/z.
Peaks indicate isotopes or fragments, relative height shows how abundant particular isotope is. - For molecular samples, the highest m/z is the molecular ion peak (M⁺). Peaks at lower m/z come from fragments of molecular ion.
What does the M⁺ peak represent in molecular samples?
The molecular ion peak shows the molecule’s relative molecular mass (Mr).
What is the equation for time of flight?
😉
How do you calculate the mass (in kg) of one ion of an isotope?
Convert the mass number (in g/mol) to kilograms per mole by dividing by 1000.
Divide by Avogadro’s constant to get the mass of a single atom or ion (in kg/mol).
