Unit 1: Atomic structure and the periodic table
discuss using atomic number as guide to an elements structure other than atomic mass
Atomic number shows the number of protons in the nucleus; hence the number of electrons and the electron configuration. The atomic number determines the position of the element in the periodic table. The chemistry of an element is strongly related to the periodic group it belongs to and also the row in the table in which it belongs. However, the atomic number does not show how many neutrons there are in the nucleus. The relative atomic mass is only a rough guide to the number of neutrons because this varies depending on the number of isotopes. Relative atomic mass is an important quantity when determining formulae and reacting masses but it is of less value than atomic number as a guide to chemistry. There are examples in the periodic table where ordering elements in terms of relative atomic mass puts elements in the wrong group (for example Te and I).
example - silicon can exist as 3 isotopes. 92.23% of silicon is 28si and 4.67% is 29si. given the ar of silicon is 28.1 calculate the abundance and isotopic mass of the third isotope.
1) first find abundance of third isotope- 100 minus 92.23 minus 4.67 = 3.1% 2) you know the ram of silicon is 28.1 so. put all into the equation 28.1 = (28x92.23) + (29 x 4.67)+ ( X x 3.1) / 100 28.1 = (2717.87 + ( X x 3.1) / 100 2810-2717.87 = X x 3.1 x = 29.719 so isotopic mass of third isotope is 30. 30si isotopic masses are usually whole numbers so round answer to nearest whole number
give the first second and third ionisation energy of NA
1)Na (g) -> Na+ (g) + e- 2)Na+ (g) -> Na2+ (g) + e- 3) Na2+ (g) -> Na3+ (g) + e-
When was daltons academic theory published and what did it contain?
1808- all elements are made up of indivisible particles called atoms. all the atoms of a given element are identical and have the same mass, the atoms of different elements have different masses, atoms can combine to form molecules in a compound, all the molecules of a given compound are identical
when was the plum pudding model published and describe it
1904. tiny balls of positive material, mass concentrated throughout atom , electrons embedded in it, sea of uniform positive charge
who invented the mass spectrometer and when
1919 F.W Aston, gave scientists an accurate method of comparing relative atom masses of atoms and molecules.
give the electronic configuration of scandium
1s2. 2s2 2p6 3s2 3p6 3d1 4s2
(ii) Explain how these ionization energies give evidence for the electronic structure of sodium. You may use a sketch graph if you wish.
2 Large jumps, 2,8,1
how many electrons can each quantum shell hold
2,8,18,32
When and What did Democritus suggest ?
2400 years ago he suggested that all substances are made of atoms however he did no experiments and had no way to tests his ideas so failed to convince others his theory was correct.
describe the structure of an atom?
3 subatomic particles - protons neutrons and electrons. electrons are outside the nucleus in orbitals. mass concentrated in centre ( nucleus), diameter of nucleus is much smaller than the rest of the atom, protons and neutrons in nucleus . large region of empty space
discuss: the chemical properties of an element are largely determined by the number of electrons in the outer shell of its atom
Elements in the same group have similar properties because they have the same outer electron configuration. Elements in the same group do not have identical properties. There are trends down groups - the number of inner full shells affects the chemical characteristics of an element. Across a period there is shift in properties from metals on the left to non-metals on the right. This reflects the effect of the increasing nuclear charge as the number of electrons in inner full shells stays the same. This increasing effective nuclear charge across a period means that electrons are held more strongly to the atom. Across a period the atoms have a declining tendency to lose electrons and form positive ions but greater tendency to gain electrons and form negative ions.
predict which element has the highest IE and explain
Helium. Ionisation energies rise from left to right across each period [1] and fall down each group [1], suggesting that the element that is top-right in the table has the highest first ionisation energy. [1] (Electrons in a helium atom are in the innermost shell with no shielding from the 2+ nuclear charge.)
how is the position of d block elements accounted for in the periodic table
In Period 4, the sub-shells fill in the order: 4s > 3d > 4p. So the 3d orbitals do not start to fill until there are two electrons in the s orbital. The same is true in Period 5. [1] Hence the d- block elements lie between the s- and p-blocks for Periods 3 and above
why are the first and second Ie of sodium smaller than that of lithium
Sodium has an additional electron shell compared to lithium. [1] The nuclear charge is greater in sodium [1], but the outermost electron is further from the nucleus [1] and the shielding effect is greater in sodium. [1] The increased distance of the electron from the nucleus and the increased shielding effect outweigh the increased nuclear charge [1] and therefore the ionisation energies in sodium are less than those of lithium
practise - explain why the relative atomic mass of copper is not a whole number
a sample of copper is a mixture of 2 isotopes in different abundances. the relative atomic mass is an average mass of these isotopes which isn't a whole number
When did modern atomic theory start to grow
about 2000 years after Democritus when scientists in Europe started to purify substances to Carry out experiments with them. they found that many substances could be broken down into simpler substances Called elements which could then be combined to make compounds.
describe orbitals
an orbital is an region of space around the nucleus that the electron moves in/ in which there is 95% chance of finding an electron. orbitals within the same sub shell have the same energy. electrons in each orbitals spin in opposite directions- spin pairing. s orbitals are spherical, p orbitals have dumbbell shapes. there are 3 p orbitals at right angles to eachother.
Explain why the melting temperature of argon is the lowest of all the elements of Period 3.
argon is monatomic
why does the 4s sub shell fill up before the 3d
as the shells get further from the nucleus they become closer in energy, the difference in energy between the second and third shell is less than the first and second. when the fourth shell is reached there is an overlap between the orbitals of highest energy in the third shell and that of the lowest energy in the fourth shell. the 4s shell has a lower energy Level than 3d sub shell even though its principal quantum number is bigger, therefore it fills first,
why does ionisation energies decrease down the group
as you go down a group it becomes easier to move electrons because elements further down have more electron shells, extra shells means a larger atomic radius so the outer electrons are further from the nucleus, also the extra inner shells shield the outer electrons from the full attraction of the nucleus . a decrease in ionisation energy going down a group provides evidence that electron shells really exist.
why do ionisation energies increase across a period
as you move across a period the general trend is for the ionisation energies to increase. this is because the number of protons is increasing which means a stronger nuclear attraction. all the extra electrons are roughly at the same energy level even if the outer electrons are in different orbital types. this means there is generally little extra shielding effect or extra distance to lessen the attraction from the nucleus .
which 2 ideas of daltons were not true?
atoms are not indivisible and all atoms of the same element are not identical
why were daltons ideas not accepted by some but accepted by others ?
atoms were too small to see modern day microscopes not invented, however atomic theory caught in because it could explain the results of many experiments
what is an isotope
atoms with the same number of protons but different number of neutrons- e.g cl35 and cl37-different mass numbers
why do lines get closer together in a line spectrum
because energy levels get closer together with increasing energy
why die elements in each group have similar properties
because they have similar electron structures
how were electrons first discovered?
by JJ Thomson in his laboratory in 1897. he connected 15000 volts across the terminals of a tube containing air, glass walls glowed bright green. rays travelling in straight lines from the negative terminal had hit the glass causing it to glow. experiments showed the narrow beam of rays could be deflected by an electric field - always bent towards positive plate
4 basic principles of electron shells
can only exist in fixed orbits, each shell has fixed energy when an electron moves between shells electromagnetic radiation is emitted or absorbed, radiation has a fixed frequency
what are the p block elements
comprises the elements in groups 3,4,5,6,7 and 0 on the right of the periodic table. include relatively un reactive metals such as tin and lead plus all the non metals. the last electron added goes into a p orbital in the outer shell
how to electrons fill shells
electrons fill up the lowest energy level first , they fill orbitals singly before they begin pairing up.
describe electron structure
electrons move around the nucleus in quantum shells ( sometimes called energy level) . shells further from the nucleus have a greater energy level than those closer. the shells contain different types of sub shell , these sub shells have different orbitals which can each hold 2 electrons
why are ionisation energies endothermic
energy is taken in by the reaction so the energy change is a positive sign.
what did Rutherford do
fired alpha rays at thin gold foil only a few atoms thick, expecting the particles to pass straight through or be deflected. results showed most of the particles went through, some were deflected and a few rebounded.
how can we predict the mass spectra of diatomic molecules example - chlorine has 2 isotopes. 35cl has an abundance of 75% and 37cl has an abundance of 25% predict the mass spectrum of cl2.(page 8 cgp)
first we express both % as a decimal. 0.75 and 0.25 then we make a table showing all the different cl2 molecules. for each molecule multiply the decimal abundance of the isotopes to get relative abundance of each one. different molecules = 35cl-35cl, 35cl-37cl, 37cl-35cl, 37cl-37cl look for any molecules in the table that are the same and add their abundances - in this case 35cl-37cl and 35cl-37cl are the same so the actual abundance for this molecule is 0.1875 `+ 0.1875 = 0.375 divide all the relative abundances by the smallest relative abundance to get the smallest whole number ratio. using the relative molecular mass of each molecule you can predict the mass spectrum, eg relative molecular mass for 35cl-35cl = 35 + 35 = 70
stages of a mass spectrometer
firstly ionised into positive ions in the gaseous vapour state- bombarded with a beam of high energy electrons knocking out one or more electrons, accelerated by a electric field and then deflects onto a detector by magnetic field or accelerates the ions and separates them via flight time through a field free region a third type (transmission quadrupole instrument, reliable easy and compact) varies the fields in the instrument to allow ions with a particular mass to charge ratio to pass through the detector at any one time. ion detector then gives an electrical signal that is converted to a digital response and stored in a computer.
why do successive ionisation energys get more endothermic
for the first ionisation energy an electron is being removed from a neutral atom. for the second and subsequent ionisation energies an electron is being removed from an ion with an increasingly positive charge. the process of removing an electron becomes more difficult and therefore more endothermic, the last electrons feel the full attraction of the positive nucleus and are the hardest to remove.
further discover of electrons?
further study showed that the rays consisted of tiny negative particles, 2000 lighter than hydrogen atoms. Thomson obtained the same electrons with different gases in the tube and when the terminals were made of different substances suggesting all atoms contained electrons. knew atoms had no charge over all so must have a positive charge to balance.
how does an emission spectrum support the idea that energy levels are discrete
has clear lines for different energy levels, it means That an electron doesn't move from one energy level to the next it just jumps with no in-between stage at all. emission spectra provide evidence that electrons exist in quantum shells
what does the development of knowledge and understanding about electronic structures illustrate
how chemists use the results of their experiments, such as the measure of ionisation energy, To devise atomic models that they can use to explain the properties of elements, it also illustrates the important distinction between evidence and experimental data on the one hand and ideas theories and explanations on the other
what problem remained for Rutherford.
if hydrogen atoms contained one proton and helium atoms contained 2 protons then the relative masses of hydrogen and helium should be one and 2 respectively . but themes of helium atoms relative to hydrogen atoms is 4 not 2. it took the discover of isotopes, neutrons and much further research before the problem was solved
why is there a drop in IE between groups 5 and 6
in general elements with singly filled or full sub shells are more stable than those with partially filled subshells so have higher first Ies. there is a drop between P and S. the shielding is identical in the P and S atoms and the electron is being removed from an identical orbital. in P's case the electron is being removed from a singly occupied orbital but in S the electron is being removed from an orbital containing 2 electrons. the repulsion between 2 electrons in an orbital means that electrons are easier to remove from shared orbitals
why is there a drop between groups 2 and 3 in ionisation energies
in general it requires moe energy to remove an electron from a higher energy subshell than a lower . but Als outer electron is in a 3p orbital rather than a 3s, the 3p orbital has a slightly higher energy than the 3s orbital so the electron is on average to be found further from the uncles. the 3p orbital has additional shielding provided by 3s2 electrons, both these factors are strong enough to override the effect of increases nuclear charge resulting in ionisation energy dropping slightly, provides evidence for sub shells
why do elements have flame colours
in their ground state atoms have their electrons in their lowest possible energy levels if an atoms electrons take in energy from their surroundings they are said the be excited and move to higher energy levels further from the nucleus, electrons release energy when they fall back down from this higher level to a Lower level . the energy levels all have certain fixed values
name one other factor that provides evidence that support our current understanding of electrons existing in fixed energy levels
ionisation energy
give examples of periodic properties
ionisation energy, atomic radius , boiling point , formulae of compounds
Explain why magnesium is a good conductor of electricity whereas sulfur is a non-conductor.
magnesium has delocalised electrons, sulfurs are fixed
describe the nuclear symbol
mass number on top ( protons and neutrons) atomic number on bottom ( protons) . negative ions have more electrons than protons and positive ions have fewer electrons than protons)
why do isotopes have different physical properties. but same chemical properties
mass numbers decide physical properties such as different densities, rate of diffusion etc because physical properties tend to depend more on the mass of an atom, isotopes have different mass numbers. isotopes have same chemical properties because it has the same configuration ( number and arrangement) of electrons.
what does quantum mean
means fixed
how to calculate the relative atomic mass Ar of an element from its isotopic abundances?
mulitply each relative isotopic mass by its % relative isotopic abundance and add up the results. divide by 100
how to work out relative atomic mass from graph
multiply each relative isotopic mass by its relative isotopic abundance and add up the results, divide by the total sum of isotopic abundances.
how can we use C14 for radio dating?
n living organisms, the relative amount of 14C in their body is approximately equal to the concentration of 14C in the atmosphere. When an organism dies, it is no longer ingesting 14C, so the ratio between 14C and 12C will decline as 14C gradually decays back to 14N. This slow process, which is called beta decay, releases energy through the emission of electrons from the nucleus or positrons. After approximately 5,730 years, half of the starting concentration of 14C will have been converted back to 14N. This is referred to as its half-life, or the time it takes for half of the original concentration of an isotope to decay back to its more stable form. Because the half-life of 14C is long, it is used to date formerly-living objects such as old bones or wood. Comparing the ratio of the 14C concentration found in an object to the amount of 14C in the atmosphere, the amount of the isotope that has not yet decayed can be determined. On the basis of this amount, the age of the material can be accurately calculated, as long as the material is believed to be less than 50,000 years old. This technique is called radiocarbon dating, or carbon dating for short.
3 factors affecting ionisation energy
nuclear charge- more protons there are the more positively charged the nucleus and stronger electron attraction diameter- attraction falls very rapidly with distance , an electron in a shell closer to the nucleus will be much more strongly attracted shielding - as the number of electrons between the outer electrons and the nucleus increases the outer electrons feel less attraction towards the nuclear charge /
what can protons and neutrons be called together
nucleon - nucleon number can be used as an alternative to mass number
what are the d block elements
occupies a rectangle between groups 2 and 3 and across periods 4,5,6. the d block elements are all metal, including titanium, iron copper and silver in which the last element added goes into a d orbital. much less reactive than s block. . many similarities across a period, loosely called transition metals.
what causes the m+1 peak
occur due to the presence of any atoms of carbon-13
what is meant by the term periodic
patterns occurring at intervals.
why does atomic radius decrease across a period
protons increase, positive charge of nucleus increases pulling electrons closer to the nucleus , shielding remains constant as extra electrons are added to same energy level
what are the s block elements
reactive metals in group 1 and 2 , e.gK, Na , Ca and Mg, the outermost electron is in an s orbital in the outer shell
what is theRAM
relative atomic mass - is the average mass of an element relative to 1/12th of the mass of an atom of the isolate carbon 12. value are relative so they don't have units. not usually a whole number because it is an average of a mixture of isotopes of different percentages
what does the periodic table show instead of mass number
relative atomic mass, can often assumed to be equal to mass number
what is meant by periodicity
repeating trends in physical and chemical properties of an element from left to right across a periodic table
describe what the lines on an emission spectrum show
represent the frequencies of light that are emitted when a n electron drops from a high energy level to a lower one
how are electrons paired
represented by arrows in energy Level diagrams. when an energy level is filled the electrons are paired up and in each of these pairs the electrons are spinning in opposite directions. chemists have found that paired electrons can only be stable when they spin in opposite directions so that the magnetic attraction resulting from their opposite spins counteracts the electrical repulsion from their negative charges
what are the 4 sub shells
s,p,d,f 1s2, 2s2, 2p6, 3s2, 3p6, 3d10, 4s2, 4p6, 4d10, 4f14
descibe each orbital In the sub shells
s= spherical, p Is made up of 1 s (2s) orbital and 3 dumbbell shaped p orbitals, the three p orbitals 2px 2py and 2pz are arranged at right angles to each other along x y and z xis.
what is shielding
shielding is the effect of inner electrons which reduces the pull of the nucleus on the electrons in the outer shell of the atom, thanks to shielding the electrons in the outer shell are attracted by an 'effective nuclear charge' which is less than the full charge on the nucleus.
what does a line spectrum show.
shows the frequencies of light emitted when electrons drop down from a higher energy level to a lower one, they appear as coloured lines on a dark background. each element has different electron arrangement and so the frequencies of radiation absorbed and edited are different, this means the spectrum for each element is unique.
Explain why the melting temperature of silicon is very much greater than that of white phosphorus.
silicon is a giant covalent lattice whereas phosphorus is made from simple molecules
Explain why the melting temperature of sodium is very much less than that of magnesium.
sodium atoms are larger than magnesium atoms. na+ rather than mg2+- smaller charge density , sodium has fewer delocalised electrons, attraction between positive ion and delocalised electrons = therefore weaker , sodium is not close packed, magneisum is. less energy to break bonds
how did Chadwick discover neutrons (1932)
studied the side effects of bombarding a beryllium atom with alpha particles, producing a new kind of radiation with no electric charge but enough energy to release protons when fired at a material such as wax. Chadwick was able to demonstrate that there must be uncharged particles in the nuclei of atoms as well as protons. soon found that neutrons had the same weight as protons
how is a CO2 atom formed
the addition of a proton and loss of a neutron from atmospheric N14. Once produced N14 often combines with the oxygen in the atmosphere to form carbon dioxide. Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is incorporated by plants via photosynthesis. Animals eat the plants and, ultimately, the radiocarbon is distributed throughout the biosphere.
what is the ionisation energy
the energy needed to remove one mole of electrons from one mole of gaseous atoms, or ions of an elements . a high ionisation energy means theres a strong attraction between the electron and the nucleus so more energy is needed to overcome the attraction and remove the electron
what are the f block elements
the f block elements occupy a low triangle across periods 6 and 7 with the d block. are all metals, last electron in f orbital. f elements are often called lanthanoids and actinides because they are the 14 elements that immediately follow Lanthanum, La, and actinium, Ac,. or the inner transition elements.
what is the relative isotopic mass
the mass of one atom of an isotope relative to 1/12th of the mass of an atom of the isotope carbon 12. the values are relative so don't have units usually a whole number
what happens in the atom when energy is emitted
the movement of electrons from higher ti lower energy levels
which line in the spectrum represents the largest emission of energy
the one with the highest frequency
how can we predict the group in which an element belongs
the quantum shells of electrons correspond to the periods of elements in the periodic table. by noting where the first big jump comes in the successive ionisation energies of an element it is possible to predict the group.
what is the electromagnetic spectrum
the range of electromagnetic radiation
relative molecular mass and how to find it
the relative molecular mass is the average mass of a molecule or formula unit compared to 1/12th of the mass of a c12 atom.
why are the second ionisation energies of lithium and sodium larger than their first
the single outer electrons of sodium and lithium are shielded from the full attraction of the atomic nucleus by inner full shells. [1] So the effective nuclear charge is 1+ and the first electron is relatively easy to remove. [1] The second electron has to come from a full shell and there is little or no shielding effect for electrons in the same shell so the nuclear attraction for the electrons in this shell are much larger. [1]
relative formula mass
the sum of the relative atomic masses of all the atoms in its formula
describe melting and boiling points across a period
the typeof bond formed between atoms of an element changes across a period. for the metals boiling points increase across a period because metallic bonds get stronger. bonds get stronger because the metal ions have increasing number of delocalised electrons and a decreasing radius- higher charge density meaning stronger attraction between metal ions so stronger metallic bonding. the elements with giant covalent lattice have strong covalent bonds with require a lot of energy to break all of them , so carbon and silicon have the highest bp in their periods. next are the simple molecular structures, melting points depending on London forces, weak and easily overcome so low bp and mp. more electrons in a molecule mean stronger LF , e.g sulphur s8 has the most electrons so its got higher mpbp than phosphorus. noble tases have lowest mp because they are individual atoms. weak lf simple molecules
explain how emission spectra provide evidence that electrons exist in quantum shells
they show that specific amounts of energy are emitted when electrons drop down from high energy levels to low ones, in between amounts of energy are never emitted which suggest that the electrons only exist at very specific energy levels, (they are discrete )
how can we identify compounds from mass spectrometry?
to find the molecular mass of a compound, look at the molecular ion peak . this is the peak with the highest M/z value. ( ignoring any smaller m+1 peaks that occur due to the presence of any atoms of carbon-13). the m/z value is the molecular mass
properties of alkali metals
very reactive because they lose their single outer electron so easily. form ions with a charge of 1+, so similar formulae of compounds, form very stable ions with electron structure like that of a noble gas
what is the M peak
when the molecules in a sample are bombarded with electrons an electron is removed from the molecule to form a molecular ion m+.
what are the y and x axis of the mass spectrometer results
y axis = abundance of ions often as a percentage. the heigh of each peak gives relative isotopic abundance, x axis are given as m/z value ( mass to charge ratio ) since the charge on the ions is mostly +1 you can often assume the x axis is simply relative isotopic mass.
how can you calculate isotopic masses from relative atomic mass?
you have to know the RAM of an element and all but one of the abundances and relative isotopic masses of its isotopes. then you can work out the abundance and relative isotopic mass of the final isotope.