Atomic Structure Flashcards
Give the relative charges of proton, neutron, and electron
Proton = +1 Neutron = 0 Electron = +1
Give the relative masses of proton, neutron, and electron
Proton = 1 Neutron = 1 Electron = 0 (1/1840)
Give the positions of proton, neutron, and electron
Proton = in the nucleus Neutron = in the nucleus Electron = around the nucleus
What are atoms made up of?
Protons, neutrons, electrons
Explain how sub-atomic particles are arranged in an atom
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.
Explain why we assume that there are the same number of protons and electrons in an atom
Because they have opposite charges of the same size and the atom is neutral
Define mass number
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
Define atomic number
The number of protons in an atom
It defines the chemical identity of an element
What determines the chemical properties of an element?
The number of electrons in the outer shell of an atom
What are isotopes?
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
What is the cause of radioactivity?
When the nucleus of unstable isotopes breaks down giving off bits of the nucleus or energetic rays
What can electrons behave as?
A particle, a wave, a cloud of charge
What instrument is used for the accurate determination of relative atomic mass?
The mass spectrometer
Briefly outline what happens in a TOF instrument
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
What are the 6 mains steps in a TOF mass spectrometer?
Vacuum Ionisation Acceleration Io drift Detection Data analysis
TOF mass spectrometer:
Vacuum
The whole apparatus is kept under a high vacuum to prevent the ions that are produced colliding with molecules from the air
TOF mass spectrometer:
Ionisation (electrospray)
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
TOF mass spectrometer:
Ionisation (electron impact)
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-
TOF mass spectrometer:
Acceleration
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.
TOF mass spectrometer:
Ion drift
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
TOF mass spectrometer:
Detection
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.
TOF mass spectrometer:
Data analysis
The signal from the detector is passed to a computer which generates a mass spectrum.
What does the peak height and horizontal scale in a mass spectrum give?
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)
What is the difference between high resolution mass spectrometry and low resolution mass spectrometry?
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
Describe how mass spectrometers are used in space
Space probes carry mass spectrometers. They are used to identify the elements in rock samples.
Why can electrons in different shells be represented on an energy level diagram?
Because electrons in different shells have differing amounts of energy
Draw the shape of an s-orbital
Book (P14)
Draw the shape of the p-orbitals
Book (P14)
What is quantum mechanics?
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.
What is an atomic orbital?
A volume in space which an electron fills.
What is ionisation energy?
The energy required to remove a mole of electrons from a mole of atoms in the gaseous state.
Describe the trend in first ionisation energies across a period.
Generally increases because the nuclear charge is increasing and this makes it more difficult to remove an electron.
Why is there a drop in first ionisation energy between magnesium and aluminium?
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.
Why is there a drop in first ionisation energy between phosphorus and sulfur?
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.
What is the trend in first ionisation energy going down a group?
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.
