atomic structure Flashcards
radius of an atom
1 x 10⁻¹⁰
describe the basic structure of an atom
a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons
where is most of the mass of an atom concentrated
in the nucleus
what is the radius of a nucleus
less than 1/10000 of the radius of an atom
how are electrons arranged
the electrons are arranged at different distances from the nucleus (different energy levels)
how can electron arrangements change
they may change with:
- the absorption of electromagnetic radiation; they move further from the nucleus (a higher energy level)
- the emission of electromagnetic radiation; they move closer to the nucleus (a lower energy level)
describe the amount of sub-atomic particles in an atom and the charge
the number of electrons is equal to the number of protons in the nucleus; atoms therefore have no overall electric charge
define atomic number
the number of protons in an atom of an element
what do all atoms of a particular element have in common
they have the same number of protons
define mass number
the total number of protons and neutrons in an atom
define isotopes
atoms of the same element that have the same number of protons but a different number of neutrons
when do atoms turn into positive ions
if they lose one or more outer electrons
what happens when new experimental evidence of the atom is released
this may lead to a scientific model being changed or replaced
what knowledge was before the discovery of the electron
atoms were thought to be tiny spheres that could not be divided
what did the discovery of the electron lead to
the plum pudding model of the atom
describe the plum pudding model
it suggested that the atom is a ball of positive charge with negative electrons embedded in it
what did the results from the alpha particle scattering experiment lead to
the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. this nuclear model replaced the plum pudding model
what did Niels Bohr do
he adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. the theoretical calculations of Bohr agreed with experimental observations
what did later experiments of the atom lead to
the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge; these particles are known as protons
what did James Chadwick do
he provided the evidence to show the existence of neutrons within the nucleus. this was about 20 years after the nucleus became an accepted scientific idea
describe radioactive decay
some atomic nuclei are unstable; the nuclei gives out radiation as it changes to become more stable; this is a random process called radioactive decay
define activity (give its units)
the rate at which a source of unstable nuclei decays; measured in becquerel (Bq)
define count-rate
the number of decays recorded each second by a detector (e.g. Geiger-Muller tube)
what ways can nuclear radiation be emitted by
- an alpha particle
- a beta particle
- a gamma ray
- a neutron
what do alpha particles consist of
two neutrons and two protons; it is the same as a helium nucleus
what are beta particles
high speed electrons ejected from the nucleus as a neutron turns into a proton
what are gamma rays
electromagnetic radiation from the nucleus
why is count rate and activity not the same thing
because count rate takes into account background radiation
describe alpha particle range in air
they are relatively large so they can only travel a few centimetres in air, before they collide with air particles and stop
describe alpha particle penetration
they are relatively large so have a very low penetrating power; they can be absorbed by a single sheet of paper
describe alpha particle ionising power
they have a great charge and a high mass so are strongly ionising; they can easily knock electrons off atoms and form ions
