2.1 Flashcards
Most elements are made up of a mixture of isotopes. Isotopes of the same element have:
- different masses
- the same number of protons and electrons
- different numbers of neutrons in the nucleus
We describe all elements using two numbers which equal the mass number and atomic number. These numbers are?
A = mass number (protons + neutrons) Z = atomic number (protons)
Why do different isotopes of the same element react in the same way?
- chemical reactions involve electrons, and isotopes have the same arrangement and number of electrons
- neutrons make no difference to chemical reactivity
Measurement of relative masses - how would you go about weighing something you cannot see? (sorry for this one Jayden)
Instead of finding the mass of atoms directly, we compare the masses of different atoms, using the idea of relative mass. The carbon-12 isotope is the international standard for the measurement of relative mass
Atomic masses are measured using a unit called the unified atomic mass unit, u.
Explain how this unit came about.
- 1 u is a tiny mass: 1.660 540 210 x 10 to the -27 kg
- the mass of an atom of carbon-12 is defined as 12 u
- so the mass of one-twelfth of an atom of carbon-12 is 1 u
Relative isotopic mass
All the atoms in a single isotope are identical. They all have the same mas, measured against carbon-12. How do you calculate the relative isotopic mass?
Bonus points: what are two important assumptions we miss out when calculating the relative isotopic mass?
For an isotope, the relative isotopic mass is the same as the mass number:
- so, for oxygen-16, the relative isotopic mass = 16.0
- for sodium-23, the relative isotopic mass = 23.0
Note that we have made two important assumptions:
1) we have neglected the tiny contribution that electrons make to the mass of an atom
2) we have taken the masses of both a proton and a neutrons as 1.0 u
Most elements contain a mixture of isotopes, each in a different amount and with a different mass. We use the term ‘weighted mean mass’ to account for the contribution made by each isotope to the overall mass of an element.
The contribution made by an isotope to the overall mass depends on…
- the percentage abundance of the isotope
- the relative mass of the isotope
Chemists then combine the contribution from each isotope to arrive at the relative atomic mass, A(little r), of an element. Remember, all masses are relative to the mass of carbon-12.
Summarise the definition of relative isotopic mass
Relative isotopic mass is the mass of an atom of an isotope compared with one-twelfth of the mass of an atom of carbon-12
Summarise the definition of relative atomic mass
Relative atomic mass is the weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12
A mass spectrometer is a piece of apparatus that can be used to find out about molecules. For example, it can be used to:
- identify an unknown compound
- find the relative abundance of each isotope of an element
- determine structural information about molecules
A mass spectrometer determines the mass of a molecule or isotope by measuring the mass-to-charge ratio of ions. How does it do this?
It does this by causing substances to become positive ions. These positive ions are then passed through the apparatus and separated according to their mass and charge. A computer within the mass spectrometer analyses the data on the ions present and produces a mass spectrum. This is similar to a complex bar graph and gives information about the abundance of ions present in the sample.
What is one of the most important uses of mass spectrometry?
To determine the isotopes present in an element. You will need to be able to analyse a mass spectrum to find the proportions of each isotope in the element being investigated.
How is the mass-to-charge ration shown in all mass spectra?
The mass-to-charge ratio in all mass spectra is shown as m/z
m is the mass
z is the charge on the ion, which is usually 1
Draw out a table with the headings: group in the periodic table; electrons in the outer shell and charge on ion.
Group //// electrons //// charge
1 1 +1
2 2 +2
3 3 +3
5 5 -3
6 6 -2
7 7 -1
What do atoms of metals in groups 1-13 do during a reaction?
- lose electrons
- form positive ions with the electron configuration of the previous noble gas in the periodic table
What do atoms of non-metals in groups 15-17 do during a reaction?
- gain electrons
- form negative ions with the electron configuration of the next noble gas in the periodic table
What do atoms of Be, B, C and Si not do during a reaction?
- do not normally form ions
- require too much energy to transfer the outer shell electrons to form ions
Chemists use a quantity called amount of substance for counting atoms. Amount of substance is:
- given the symbol n
- measured using a unit called the mole (abbreviated to mol)
What is the amount of substance based on?
Amount of substance is based on a standard count of atoms called the Avogadro constant, N(little A). The Avogadro constant is 6.02 x 10 to the 23 mol to the minus 1
In short, define amount of substance
Amount of substance is the quantity that has moles as its unit. Chemists use ‘amount of substance’ as a way of counting atoms. A mole is the amount of any substance containing as many particles as there are carbon atoms in exactly 12g or the carbon-12 isotope.
In short, define molar mass
Molar mass, M, is the mass per mole of a substance. The units of molar mass are g mol to the -1
What formula links the amount of substance, n, mass, m and molar mass, M?
no. of moles = mass (g) / molar mass (g mol-1)
OR
n = m/M
What is an empirical formula?
An empirical formula is the simplest way of showing a chemical formula. It shows the ratio between elements, rather than actual numbers of atoms of each element.
Why is empirical formulae always used for compounds with giant structures (like NaCl)?
If you wanted to show the formula for NaCl using the actual number of ions bonded together, it would run into millions and millions. The formula would also depend on the size of the salt crystal. Structures like this are called giant structures.
What are the three steps to calculate empirical formula of a compound?
- Divide the amount of each element present by its molar mass -this will give you the molar ratio
- Divide the answer for each element by the smallest number - this ensures your ratio is in the format 1:x
- If necessary, multiply the answer by a suitable value to make sure the ratio is in whole numbers only. For example to have a whole number ratio you would need to multiply the ratio 1 : 1.5 to make it 2 : 3
What is a molecular formula?
Molecular formulae are used for compounds that exist as simple molecules. A molecular formula tells you the number of each type of atom that make up a molecule.
At RTP how much space does a mole of gas occupy?
This is also the definition of molar gas volume
24.0 dm cubed (24000 cm cubed)
the volume per mole of gas molecules is 24 dm cubed mol-1
What is the formula triangle for calculating amounts using gas volumes?
Volume (in dmcubed)
number of moles / 24
In what five ways are gases assumed to behave?
1) they are in continuous motion and do not experience any intermolecular forces
2) they exert pressure when the collide with each other and the walls of containers
3) all collisions between gas molecules and between gas molecules and container walls are elastic - they do not cause kinetic energy to be lost
4) the kinetic energy of gases increases with increasing temperature
5) gas molecules are so small compared to the size of any container they are found in that any differences in sizes of different gas molecules can be ignored
The relationship between the volume, pressure, temperature and number of moles of an ideal gas can be described by the ideal gas equation. What is this equation?
pV = nRT
where p is pressure V is volume n is the number of moles R is the gas constant, with a value of 8.314 J mol-1 K-1 T is temperature
