Atomic Structure and Calculations

what is an isotope (1)

atoms with the same number of protons but different number of neutrons

What did Jason dalton discover (2)

1. Atoms are spheres

2. Each element is made up from different spheres

What did Jj Thompson discover (3)

1. The electron

2. That atom was made up of other particles

3. Developed plum pudding model

What did Ernest Rutherford discover (4)

1. The nucleus, it was very small and positively charged

2. Concluded atoms were made up of mostly empty space

3. Fired Alpha particles at gold leaf, which went through, proving it was mostly empty space

4. Some deflected proving the positive charge in centre

What did Bohr discover (1)

1. Electrons traveled in orbitals

what are the 4 steps of time of flight spectrometry (4)

• ionisation

• acceleration

• flight tube

• detection

what are the two ways to ionise an atom (2)

• electron impact

• electrospray ionisation

explain what electron impact is (4)

• a vaporised sample is injected at low pressure

• an electron gun fires high energy electrons at the sample

• this knocks out an outer electron

• this forms positive ions with different charges

what type of elements is electron impact used for (1)

elements/ substances with a low formula mass

what is a limitation of electron impact (1)

can cause larger ions to fragment

explain electro spray ionisation (5)

• the sample is dissolves in a volatile, polar solvent

• its injected through a fine needle giving a fine mist or aerosol

• the tip of needle has high voltage

• the sample gains a proton at the tip of needle

• the solvent evaporates away while the ions move through towards a negative plate

why is electro spray ionisation used for larger organic molecules (2)

• is a softer technique

• so it does not cause fragmentation

what is a mass spectrometer used for (2)

• to determine all the isotopes present in a sample of an element

• to therefore identify the element

why does a spectrometer need to be in a vacuum (1)

otherwise air particles would ionise and register on the detector

explain the relationship between mass and velocity (1)

lower mass means faster velocity

KE equation;

KE = ½ mv²

time, distance and velocity

t = d/v

velocity in terms of KE;

what occurs in the flight tube in spectrometry (2)

• the positive ions with smaller mass will move faster than the larger mass ones as they all have the same kinetic energy

• the ions are distinguished by different flight times

how are the ions detected in spectrometry (1)

ions reach negative plate which generates a small current which is fed to a computer for analysis

how is relative abundance determined in mass spectrometry (1)

the size of the current is proportional to the abundance of the species

relative atomic mass equation ;

RAM = total (isotopic mass X % abundance) / 100

name the sub energy levels and how many electrons they hold (4)

• s- 2 electrons

• p- 6 electrons

• d- 10 electrons

• f- 14 electrons

what is the electronic structure of the d-block elements (1)

• when d-block elements form ions, they lose electrons from 4s electrons first

what is the definition of first ionisation energy (1)

is the enthalpy change when one mole of a gaseous atom forms one mole of gaseous ions with a single positive charge

what is the definition of second ionisation energy (2)

the enthalpy change when one mole of gaseous ions with a single positive charge forms when one mole of gaseous ions with a double positive charge

what are the factors that affect first ionisation energy (3)

• the attraction of the nucleus

• the distance of the electrons from the nucleus

• shielding of the attraction of the nucleus

how does attraction from the nucleus affect the ionisation energy (1)

• the greater the attraction between the nucleus and outer electron, the higher the ionisation energy will be

how does the distance of the electron from the affect the ionisation energy (1)

the bigger the atom the further the outer electrons are from the nucleus and the weaker the attraction to the nucleus

how does shielding affect the ionisation energy (1)

the electron in an outer shell is repelled by electrons in complete inner shells, weakening the attraction of the nucleus

why are successive ionisation energies always larger (3)

• when the first electron is removed, a positive ion is formed

• the ion increases the attraction on the remaining electrons

• so the energy required to remove the next electron is larger

what is periodicity (1)

a repeating pattern across a period

why do first ionisation energies decrease down a group (2)

• the outer electrons are found in shells further from the nucleus

• and are more shielded so the attraction of the nucleus becomes smaller

why is there a general increase in the first ionisation energy across a period (2)

• the electrons are being added to the same shell which has the same distance from the nucleus and same shielding effect

• the number of protons increase, making the attraction of the nucleus greater

define avogadro’s number (1)

one mole of any specified entity contains 6.022 X 10²³ of that entity

define relative atomic mass (1)

average mass of one atom compared to one twelfth of the mass of a carbon 12 atom

define relative molecular mass (1)

the average mass of a molecule compared to one twelfth of the mass of a carbon 12 atom

define mole (1)

the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon 12

moles equations;

• concentration = moles / volume

• moles = mass / mr

how do convert degrees into kelvins (1)

add 273

gas equation ;

PV = nRT

where R is 8.31

density;

density = mass / volume

define the empirical formula (1)

the simplest ratio of atoms of each element in a compound

define molecular formula (1)

the actual number of atoms of each element in the compound

percentage yield ;

percentage yield = (actual yield / theoretical yield) X 100

percentage atom economy ;

percentage atom economy = (mass of useful products / mass of all reactants) X 100

why do chemists want a high percentage yield (1)

it means there has been efficient conversion of reactants to products

why do chemists want high atom economy(1)

so that the maximum mass of reactants ends up in the desired product

state the features of the current model that are not shown in the rutherford model (2)

• current model includes neutrons and protons whereas rutherford model does mot include either

• current model shows electrons in different orbitals whereas the rutherford model does not

define the mass number of an atom (1)

number of protons and neutrons

give two reasons why it is necessary to ionise the isotopes of an element before they can be analysed in a TOF mass spectrometer

• ions will interact with and be accelerated

• ions create a current when hitting the detector

describe how molecules are ionised using electrospray ionisation

• sample is dissolved in a volatile solvent

• injected through needle at high voltage

• each molecule gains a proton

give the first ionisation energy equation for Fe

Fe(g) —> Fe+(g) + e-

the first ionisation energies of the elements in Period 2 change as the atomic number increases.

explain the pattern in the first ionisation energies of the elements from lithium to neon (6)

• 1st IE increases

• due to proton increase and electrons are in same shell

• therefore the elements have similar shielding therefore stronger attraction between nucleus and outer e-

• B lower than Be

• as its outer electron is in 2p

• O is lower than N

• as 2 electrons in 2p need to pair and pairing causes repulsion

give 2 differences between the plum pudding model and the modern model of atomic structure (2)

• nucleus in modern model contains protons and neutrons

• electrons in modern model are arranged in orbitals

explain which one of the elements Mg/Al has lower first ionisation energy (3)

• Al

• outer electron in 3p sublevel

• higher in energy as its further from nucleus so easier to remove

state and explain if the chemical properties of isotopes differ (2)

• no difference in chemical properties

• because all isotopes have the same electron configuration

state and explain the trend in the 1st IE of the elements in group 2 from magnesium to barium (3)

• IE decreases

• as atoms get bigger due to more energy shells

• therefore weaker attraction of ion to lost electron

explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer (2)

• only ions will interact with and be accelerated by an electric field

• only ions will create a current when hitting the detector

outline how the TOF mass spectrometer is able to separate two species to give two peaks (4)

• positive ions are accelerated by an electric field

• to a constant KE

• the positive ions with lower m/z move faster than those wit greater m/z

• therefore they arrive first at the detector

state and explain the effect that rinsing a burette will have on the value of the titre (2)

• titre would increase

• as solution will be more dilute

explain the effect on the Mr caused by liquid in a gas syringe not vaporising (2)

• calculated mr will be greater than actual

• as mass recorded higher than mass of gas

what is the importance of percentage yield

allows you to calculate the max amount of product possible in the reactions

what is the importance of atom economy

allows you to minimise the amount of by products by maximising the mass of atoms that end up in the desired product

define the term concordant titres

titres that are within 0.1cm³ of each other

suggest the reasons, within an gaseous experiment, why the Mr of value determined from the experimental results differ from the actual Mr

• apparatus inaccuracy

• volume of gas syringes is greater than true volume therefore if V is too large, Mr is too small

• the temp measures is less than the temp of the gas in the syringe therefore if T is too small, Mr is too small

• measured mass of liquid transferred to the syringe is less than the actual mass transferred

suggest and explain a safety precaution that the student should take when using a toxic gas in an experiment (2)

• carry out in a fume cupboard

• to avoid toxic vapour