1.2 - Basic Ideas About Atoms Flashcards
(a)
Atomic structure
Proton:
relative mass 1, relative charge +1
Neutron: relative mass 1, relative charge 0
Electron:
relative mass negligible, relative charge -1
(a)
Relative atomic mass
number of protons
(a)
Mass number
sum of protons and neutrons
(a)
Isotopes
Atoms of the same element with different numbers of neutrons.
(a)
Alpha (α) particles
positively charged helium nuclei, mass of four units, stopped by a piece of paper, strongly ionising
(a)
Beta (β) particles
Negatively charged electrons, negligible mass, stopped by 0.5 cm of aluminium
(a)
Gamma (γ) radiation
very high energy electromagnetic
radiation, > 2cm of lead required to stop it, weakly ionising.
(a) Equations
Alpha decay (α)
A helium nucleus is produced during this decay, so the atomic number of the original element decreases by two, and the mass number decreases by four.
(a)
Alpha decay example
(238,92) U → (,) Th + (,) He
(238,92) U → (234,90) Th + (4,2) He
(a) Equations
Beta decay (β)
The atomic number of the original
element increases by one, and has the same mass number
(a)
Beta Decay Example
(14,6) C → (,) N + (,) β
(14,6) C → (14,7) N + (0,-1) β
(a) Equations
Positron emission (β+
decay)
The atomic number of the original
element decreases by one, and
has the same mass number.
(a)
Positron emission example
(11,6) C → (,) B + (,) β
(11,6) C → (11,5) B + (0,1) β
(a) Equations
Electron capture (inverse β decay)
The atomic number of the original element decreases by one, forming a different element and has the
same mass number.
(a)
Electron capture example (41,20) Ca → (,) e + (,) K
(41,20) Ca → (0,-1) e + (41,19) K
(b)
behaviour of α-radiation in electric and magnetic field and relative penetrating power
α-particles are positive, heavy and slow moving and are attracted slightly to the negative plate
of an electric field.
(b)
behaviour of β-radiation in electric and magnetic field and relative penetrating power
β-particles are light and fast moving and show considerable deviation towards the positive plate of an electric field.
(b)
behaviour of γ-radiation in electric and magnetic field and relative penetrating power
γ-radiation is electromagnetic radiation of short wavelength and is unaffected by an electric
field.
(c)
Half-life of radioactive decay
The half-life of a radioisotope is the time taken for its radioactivity to fall to half of its initial value
(c) Half-life calculations
Ra-226 has a half-life of 1600 years.
How many years will it take to decay to 1/16 of its original value?
1 → 1/2 → 1/4 → 1/8 → 1/16
Four half-lives so 4 × 1600 6400 years
(c) Half-life calculations
Ra-226 has a half-life of 1600 years.
If the initial mass of Ra-226 was 20.0g, what mass would be
remaining after 4800 years?
4800/1600 = 3 half-lives
20.0 →10.0 →5.0 →2.5
The mass halves three times 2.5 g remains
(d)
Adverse consequences for living cells of exposure to radiation
Ionising radiation can damage the DNA of a cell. Damage to the
DNA may lead to changes in the way the cell functions, which can
cause mutations and the formation of cancerous cells at lower
doses or cell death at higher doses. Exposure to high levels of
radiation can cause radiation burns and death.
(d)
Use of
radioisotopes
Treatment of cancer
Tracer
Calculating age of plant and animal remains
Production of electricity.
Measuring the thickness of metal foil.
(e)
Ionisation energy
The energy required to remove one mole of electrons from one mole of atoms in the gaseous state.
(e)
Factors that influence the attraction of the ionisation energy
- the size of the positive nuclear charge
- the distance of the outer electron from the nucleus
- the shielding effect of electrons in fully occupied inner shells.
(e)
Trend in ionisation energy across a period
Ionisation energy increases across a period because the number of protons increases and electron shielding remains very similar. This means across the period the electrons require more energy to overcome the strengthening nuclear attraction.
(e)
Trend in ionisation energy down a group
Ionisation energy decreases down a group. This is because the atomic radius and electron shielding increases so the nuclear attraction with the electron gets increasingly weaker making the electron easier to remove.
(f)
Link between successive ionisation energy values and electronic structure
Successive ionisation energies always increase because:
* there is a greater ‘effective’ nuclear charge as the same number
of protons are holding fewer and fewer electrons
* as each electron is removed, each shell will be drawn slightly
closer to the nucleus
* as the distance of each electron from the nucleus decreases,
the nuclear attraction increases.
(g)
s orbital
- spherical
- can hold up to two electrons
(g)
d orbital
- five different orbitals
- can hold up to 10 electrons in total (two in each orbital)
(g)
p orbital
- dumb-bell shaped lobes at right angles
- can hold up to six electrons in total (two in each orbital)
(g)
order of blocks
1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,5d,6p,7s,6d
(h)
Origin of emission and absorption spectra in terms of electron transitions between atomic energy levels
When an electron moves down an energy level, it releases a photon to conserve energy. Conversely, when an electron gains energy to move up an energy level, it absorbs a photon. The specific wavelengths and energies of these photons are observed in emission and absorption spectra.
(i)
Atomic emission spectrum of the hydrogen atom
Emission spectra - displays lines at the specific frequencies of emitted photons.
Absorption spectra - displays an entire spectrum with black lines for the ‘missing’ frequencies of the absorbed photons.
(j)
Relationship between energy and frequency
(E = hf)
Energy (J) = Planck’s constant (m2 kgs-1) x Frequency of photon (Hz)
(j)
Relationship between frequency
and wavelength
(f = c/λ)
Frequency (Hz) = Speed of light (m/s)/ Wavelength (m)
(j)
Equation between energy, frequency and wavelength
E= h (c/λ)
(h)
Order of increasing energy
Infrared < visible < ultraviolet light
(l)
Significance of the frequency of the convergence limit of the Lyman series and its relationship with the ionisation energy of the hydrogen atom
The frequency of the convergence limit of the Lyman series can be used in the equation E=hv to calculate the first ionisation energy for hydrogen.