Physical Chemistry A1

Question 1

First model of an atom.

Answer 1

In the 1800s, Dalton proposed that all atoms of one element are the same and that atoms of one element are different from the atoms of another element. Atoms in his model were tiny and indivisible.

Question 2

Second model of the atom.

Answer 2

In 1897, Thomson discovered the first sub-atomic particle - the electron. Thomson proposed the plum pudding model where negatively charged electrons move in a ‘sea’ of charge in a positively charged atom.

Question 3

Third model of the atom.

Answer 3

In 1911, Rutherford carried out experiments to find that most of the mass of an atom is concentrated in the centre, where negatively charged electrons orbit the central positively charged nucleus. The positive and negative charges balance to make the atom neutral.

Question 4

Fourth model of the atom.

Answer 4

In 1913, Bohr suggested that electrons orbit the nucleus on paths. Bohr’s planetary model provided an explanation for the difference in energy of electrons at different distances from the nucleus.

Question 5

Modern model of the atom.

Answer 5

The modern model of the atom states that it is composed of sub-atomic particles - protons, neutrons, and electrons. Protons and neutrons are found in the nucleus and are themselves made up of smaller particles, whereas electrons orbit the nucleus.

Question 6

Describe the structure of the atom.

Answer 6

Atoms are made up of three sub-atomic particles:
Protons and neutrons make up the nucleus and are found in the centre of an atom where most of the mass is concentrated.
Electrons orbit the nucleus in shells and do not contribute much to the mass of the atom.
Protons - mass = 1 charge = +1
Neutron - mass = 1 charge = 0
Electrons - mass = 1/1840 charge = -1

Question 7

What are isotopes?

Answer 7

Isotopes are atoms with the same number of protons and a different number of neutrons. They have different mass numbers but the same atomic number because atoms of the same element must have the same number of protons.
Isotopes of the same element have the same electronic configuration so react in the same way in chemical reactions.

Question 8

What is mass spectrometry?

Answer 8

It is a form of molecular chemical analysis that allows the masses of individual molecules or isotopes to be determined. Mass spectrometry can be used to provide structural information, identify an unknown compound, or determined the relative abundance of each isotope of an element.

Question 9

What are the stages of time of flight mass spectrometry?

Answer 9

1. Ionisation - the sample is dissolved in a volatile solvent and ejected as tiny droplets through a hollow needle. The needle is connected to a positive terminal of a high voltage supply. This produces tiny positively charged droplets as the atoms in the sample lose an electron, and the solvent evaporates.
2. Acceleration - positive ions are accelerated towards a negatively charged plate to give all ions constant kinetic energy. Therefore, the velocity of each ion will depend on its mass. Lighter and more highly charged ions will accelerate more.
3. Ions drift - ions pass through a hole in the negative plate, forming a beam which travels along the flight tube to a detector.
4. Detection - the positive ion picks up an electron which causes a current to flow. Flight times are recorded.
5. Data analysis - the signal from the detector passes to a computer which generates a mass spectrum.

Question 10

What is a mass spectra?

Answer 10

Mass spectrometry produces a graph of relative abundance against mass/charge ratio that can be used to measure relative atomic masses. This can be done to a high level of precision (up to 5 decimal places) or at a low resolution (1 decimal place) depending on the apparatus used.

Question 11

What are sub-shells?

Answer 11

Electron shells are made up of atomic orbitals, which are regions in space where electrons may be found. Each shell is composed of one or more orbitals and each orbital can hold one pair of electrons. There are four main types of orbitals s, p, d and f.
s orbitals are spherical and can hold two electrons.
p orbitals are dumbbell shaped and can hold two electrons but always come in groups of three.
d orbitals have a more complex shape, they hold up to two electrons but always come in groups of five.

Question 12

What is electron spin?

Answer 12

Electrons have an intrinsic property called spin that is caused by the two negative charges of electrons in an orbital repelling each other. Electron spin can either be up or down but two electrons in the same orbital must spin in opposite directions in order to minimise the repulsion.

Question 13

Shells and sub-shells are filled with electrons according to a set of rules, what are these rules?

Answer 13

• Atomic orbitals with the same energy are occupied individually first before pairing.
• The lowest available energy level is filled first.
• No more than two electrons can fill an atomic orbital

Question 14

Define relative atomic mass.

Answer 14

The ratio of the average mass of an atom of an element to 1/12th of the mass of an atom of carbon 12.

Question 15

Define relative molecular mass.

Answer 15

The ratio of the average mass of a molecular of an element or compound to 1/12th of the mass of an atom of carbon 12.

Question 16

Define a mole.

Answer 16

The amount of any substance that contains the same number of atoms or particles as 12g of carbon-12

Question 17

What is avogadros constant?

Answer 17

One mole of a substance contains a number of particles equal to the avogadro constant: 6.022 x 10^23 mol^-1.

Question 18

What assumptions are made about an ideal gas?

Answer 18

• molecules are in constant and random motion.
• intermolecular forces between the gas molecules are negligible.
• the volume of the molecules themselves, relative to the volume of their container, is negligible.