Discovery of the atom
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atom
smallest unit quantity of an element that may exist alone or in chemical combintion with another element
atomic number
[z] - no. of protons or electrons
mass number
[A] - no. of protons and neutrons
isotope
forms of the same element that contain equal number of protons but different number of neutrons in their nuclei
how many atomic mass units (u) are equal to the mass of 1 atom of carbon-12?
12
what is 1u/Dalton (Da) equal to
1/12 x mass of 1 atom of carbon-12
how many atoms is equal to 1 mol of carbon-12
(6.022 x 10^23) = Avogadro’s constant
relative atomic mass
[Ar] - ratio of average mass of atoms of an element in a given sample to 1u
evidence for isotopes
mass spectrometry
key dates for model of atom discovery
1800s - John Dalton - each element has different atom type 1897 - J.J. Thomson - discovered the electron 1904 - Plum pudding model 1910 - Robert Millikan - charge on electron 1909-1911 - Ernest Rutherford - Gold-foil experiment -discovery of nucleus - disproved plum pudding model -proposed planetary model of atom (small positively charged nucleus balanced by charge of electron) 1932 - James Chadwick - discovery of neutron
conclusions from gold-foil experiment
• most alpha-particles passed through sheet ∴ atom mostly composed of empty space • nucleus = small and dense
discrete frequencies
missing characteristic of H when an electric discharge is passed through a sample of hydrogen
emission spectrum of hydrogen
recorded by passing an electric discharge through hydrogen gas splits molecules into atoms electrons within atoms get excited when these electrons return to ground state, they emit energy
electromagnetic radiation
wave with electric and magnetic properties
frequency
no. of wave crests that pass a fixed point per second symbol = V units = Hz
what happens to the wavelength as frequency increases?
decreases
what happens to the energy of EM radiation as frequency increases?
increases
what did Max Planck propose about EM radiation
[1900] -EM radiation could only be emitted or absorbed in quanta (= discretes packets with energy stored in them) -quanta = photons (units of light)
what does the energy of a photon depend on?
the frequency of the radiation high energy = high frequency (more dangerous i.e. skin damage + UV light) = short radiation wavelength
what did Albert Einstein propose about light?
[1905] - light energy was made up of photons explained the photoelectric effect = the way certain metals release electrons when UV falls on them each individual photon needs to have enough energy to remove e- from metal (radiation must reach threshold frequency, v)
work function
[Φ] = min. energy needed to eject electron from atom
wave-particle duality
light can demonstrate both wave-like and particle-like characteristics
Balmer series
hydrogen emission frequencies observed in the visible region of EM spectrum (n = 2) Lyman series - n = 1 Pashcen series - n = 3 Brackett series - n = 4 Pfund series - n = 5
emission spectrum
coloured lines on dark background