Atomic Structure

Question 1

Give the relative charges of proton, neutron, and electron

Answer 1

Proton = +1
Neutron = 0
Electron = +1

Question 2

Give the relative masses of proton, neutron, and electron

Answer 2

Proton = 1
Neutron = 1
Electron = 0 (1/1840)

Question 3

Give the positions of proton, neutron, and electron

Answer 3

Proton = in the nucleus
Neutron = in the nucleus
Electron = around the nucleus

Question 4

What are atoms made up of?

Answer 4

Protons, neutrons, electrons

Question 5

Explain how sub-atomic particles are arranged in an atom

Answer 5

The protons and neutrons are in the centre of the atom, held together by a force called the strong nuclear force. This is much stronger than the electrostatic forces of attraction that hold electrons and protons together in the atom, so it overcomes repulsion between the protons in the nucleus. It acts only over very short distances, that is, within the nucleus.

The nucleus is surrounded by electrons. Electrons are found in a series of levels (or orbits or shells), which get further and further away from the nucleus.

Question 6

Explain why we assume that there are the same number of protons and electrons in an atom

Answer 6

Because they have opposite charges of the same size and the atom is neutral

Question 7

Define mass number

Answer 7

Total number of protons plus neutrons in the nucleus

It is the nucleons that are responsible for almost all of the mass of an atom because electrons weigh virtually nothing

Question 8

Define atomic number

Answer 8

The number of protons in an atom

It defines the chemical identity of an element

Question 9

What determines the chemical properties of an element?

Answer 9

The number of electrons in the outer shell of an atom

Question 10

What are isotopes?

Answer 10

Atoms with same number of protons but different numbers of neutrons

Different isotopes of the same element react chemically in exactly the same way as they have the same electron configuration

Atoms of different isotopes of the same element vary in mass number because of the different number of neutrons in the nuclei

Question 11

What is the cause of radioactivity?

Answer 11

When the nucleus of unstable isotopes breaks down giving off bits of the nucleus or energetic rays

Question 12

What can electrons behave as?

Answer 12

A particle, a wave, a cloud of charge

Question 13

What instrument is used for the accurate determination of relative atomic mass?

Answer 13

The mass spectrometer

Question 14

Briefly outline what happens in a TOF instrument

Answer 14

The substance(s) in the sample are converted to positive ions, accelerated to high speeds (which depend on their mass to charge ratio), and arrive at a detector

Question 15

What are the 6 mains steps in a TOF mass spectrometer?

Answer 15

Vacuum
Ionisation
Acceleration 
Io drift
Detection 
Data analysis

Question 16

TOF mass spectrometer:

Vacuum

Answer 16

The whole apparatus is kept under a high vacuum to prevent the ions that are produced colliding with molecules from the air

Question 17

TOF mass spectrometer:

Ionisation (electrospray)

Answer 17

The sample is dissolved in a volatile solvent and forced through a fine hollow needle that is connected to the positive terminal of a high voltage supply. This produces tiny positively charged droplets that have lost electrons to the positive terminal

The solvent evaporates from the droplets into the vacuum and the droplets get smaller and smaller until they may contain no more than a single positively charged ion

Question 18

TOF mass spectrometer:

Ionisation (electron impact)

Answer 18

The sample is vaporised and high energy electrons are fired at it from an electron gun, which is a hot wire filament with a current running through it that emits beam of high energy electrons

This usually knocks off one electron from each particle forming a 1+ ion

X(g) + e- ——> X+(g) + 2e-

Question 19

TOF mass spectrometer:

Acceleration

Answer 19

The positive ions are attracted towards a negatively charged plate and accelerate towards it. Lighter ions and more highly charged ions achieve a higher speed.

Question 20

TOF mass spectrometer:

Ion drift

Answer 20

The ions pass through a hole in the negatively charged plate, forming a beam and travel along a tube, called the flight tube, to a detector

Question 21

TOF mass spectrometer:

Detection

Answer 21

When ions with the same charge arrive at the detector, the lighter ones are first as they have higher velocities. The flight times are recorded. The positive ions pick up an electron from the detector, which causes a current to flow.

Question 22

TOF mass spectrometer:

Data analysis

Answer 22

The signal from the detector is passed to a computer which generates a mass spectrum.

Question 23

What does the peak height and horizontal scale in a mass spectrum give?

Answer 23

Peak height = the relative abundance of each isotope

Horizontal scale = the m/z value (for a singly charged ion this is numerically the same as the mass number)

Question 24

What is the difference between high resolution mass spectrometry and low resolution mass spectrometry?

Answer 24

High resolution = measures relative atomic masses to five decimal places of an atomic mass unit

Low resolution = measures relative atomic masses to one decimal place

Question 25

Describe how mass spectrometers are used in space

Answer 25

Space probes carry mass spectrometers. They are used to identify the elements in rock samples.

Question 26

Why can electrons in different shells be represented on an energy level diagram?

Answer 26

Because electrons in different shells have differing amounts of energy

Question 27

Draw the shape of an s-orbital

Answer 27

Book (P14)

Question 28

Draw the shape of the p-orbitals

Answer 28

Book (P14)

Question 29

What is quantum mechanics?

Answer 29

A theory that describes the atom mathematically with an equation (the Schrodinget equation).

The solutions to this equation give the probability of finding an electron in a given volume of space called the atomic orbital.

Question 30

What is an atomic orbital?

Answer 30

A volume in space which an electron fills.

Question 31

What is ionisation energy?

Answer 31

The energy required to remove a mole of electrons from a mole of atoms in the gaseous state.

Question 32

Describe the trend in first ionisation energies across a period.

Answer 32

Generally increases because the nuclear charge is increasing and this makes it more difficult to remove an electron.

Question 33

Why is there a drop in first ionisation energy between magnesium and aluminium?

Answer 33

Because the outer electron in aluminium is in a 3p orbital which is of a slightly higher energy than the 3s orbital so less energy is needed to remove it.

Question 34

Why is there a drop in first ionisation energy between phosphorus and sulfur?

Answer 34

In phosphorus, each of the three 3p orbitals contains just one electron, while in sulfur, one of the 3p orbitals must contain 2 electrons. The repulsion between these paired electrons make it easier to remove one of them, despite the increase in nuclear charge.

Question 35

What is the trend in first ionisation energy going down a group?

Answer 35

There is a general decrease. Although the nuclear charge increases, the actual positive charge ‘felt’ by an electron in the outer shell is less than the full nuclear charge. This is because of the effect of the inner electrons shielding the nuclear charge.