Atomic structure and the periodic table

Question 1

Relative atomic mass (Ar) definition

Answer 1

Weighted mean mass of an atom of an element compared to 1/12th the mass of an atom of C-12

Question 2

Relative isotopic mass definition

Answer 2

Weighted mean mass of an atom of an isotope compared to 1/12th of the mass of an atom of C-12

Question 3

Relative molecular mass (Mr) definition

Answer 3

Mean mass of a molecule or formula unit compared to 1/12th of the mass of an atom of C-12

Question 4

Relative molecular mass calculation

Answer 4

Used when referring to simple molecules
- add up the relative atomic mass values of all atoms in the molecule

E.g. Mr(H20) = (2x1)+(16x1) = 18

Question 5

Relative formula mass calculation

Answer 5

Used for compounds that are ionic (or giant covalent)

-add up the relative atomic masses of all the ions or atoms in the formula unit

E.g. Mr(CaF2) = 40.1 + (2×19)= 78.1

Question 6

How to work out relative atomic mass (Ar) from isotopic abundances

Answer 6

relative atomic mass
= (abundanceA x m/zA)+(abundanceB x m/zB) / total abundance

-multiply each relative isotopic mass by its % relative isotopic abundance, and add them up

-divide by 100

Question 7

Relative isotopic abundance definition

Answer 7

The percentage of atoms with a specific atomic mass found in a sample of an element

Question 8

What is the relative mass of an electron?

Answer 8

1/2000

Question 9

Isotope definition

Answer 9

Atoms of an element that contain the same number of protons and electrons but a different number of neutrons

Question 10

Mass number definition

Answer 10

The sum of the number of protons and the number of neutrons

Question 11

Atomic/proton number definition

Answer 11

The number of protons in the nucleus of an atom of an element

Question 12

summary of steps of mass spectrometry

Answer 12

-sample containing all the different isotopes of an element is vapourised and placed into the sample chamber
-atoms are ionised and convereted to positive ions, which are attracted to a negatively charged plate, causing ions to accelerate and their kinetic energy to increase
ions with the same charge have the same kinetic energy
-ions stop accelerating and drift down towards the detector at different velocities (lighter ion move faster)
-when they reach the detector each ion gains positive ions, the transfer of electrons causing a current to flow
-time taken to reach the detector is used to determine isotope mass, and current produced determines abundance of each isoptope (greater abundance produces greater current)

Question 13

predicting mass spectra for diatomic molecules

Answer 13

-express each percentage abundance as a decimal
-create a table and multiply them to get the relative abundance for each possibility of molecule
-divide all relative abundances by the smallest to get the smallest ratio
-for each possibility of molecule add the masses to get the relative isotopic mass
-draw mass spec (m/z on x, abundance on y)

example:
O16 = 98% = 0.98
O18 = 2% = 0.02

combinations = 016-O18 O18-O18 O16-O16
so relative abundance of a O16-O18 moleculefor example = 0.98x0.02=0.0196

Question 14

using mass spectrometry to identify compounds

Answer 14

-molecules in a compound are bombarded with electrons, an electron is removed from the molecule to form a molecular ion, M+(g)
-to fine the relative molecular mass of a compound find the moleclar peak on the mass spectrum (the peak with the highest M/Z value)

Question 15

How to read a mass spectra graph

Answer 15

-peaks show the fragments of the original molecule
-the last peak is the M+1 peak OR the molecular ion peak, which equals the relative molecular mass of the molecule

Question 16

How to find the relative molecular mass of a molecule from mass spectrometry

Answer 16

The peak with the highest m/z value is the relative molecular mass