atomic structure Flashcards
what was the Greek philosophers’ model of the atom
Greek philosophers had a model in which matter was made up of a single continuous substance that produced the four elements - earth, fire, water and air
the idea that matter was made of individual atoms was not taken seriously for another 2000 years
what were the steps taken that lead to the current atom model
1661:
Robert Boyle proposed there were some substances that could not be made more simpler. These were the chemical elements, as we know them
1803:
John Dalton suggested elements were composed of indivisible atoms.
All the atoms of a particular element had the same mass and atoms of different elements had different masses.
Atoms could not be broken down
1896:
Henri Becquerel discovered radioactivity. Radioactivity showed that particles could come from inside the atom. Therefore the atom was not indivisible
1897:
J.J Thomson discovered the electron. This was the first sub- atomic particle to be discovered. He showed that the electrons were negatively charged and electrons from all elements were the same
This lead to the creation of the plum- pudding model (J.J Thomson)
1911
Ernest Rutherford and his team found that most of the mass and all the positive charge of the atom in a tiny nucleus
what is the plum pudding model
as electrons are negatively charged, there had to be some source of positive charge. Furthermore, electrons are much lighter than the whole atom meaning there much be something that account for the rest of the atom.
Thompson therefore suggested that the electrons were located within the atom in circular arrays surrounded by positive charge, like plums in a pudding of positive charge
what are subatomic particles
the fundamental particles that atoms are made up of
these particles are:
protons
neutrons
electrons
the protons and nucleus make up the neutrons (sometimes referred to as nucleons)
electrons surround the nucleus
what is the mass of a proton
1.673 x 10 -27
what is the mass of a neutron
1.675 x 10 -27
what is the charge of electrons and protons
+/- 1. 602 x 10-19
what is the mass of an electron
0.911 x 10 -3 (very nearly 0)
what is the position of the electons
around the nucleus
what is the position of the neutrons and protons
in the nucleus
what is the charge of a neutron
0
what are the relative values of the mass and charges of protons neutrons and electrons
as the actual charges and masses are small, we use relative values
relative charges of proton and electron: +/- 1
relative mass of proton and neutron : 1
relative mass of electron:
1/1840
relative charge of neutrons: 0
what is the arrangement of subatomic particles
protons and neutrons are in the centre of the atom, held together by a force called the strong nuclear force.
electrons surround the nucleus
electrons are found in a series of levels (referred to as orbitals or shell)
the levels get further and further away from the nucleus
what are the nuclear forces
protons and neutrons are in the centre of the atom, held together by the strong nuclear force. This force is stronger than the electrostatic forces of attraction that hold electrons and protons together i the atom, so it overcomes the repulsion between the protons in the nucleus
- force only acts within the nucleus
what is the mass spectrometer
the mass spectrometer is the most useful instrument for the accurate determination of relative atomic Ar (relative atomic mass)
on what scale are relative atomic masses measured on
relative atomic masses are measured on a scale which the mass of an atom of C12 is defined as exactly 12
No other isotope has a realtive atomic mass that is exactly a whole number. This is because neither the proton or the neutron has a mass of exactly 1
what is the equation for the relative atomic/molecular mass
average mass of 1 atom/ 1/12 of mass of atom
average mass of 1 molecule/ 1/12 of mass of atom
what principle do all mass spectrometers follow
there are several types of mass spectrometer but all work on the principle of forming ions
- separating ions according to the ratio of their charge to their mass
what happens in a time of flight mass spectrometer
substance(s) in the sample are converted to positive ions
accelerated to high speed which depend on their mass to charge ratio)
they then arrive at the detector
what are the steps behind the flight mass spectrometer
- IONISATION
- ACCELERATION
- FLIGHT TUBE
- DETECTION
how are large molecules ionised in TOF
IONISATION
two types of ionisation:
ELECTROSPRAY
Is for larger molecules. 1.The sample is dissolved in a volatile solvent
2.forced through a fine hollow needle that is connected to the positive terminal of a high voltage supply. This produces tiny, positive charged droplets that have gained a proton from the solvent
3.Solvent evaporates from the droplet into the vacuum and the droplets get smaller and smaller until they can contain no more a singley positively ion
how are smaller molecules/ atoms ionised in TOF
the 2nd type of ionsiation is ELECTRON IMPACT
the sample must be a gas
1.sample is vaporised and high energy electrons are fired at it from an electron gun An electron gun is a hot wire filament with a current running through it that emits beam of high energy electrons
This usually knocks off one electron from each particle forming a 1+
the equation:
e.g.x(g) - x+ +e-
what happens in the 2nd stage of ionisation
the 2nd stage is ACCELERATION
The positive ions are accelerated using an electric field so that they have the same kinetic energy
Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass
- lighter particles = faster velocity
- heavier particles = slower velocity
what happens in the 3rd stage of TOF spectrometer
The positive ions travel through a hole in the negatively charged plate into a tube. The time of flight of each particle through this flight tube depends on its velocity which in turn depends on its mass
equation to work out velocity of ions travelling through the flight tube :
t= d/v
what happens in the 4th and final stage of TOF
DETECTION
the positive ions hit the negatively charged electric plate. When they hit the electron plate, the positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and hence a electric current that is measured. The size of the current gives a measure of the number of ions hitting the plate
what doe the computer do with the data collected at the end of TOF
a computer uses this data to produce a mass spectrum. This shows the mass to charge ratio and the abundance of each ion that reaches the detector
given that all ions produced by electospray and most ions by electron ionsistaion have a 1+ charge, the mass to charge ratio is effectively the mass of each ion
what is the relative atomic mass
the relative atomic mass is:
mean mass of an atom/ mass of 1 atom x 12
what is the atomic number Z
atoms consist of a tiny nucleus made up of protons and neutrons that is surrounded by electrons
The number of protons in the nucleus is called the atomic number/ proton number Z
why are atoms neutral
the number of electrons in the atom is equal to the proton number, so atoms are electrically neutral
what determines the chemical properties of an atom
the number of electrons in the full outer shell of an atom determines chemical properties of an element ( how it reacts) and what sort of element it is
what does the atomic number of an element show
chemical identity of an element and the number of protons
what are isotopes
every single atom of any particular element has the same number of protons in its nucleus therefore the same number of electrons. However, the number of neutrons may vary
Atoms with the same number of protons but different numbers of neutrons are isotopes
how do different isotopes of the same element react
different isotopes of the same element react chemically in exactly the same way as they have the same electron configuration
how are electrons arranged
electrons are arranged in electrons shells (energy levels), which themselves have sub shells
what are the different sub shells
there are 4 sub shells
s,p,d,f which increase in energy level
what do each sub levels consists of
each sub level consists of electron orbitals
what are orbitals
regions in space in which the electrons spends most of its time/ most likely to be in
how many electrons can orbitals consists of
each orbital can hold up to 2 electrons with opposite spin
how are electron orbitals filled up
1) AUFBAU PRINCIPLE
eletrons enter the lowest orbital available
2)HUND’S RULE
electrons prefer to occupy orbitals on their own and only pair up when no empty orbitals of the same energy are available
this is the order that orbitals fill up:
1s 2s 2p 3s 3p 4s 3d 4p
what is the maximum number electrons in each sub level
s- 2
p- 6
d- 10
f- 14
what electron is lost first when positive ions are formed
in ions, the electron in the highest energy levels are lost first
however there are some exceptions to this rule:
when losing electron, electrons are lost from 4s before 3d ( the energy levels are close, and when electrons fill, 4s goes before 3d)
what are the two elements that do not follow the trend
Cr: 1s2 2s2 2p6 3s2 3p6 4s1 instead of 1s2 2s2 2p6 3s2 3p6 4s2 3d4
Cu :1s2 2s2 2p6 3s2 3p6 4s1 3d10 instead of 1s2 2s2 2p6 3s2 3p6 4s2 3d9
what is ionisation energy
electrons can be removed from atoms and the energy it takes to remove them can be measured. This is called ionisation energy because as the electrons are removed, the atoms become positive ions
DEFINITION:
ionisation energy is the energy required to remove a mol of electrons from a mol of atoms in the gaseous state and is measured in kJ mol -1
what are successive ionisation energies
you can measure the energies to remove the electrons one by one from an atom, starting from the outer electron and working inwards
1st electron needs the least energy to remove it because it is being removed from a neutral atom. This is called the first IE (ionisation energy)
2nd electron needs more energy than the first because it is being removed form a 1+ ion. This is the 2nd IE.
the third needs more and so on
These are called successive ionisation energies
what does the pattern of ionisation energies across a period provide evidence for
provides evidence for electron energy sub- levels
how can you find the number of electrons in any element without using a periodic table
you can find the number of electrons in each main level of any element by looking at the jumps in successive ionisation energies
what is the general trend for ionisation energies across in the periodic table
Ionisation energies gradually increase across a period because the nuclear charge is increasing and this makes it more difficult to remove an electron. There is still the same number of shells - same shielding
This increase, however, is not regular - this is evident if you were to plot the ionisation energies of element across a period.
why is the increase in ionisation energies across a period not regular
e.g. from magnesium ( 1s2 2s2 2p6 3s2) to aluminium (1s2 2s2 2p6 3s2 3p3), the ionisation energies actually go down despite this increase in nuclear charge.
This is because the outer electron in aluminium is in a 3p orbital which is slightly higher than the 3s orbital. It, therefore, needs less energy to remove it
there is also a dip in ionisation energy between phosphorous ( 1s2 2s2 2p6 3s2 3p3) and sulfur (1s2 2s2 2p6 3s2 3p4).
In phosphorus, each of the three 3p orbitals contains just one electron, while in sulfur, one of the orbitals contain two electrons.
The repulsion between these paired electrons makes it easier to remove them, despite the increase in nuclear charge
what is the general trend in ionisation energies down a group in the periodic table
there is a general decrease in first ionisation energies down a group.
This is because the outer electron is in the main energy level that gets further from the nucleus in each case
- more shielding
- fewer forces of attraction felt between the positive nucleus and negative electrons
time of flight calcualtion (work out the length of the tube
1) find the mass of one atom by dividing the atomic number by 1000 then dividing that number by AVAGADRO’S CONSTANT
2) work out the velocity by using the KINETIC ENERGY EQUATION
3) work out the distance (in metres) by using the SPEED EQUATION ( distance = time x speed)
