Atomic Structure - Unit 1, Section 1

Question 1

The structure of the atom

Answer 1

made up of protons, neutrons and electrons.

protons - 1, +1
neutrons - 1, 0
electrons - 1/2000, -1

most of the mass is concentrated in the nucleus - protons and neutrons found.

Question 2

what is the mass number? (A)

Answer 2

total number of protons and neutrons in the nucleus of an atom.

Question 3

what is the atomic number? (Z)

Answer 3

the number of protons in the nucleus of an atom.
it identifies the element.

Question 4

what is an ion?

Answer 4

an atom that has become positively or negatively charged by either gaining or losing electrons.

Question 5

what is an isotope?

Answer 5

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

Question 6

similarities and differences between isotopes

Answer 6

they have the same electron configuration so therefore have the same chemical properties.

have different physical properties as this tends to depend more on the mass of the atom.

Question 7

John Dalton’s atomic model - 19th Century

Answer 7

described atoms as solid spheres

different spheres made up the different elements

Question 8

J.J Thomson’s atomic model - 1897

Answer 8

concluded that an atom must contain even smaller, negatively charged particles - electrons.

made the ‘plum pudding model’ - solid sphere with negative electrons spread throughout.

Question 9

Ernest Rutherford’s atomic model - 1909

Answer 9

gold foil experiment with Geiger and Marsden.

they fired positively charged alpha particles at a thin sheet of gold.

the plum pudding model would mean that most of the alpha particles would be deflected slightly by the positive ‘pudding’.

however, most of the particles passed straight through and only a small number were deflected, meaning the plum pudding model was incorrect.

made the nuclear model of the atom - a tiny, positively charged nucleus at the centre, surrounded by a cloud of negative electrons. Most of the atom is empty space.

Question 10

Neils Bohr’s atomic model

Answer 10

scientists realised that electrons in a cloud would spiral into the nucleus and cause the atom to collapse, making Rutherford’s model incorrect.

Bohr proposed new model with 4 principles:

• electrons only exist in fixed orbits (shells)
• each shell has a fixed energy
• when an electron moves between shells, electromagnetic radiation is emitted or absorbed.
• because the energy of shells is fixed, the radiation will have a fixed frequency.

the frequencies were already known from experiments and the Bohr model fitted these observations.

Question 11

Other atomic models - Quantum model

Answer 11

Bohr model wasn’t 100% right as scientists discovered that not all electrons in a shell had the same energy.

they refined the model and included sub-shells.

Today’s model is based on quantum mechanics. The quantum model explained observations that cannot be accounted for by the Bohr model.

Question 12

what is Relative Atomic Mass? (Ar)

Answer 12

the average mass of an atom of an element relative to 1/12th the mass of an atom of carbon-12.

Question 13

what is relative isotopic mass?

Answer 13

the mass of an atom of an isotope of an element relative to 1/12th the mass of an atom of carbon-12.

Question 14

calculating relative atomic mass

Answer 14

isotopic masses x percentages / 100

Question 15

calculating relative molecular mass (Mr)

Answer 15

the average mass of a molecule relative to 1/12th the mass of an atom of carbon-12.

add up all relative atomic mass values of all the atoms in the molecule.

Question 16

calculating relative formula mass

Answer 16

the average mass of a formula unit relative to 1/12th the mass of an atom of carbon-12.

add up all the relative atomic masses of all the ions in the formula unit.

Question 17

what is a mass spectrometer?

Answer 17

a machine used to analyse elements or compounds. it can provide information about the relative atomic mass of an element and the relative abundance of its isotopes, or the relative molecular mass of a molecule if you use it to analyse a compound.

Question 18

how a TOF mass spectrometer works

Answer 18

IONISATION x2
- the sample needs to be ionised before it enters the mass spectrometer.
-Electrospray ionisation = the sample dissolved in a solvent and pushed through a small nozzle at high pressure. A high voltage is applied to it, causing each particle to gain a H+ ion. The solvent is then removed, leaving a gas made up of positive ions.
- Electron impact ionisation = the sample is vaporised and an electron gun is used to fire high energy electrons at it. This knocks one electron off each particle so they become 1+ ions.

ACCELERATION
- the positive ions are accelerated by an electric field. The electric field gives the same kinetic energy to all the ions. The lighter ions experience a greater acceleration than heavier ions as they accelerate more easily.

ION DRIFT
- the ions enter a region with no electric field. They drift through at the same speed as they left the electric field, so the lighter ions will be drifting at higher speeds.

DETECTION
- because the lighter ions travel through the drift region st higher speeds, they reach the detector first. The detector detects the current created when the ions hit it and records how long they took to pass through the spectrometer. This data is then used to calculate the mass/charge values needed to produce a mass spectrum.

Question 19

what is a mass spectrum?

Answer 19

a type of chart produced by a mass spectrometer. it shows information about the sample that was passed through the spectrometer.

Question 20

Interpreting a mass spectrum

Answer 20

• if the sample is an element, each line will represent a different isotope of the element.
• the y-axis gives the abundance of ions as a percentage.
• for an element, the height of each peak shows the relative isotopic abundance.
• the x-axis units are given as a ‘mass/charge’ ratio. you can assume that the x-axis is the relative isotopic mass.

Question 21

identifying elements

Answer 21

mass spectrometry can be used to identify elements.

elements with different isotopes produce several lines in the mass spectrum as they all have different masses.
this produces characteristic patterns which can be used as ‘fingerprints’ to identify certain elements.

Question 22

calculating relative atomic mass (Ar) using mass spectra

Answer 22

1. for each peak, read the % relative isotopic abundance from the y-axis and the relative isotopic mass from the x-axis. Multiply them together to get the total relative mass for each isotope.
2. add up these totals
3. divide by 100

Question 23

electron configuration order

Answer 23

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f.

Question 24

electron configuration rules

Answer 24

1. fill up lowest energy sub-shells first.
2. fill orbitals in a sub-shell singly before they start sharing.

Question 25

Electron configuration of Chromium (Cr)

Answer 25

1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1.

donates one of its 4s electrons to the 3d sub shell as it is more stable.

Question 26

Electron configuration of Copper (Cu)

Answer 26

1s2, 2s2, 2p6, 3s2, 3p6, 3d10, 4s1.

donates one of its 4s electrons to the 3d sub shell as it is more stable.

Question 27

electron configuration of transition metals - odd thing

Answer 27

they lose their 4s electrons before their 3d electrons.

Question 28

electronic structure and chemical properties

Answer 28

the number of outer shell electrons decides the chemical properties of an element.

the S block elements (Group 1 and 2) have 1 or 2 outer shell electrons which are easily lost to form positive ions with a noble gas configuration.

the P block elements (Groups 5,6 and 7) can gain 1,2 or 3 electrons to form negative ions with a noble gas configuration.
Groups 4-7 can also share electrons when they form covalent bonds.

Group 0 elements have completely filled S and P sub shells so don’t gain, lose or share electrons.

Question 29

Ionisation

Answer 29

when electrons have been removed from an atom or molecule.

Question 30

first ionisation energy

Answer 30

the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

Question 31

Important points about ionisation energies

Answer 31

• must use the gas state symbol
  -always refer to 1 mole of atoms
• the lower the ionisation energy, the easier it is to form a positive ion

Question 32

factors affecting ionisation energy

Answer 32

1. nuclear charge - the more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons.
2. distance from nucleus - an electron close to the nucleus will be much more strongly attracted than one further away.
3. shielding - as the number of electrons between the outer electrons and nucleus increases, the outer electrons feel less attraction to the nucleus. this lessening of the pull of the nucleus is as a result of shielding.

Question 33

second ionisation energy

Answer 33

the energy needed to remove one electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions.

Question 34

general equation for successive ionisation energies

Answer 34

X(n-1)+ (g) -> Xn+ (g) + e-

Question 35

Ionisation trends down Group 2

Answer 35

• 1st ionisation energy decreases down Group 2.
• each element going down the group has one more electron shell than the one above. The extra shell will shield the outer electrons from the attraction of the nucleus. Also, the extra shell means the outer electrons will be further from the nucleus so the nucleus’s attraction will be reduced. Both these factors make it easier to remove outer electrons, resulting in lower ionisation energies.

Question 36

Ionisation trends across periods

Answer 36

as you move across a period, the general trend is for ionisation energies to increase. this can be explained by the fact that the number of protons is increasing, which means a stronger nuclear attraction.

Question 37

Drop between Group 2 and 3 - ionisation trends

Answer 37

Aluminium’s (G3) 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 to be found further from the nucleus, as the 3p orbital has additional shielding provided by the 3s electrons - Lower ionisation energy than Mg when it is expected to be higher.

Question 38

Drop between Group 5 and 6 - ionisation trends

Answer 38

The shielding is identical in the phosphorus (G5) and sulfur (G6) atoms and the electron is being removed from identical orbitals.
In phosphorus’s case, the electron is being removed from a singly occupied orbital, whereas in sulfurt it is being removed from a fully occupied orbital. The repulsion between the 2 electrons in this orbital means that the electrons are easier to remove, therefore sulfur has a lower ionisation energy.