atomic structure Flashcards
(34 cards)
what does an atom consist of?
a nucleus containing protons, neutrons surrounded by electrons
what did Democritus discover?
atom is the smallest and its indivisible
what did John Dalton discover?
all matter was composed of atoms, indivisible and indestructible building blocks, each element has different size atoms and masses
what did j.j Thompson discover?
the plum pudding model : atoms are a ball of positive charge with negative electrons embedded in it
what did Rutherford discover
did the alpha particle scattering experiment with gold foil which proved that the plum pudding model was wrong. he fired positively charged alpha particles at the gold foil. the results showed most passed straight through and some were deflected back, proving that the plum pudding model couldn’t be right
what did Niels Bohr discover?
the nuclear model, that electrons can only exist in fixed orbits or shells so each shell has a fixed energy
why was the nuclear model not accurate?
not all electrons in the same shell had the same energy so this model is wrong. its refined to be right by adding more subshells. this explains why some elements are inert. Elements reactivity is due to its electrons, so when the shell is full its inert and does not react.
what did James Chadwick discover?
provided the evidence to show the existence of neutrons within the nucleus and has a mass
what are the relative mass for electrons, protons and neutrons?
- electrons: 1/1840 - negligible
2.neutrons: 1 - protons: 1
what are the relative charges for protons, neutrons and electrons?
- electrons: -1
- protons: +1
- neutrons 0
what is the atomic number
number of protons in the nucleus
what is the mass number
total number of protons and neutrons in the atom
what are isotopes?
-Isotopes are atoms with the same number of protons, but different numbers of neutrons.
-Isotopes have similar chemical properties because they have the same electronic structure.
They may have slightly varying physical properties because they have different masses.
what is the relative atomic mass?
the relative atomic mass of an element is the average mass of an atom, compared to 1/12th the mass of a carbon-12 atom
how do you calculate the relative atomic mass of an element?
if percentage abundance is used: R.A.M = (isotopic mass x % abundance) divide 100
if relative abundance is used: R.A.M = (isotopic mass x relative abundance) divide
total relative abundance
what is the time of flight spectrometer used for
-The mass spectrometer can be used to determine all the isotopes present in a sample of an element and to therefore identify elements and used to determine relative molecular mass
- It needs to be under a vacuum otherwise air particles would ionise and register on the detector
what are the two ways that a sample can be ionised?
Two of these techniques are electron impact and electrospray ionisation
what happens in electron impact?
-A vaporised sample is injected at low pressure
-An electron gun fires high energy electrons at the sample
-This knocks out an outer electron
-Forming positive ions with different charges
e.g. Ti (g)– Ti+(g)+ e–
- Electron impact is used for elements and substances with low formula mass. Electron impact can cause larger organic molecules to fragment.
what happens in the electrospray ionisation?
-The sample is dissolved in a volatile, polar solvent
-injected through a hypodermic needle giving a fine mist or aerosol ( because its all positive so it repels)
-the tip of needle has high voltage
-at the tip of the needle the sample molecule, M, gains a proton, H+, from the
solvent forming MH+
-M(g) + H+ – MH+(g)
-The solvent evaporates away while the MH+
ions move towards a negative plate
-Electro spray ionisation is used preferably for larger organic molecules. The ‘softer’ conditions of this technique mean fragmentation does not occur
what happens in stage 2 of acceleration in the TOF?
-Positive ions are accelerated by an electric field
-To a constant kinetic energy through attraction to a negative charged plate KE = ½MV^2
KE = kinetic energy of particle (J)
m = mass of the particle (kg)
v = velocity of the particle (ms–1)
-Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass. Lighter particles
have a faster velocity, and heavier particles have a slower velocity.
what happens in stage 3 of flight tube?
*The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move
faster.
*The heavier particles take longer to move through the drift area.
*The ions are distinguished by different flight times
-t= d/v
t = time of flight (s)
d = length of flight tube (m)
v= velocity of the particle (m s–1)
what happens in stage 4 of detection in TOF?
- positive ions hit a negative metal plate and is detected as they gain electrons
- the electrons move from the plate to the ion because of force of attraction
- this produces an electric current (movement of electrons)
- the size of current is proportional to abundance of each isotope
what’s the definition of the first ionisation energy change?
the energy needed to remove one mole of electrons from one mole of atoms in their gaseous state to form one mole of 1+ ions (also in their gaseous state)
- removes one electron only in the outer shell and requires energy to overcome the force of attraction
what is the definition of the second ionisation energy?
The second ionisation energy is the enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions with a double positive charge
