Chemistry Unit 1

Unit 1.2: Atomic Structure

Key Words

Atomic number – number of protons in the nucleus of an atom

Mass number – the number of protons + the number of neutrons in the nucleus of an atom

Isotopes – atoms having the same number of protons but different numbers of neutrons

Ion – is a particle where the number of electrons does not equal the number of protons

Alpha particles – helium nuclei (two protons and two neutrons) – they are the least penetrating (positive charge)

Beta particles – streams of high – energy electrons (negative charge)

Gamma rays – high electromagnetic radiation which have no charge (most penetrating)

Half-life is the time taken for half the atoms in a radioisotope to decay or the time taken for the radioactivity of a radioisotope to fall to half its initial value.

Atomic orbital – is a region in an atom that can hold up to two electrons with opposed spins

Electronic configuration – is the arrangement of electrons in an atom

Successive ionisation energies – a measure of the energy needed to remove each electron in turn until all the electrons are removed from an atom

The molar first ionisation energy of an element is the energy required to remove one mole of electrons from one mole of its gaseous atoms. (ALWAYS POSITIVE)

Electron shielding or screening – is the repulsion between electrons in different shells. Inner shell electrons repel outer shell electrons.

Nuclear charge – each proton carries a positive charge of +1, as the number of protons increase, so will the nuclear charge.

Types of radioactive emissions and their behaviour

When alpha, beta, gamma radiation pass through matter, they tend to knock of electrons out of atoms and thus make them ionising. Alpha is the most ionising, gamma is the least.

Ionising radiation: they can knock electrons out of atoms and form charged particles.  It involves the transfer of energy from the radiation passing through the matter to the matter itself. As alpha particles is the most ionising, the transfer of energy happens more rapidly and so is the least penetrating.

Effect on mass and atomic number

·         When an element emits an alpha particle, its mass number decreases by 4 and it atomic number decreases by 2.

·         When an element emits a beta particle its mass number is unchanged and its atomic number increases by 1.

·         In electron capture, one of the orbital electrons is captured by a proton, forming a neutron and emitting an electron neutrino.

·         Beta decay/positron emission is where a proton is converted into a neutron while releasing a positron and an electron neutrino.

Half life

Find the time for the radioactive sample to fall to a certain fraction of its initial value.

Find the mass of a radioactive isotope remaining after a certain length of time given the initial mass.

Consequences for living cells

Radioactive emissions are all potentially harmful. However, we all receive harmless background radiation.…