atomic structure

Question 1

the structure of the atom:

Answer 1

atoms are made up of 3 types of particle- protons, neutrons and electrons. protons and neutrons exist in the nucleus of the atom whereas electrons exist in shells and sub shells around the nucleus.

protons: relative mass is 1, relative charge is +1

neutrons: relative mass is 1, relative charge is 0

electrons: relative mass is 1/1840, relative charge is -1

Question 2

mass number and atomic number:

Answer 2

mass number is the top number next to an element on the periodic table, it is the total number of protons and neutrons in the nucleus of an atom.

atomic number is the bottom number next to an element on the periodic table, it is the number of protons in the nucleus of an atom.

Question 3

atoms:

Answer 3

for neutral atoms, which have no overall charge, the number of electrons is the same as the number of protons. the number of neutrons is (mass number-atomic number).

Question 4

ions:

Answer 4

atoms form ions by gaining or losing electrons. ions have different numbers of protons to electrons- negative ions gain electrons therefore will have more electrons than protons, positive ions lose electrons therefore will have more protons than electrons.

Question 5

isotopes:

Answer 5

isotopes of an element are atoms with the same number of protons but a different number of neutrons (e.g. Cl^35 and Cl^37). different isotopes will have different physical properties (density and rate of diffusion) but the same chemical properties due to the same electron configuration.

Question 6

atomic models:

Answer 6

dalton: at the start of the 19th century, dalton described atoms as solid spheres and said that different spheres made up the
different elements.

thomson: in 1897, j.j.thomson concluded from his experiments that an atom must contain even smaller, negatively charged particles- electrons. the new model was known as the plum pudding model.

rutherford: in 1909 ernest rutherford and his students conducted their famous gold foil experiment, they fired positively charged alpha particles at a very thin sheet of gold, they were expecting most alpha particles to be deflected slightly but in fact most of the alpha particles passed straight through and a very small number deflected backwards, rutherford came up with the nuclear model. in this there is a tiny, positively charged nucleus at the centre, surrounded by a ‘cloud’ of negative electrons, most of the atom is empty space.

bohr: scientists realised that a ‘cloud’ around the nucleus of an atom would quickly spiral down into the nucleus, causing the atom to collapse. neils bohr proposed a new model of the atom with 4 basic principles- electrons exist in fixed orbits (shells) and not anywhere in between, each shell has a fixed energy, when an electron moves between shells electromagnetic radiation is absorbed or emitted, the radiation will have a fixed frequency.

scientists later discovered that not all electrons in a shell had the same energy so they refined bohrs model to include sub shells.

Question 7

relative mass:

Answer 7

relative atomic mass: the average mass of an atom of an element on a scale where an atom of carbon-12 is exactly 12.

relative isotopic mass: the mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is exactly 12.

relative atomic mass= (isotopic masses x percentages) / total percentage

relative molecular mass: the average mass of a molecule on a scale where an atom of carbon-12 is exactly 12.
to find the Mr: add up the atomic masses of all the atoms in the molecule.

relative formula mass: the average mass of a formula unit on a scale where an atom of carbon-12 is exactly 12

Question 8

mass spectrometer stages:

Answer 8

1.ionisation:
electrospray ionisation- the sample is 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.

2.acceleration:
the positive ions are accelerated by an electric field that gives the same kinetic energy to all the ions. the lighter ions experience a greater acceleration than the heavier ions.

3.ion drift:
the ions enter a region with no electric field, they drift through it at the same speed they left the electric field so the lighter ions will be drifting at higher speeds.

4.detection:
lighter ions reach the detector in less time than the heavier ions. the detector detects the current created when the ions hit it and records how long it took for them to pass through the spectrometer, this data is then used to calculate the mass/charge values needed to produce a mass spectrum.

Question 9

interpreting a mass spectrum:

Answer 9

the y-axis gives the abundance of ions, often as a percentage. for an element, the height of each peak gives the relative isotopic abundance (the relative amount of each isotope present in a sample).
the x-axis units are given as a mass/charge ratio.

if electron impact ionisation was used: mass/charge ratio of each peak is the same as the relative mass of that isotope.
if electrospray ionisation was used: mass/charge ratio of each peak would be one unit greater than the relative mass of each isotope as a H+ ion was added.

Question 10

calculating relative atomic mass using mass spectra:

Answer 10

1. for each peak, read the % relative isotopic abundance from the y-axis and the mass/charge value from the x-axis and multiply them together to get the total relative mass for each isotope.
2. add up these totals.
3. divide by 100 since percentages were used.

Question 11

electronic structure:

Answer 11

in the currently accepted model of the atom, electrons have fixed energies. they move around the nucleus in certain regions of the atom called shells or energy levels. the further a shell from the nucleus, the higher its energy.
experiments show that not all the electrons in a shell have exactly the same energy- shells are divided into sub shells. different electron shells have different numbers of sub shells which each have a different energy. there are 4 different sub shells: s,p,d and f.
the sub shells have different orbitals which can hold up to 2 electrons.

s- 1 orbital
p- 3 orbitals
d- 5 orbitals
f- 7 orbitals

Question 12

showing electron configurations:

Answer 12

1. sub shell notation
2. arrows in boxes
3. energy level diagrams

rules:
-electrons fill up the lowest energy sub shells first.
-electrons fill orbitals in a sub shell singly before they start sharing.

Question 13

electron configurations of transition metals:

Answer 13

chromium and copper donate one of their 4s electrons to the 3d sub shell.

chromium= 1s2.2s2.2p6.3s2.3p6.3d5.4s1
copper=
1s2.2s2.2p6.3s2.3p6.3d10.4s1

Question 14

ionisation energies:

Answer 14

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

factors affecting ionisation energy
-nuclear charge: the more protons in the nucleus, the stronger the attraction for the electrons.
-distance: the further away from the nucleus an electron is, the weaker the attraction.
-shielding: the more shielding, the weaker the attraction.

Question 15

ionisation trends:

Answer 15

ionisation energy will increase the more electrons you remove as there will be a stronger attraction to nucleus each time.

down groups: ionisation energy decreases- more shielding, further distance, more protons.

across periods: generally increases but between 2and3 and 5and6 there is a small drop.
•2and3- for 3 the electrons are in the p orbital, therefore have extra shielding from the s orbital and is also further from the nucleus.
•5and6- for 5 the electron is being removed from a singly filled orbital whereas for 6 the electron is being removed from an orbital containing 2 electrons, these 2 electrons will repel each other making it easier for one to be removed.