sem 2 exam (unit 1 content) Flashcards
avogadro’s number
6.022 x 10^23 particles
mole
a precisely defined quantity of matter, equal to Avogadro’s number of particles/atoms/molecules/formula units
atoms
the smallest, indivisible building block of matter that can exist stably and independently, uniquely defining a chemical element
subatomic particles
Particle Location Relative Mass Charge
Proton (p+) Nucleus 1 +1
Neutron (n0) Nucleus 1 0
Electron (e-) Electron Cloud 1/1836 -1
isotopes
variations of same element with different numbers of neutrons
- same chemical properties e.g. reactivity, bonding, electron structure
- different physical properties e.g. density, mass, half-life, size, MP/BP
relative atomic mass
the weighted average of all elemental isotopes (calculated using relative isotopic abundance), as relative to 1/12 of the mass of a carbon-12 atom
mass number
the mass of one specific isotope of an element, generally in whole numbers (unlike relative atomic mass which is in decimal)
mass spectrometry
an accurate instrumental technique used to measure relative isotopic mass and relative abundance within a sample, and thus calculate relative atomic mass
steps of mass spectrometry
- vaporisation: sample enters the spectrometer in a gaseous form, after being vaporised in a vacuum chamber
- ionisation: the vapour is passed through a high-energy electron beam, where collisions with the beam result in loss of one (or sometimes two) electron/s, thus forming cations
- acceleration: resulting cations are accelerated by an electric field to form a high speed beam of positive ions
- detection: the high speed beam of positive ions are directed through a strong magnetic field, perpendicular to the ion’s path, as generated by the electromagnet, where ions are deflected into circular paths of different radii based on mass; lower mass / lighter ⇒ more deflection ⇒ smaller radius
- deflection: ions are collected with the current measured, then being graphed as relative abundance (y) over m/z mass to charge ratio (x) ⇒ most cations formed have +1 charge so the m/z ratio is usually numerically equal to mass (m) of the various ions; height of peak in graph is actually the relative intensity (proportional to relative abundance)
atomic absorption spectra
element-specific frequencies of electromagnetic radiation (light) at which energy is absorbed when transitioning up from a ground to an excited state
electron promoted to a higher level
continuous spectrum of light has specific frequencies (black lines) of light missing; gaps between the energy levels (absorbed by atom)
atomic emission spectra
element-specific frequencies of electromagnetic radiation (light) at which energy is emitted when transitioning down from an excited to a ground state
only specific frequencies of light emitted as coloured lines on a black background; the frequencies shown = energy emitted
flame test (basic emission spectroscopy)
heat a sample of chosen substance using a flame, thus exciting the e-
since the excited state is unstable, the electrons eventually drop back to ground state
energy is emitted as photons at characteristic frequencies
combinations of photon frequencies produce coloured light
the light is viewed through a spectroscope monochromator
emission lines can be matched to identify the element (QUALITATIVE)
uses: identification of unknown metals (metallic cations), firework displays, flares
atomic absorption spectroscopy
element must be previously determined through other qualitative methods (since this method determines concentration using a hollow cathode lamp of the same element)
sample is vaporised through an atomiser, resulting in a flame with hydrocarbon fuel, oxidants, and a gaseous form of the tested element
light from the lamp passes through the atomised sample; only the element tested would be able to absorb light; frequencies corresponding to energy levels as atoms
unabsorbed light is focused through a slit and enters a monochromator, separating wavelengths of interest
the selected wavelength goes through a detector, which numerically depicts the intensity of the light (i.e. measures the unabsorbed light), producing an absorbance value
absorbance value is directly proportional to elemental concentration
using many samples with known elemental concentration, a standard calibration curve is created using line of best fit, and the unknown sample’s absorbance value is compared, using interpolation to determine its concentration
strengths of flame tests
- quick and easy test for metal atoms
- convenient (easy to access materials)
weakness of flame tests
- qualitative data only (subjective)
- only a small range of metals are detectable with a flame test
(emissions may not be on the visible light spectrum) - metals in low concentrations may be difficult to observe
- mixtures of metals will produce confusing results
- used a standard flame
=> luminous with orangish hue that may obscure emitted colours
=> perhaps not hot enough to achieve proper excitation of metal
advantages of AAS
- quantitative data (comparable, easy to analyse)
- can handle/process mixtures of many metals
- highly selective for one metal to be tested
- can test larger range of elements
- very sensitive to low concentrations
ionisation energy
energy required for the process by which atoms lose electrons and ionise IN A GASEOUS STATE
electronegativity
the ability of an atom in a molecule to attract a pair of electrons in a covalent bond towards itself, depending on atomic radius and number of unshielded protons
first ionisation energy
energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of ions (M → M+ + e-); measure of strength of attraction between valence electrons and the nucleus
atomic radius
distance between nucleus and valence electrons
valency
a measure of an atom’s bonding capacity (combining power)
bonding
the forming of chemical bonds (either ionic, covalent network, covalent molecular, or metallic) to attain stability by having a full outermost valence shell
ion
atoms, or groups of atoms that are electrically charged due to the loss or gain of electrons
ionic bonding
After ions are formed (due to the gain or loss of electrons) to attain noble gas configurations, the ionic particles become electrically charged, yet are perfectly stable particles, capable of existing independently. However, whenever ions exist within a specific distance to each other (specifically a positively charged cation and a negatively charged anion), the electrostatic attraction between positive and negative charges is what holds the ion together to form an ionic lattice. An ionic lattice is a giant, theoretically endless crystalline structure, consisting of a consistently repeated pattern of cations, surrounded by anions, surrounded by anions, and so on.