S1.2

Question 1

brief history of the atom

Answer 1

Democritus states the existence of atoms (based on philosophy)
Dalton’s atomic theory => first scientific theory of atoms
JJ Thomson discovers the electron using a cathode ray => “plum pudding” atomic model
Rutherford’s gold foil experiment => Rutherford’s nuclear model (atom is mostly empty, positive charge is concentrated in one area, electrons occupy most of the volume)

Question 2

atoms are divisible into subatomic particles:

Answer 2

electrons (electron cloud, charge = -1)
protons (nucleus, +1)
neutrons (nucleus, 0)

Question 3

how do elements differ?

Answer 3

because they contain different numbers of protons

Question 4

atomic number

Answer 4

A = number of protons and neutrons in the nucleus of an isotope: A = p + n
element-A (the element has a mass number A)
superscript

Question 5

atomic number

Answer 5

Z = number of protons in the nucleus of each atom of an element
subscript

Question 6

isotopes

Answer 6

⇒ atoms of the same element with different numbers of neutrons and, thus, different mass numbers
same chemical properties (due to the same el. configuration), differing physical properties (f.i. boiling point)

Question 7

natural abundance

Answer 7

NA = percentage of the atom of an isotope among all of the atoms of the given element

Question 8

protium

Answer 8

hydrogen isotope H-1
1 proton + 1 electron

Question 9

deuterium

Answer 9

hydrogen isotope H-2
1 proton + 1 electron + 1 neutron

Question 10

tritium

Answer 10

hydrogen isotope H-3
1 proton + 1 electron + 2 neutrons

Question 11

relative atomic mass

Answer 11

Ar = the ratio of the average mass of the atom to the atomic mass unit (1/12 of a carbon-12 atom)
based on the NA of each isotope of that element in nature: Ar = Ar(el)1⋅rel.ab1 + Ar(el)2⋅rel.ab2 + … + Ar(el)n⋅rel.abn
if not stated otherwise, assume that Ar of an isotope is equal to mass number

Question 12

mass spectrometer

Answer 12

= instrument used to determine the abundance of various isotopes of an element based on the mass-to-charge ratio of isotopes within a sample of an element

Question 13

stages of mass spectra

Answer 13

1. vaporisation ⇒ substance is vaporised to produce gaseous molecules in order to create a lot of space between atoms and therefore neglect intermolecular forces
2. ionization ⇒ high-energy electrons are fired at the gaseous molecules, forming gaseous (molecular) ions with a positive charge
3. acceleration ⇒ gaseous ions are accelerated in an electric field
4. deflection ⇒ gaseous ions are deflected by an electromagnet, the degree of this deflection depends on the mass-to-charge ratios of these ions
5. detection ⇒ gaseous ions are detected and a mass spectrum (graph) is produced

Question 14

which particles deflect most in a mass spectrometer

Answer 14

lower mass-to-charge ratios (small mass) are deflected most
higher mass-to-charge ratios (large mass) are deflected the least
larger positive charges are deflected most

Question 15

detection in mass spectrometry

Answer 15

• gaseous ions are detected and a mass spectrum (graph) is produced
• detectors measure the general intensity, the highest of which is adjusted to 100% ⇒ relative abundance = relative intensity/mass
• mass spectrometers are only able to detect 1+ charges ⇒ mass to charge ratio determines the mass