2.1.1 Atomic structure and isotopes

• protons + neutrons found in nucleus • electrons orbit nucleus in shells • nucleus is small and dense and accounts for almost all of atoms mass • most of atom consists of empty space with positive nucleus region in centre surrounded electron shells

p - 1 n - 1 e - 1/1836

p - +1 n - 0 e - -1

number of protons

• number of protons + neutrons

• number of protons

• the number of protons

• atoms of the same element with different number of neutrons but same protons > and so different masses

• because the actual mass + charge is very small number

• chemical reactions involve the electrons • isotopes have same number of electrons and therefore they react in the same way > there may be some small differences in physical properties e.g. mp / bp

• a charged atom • number of electrons is different than protons

• the international standard for the measurement of atomic mass

• the mass of an isotope relative to 1/12th the mass of an atom of carbon-12

• the weighted mean mass of an atom of an element relative to 1/12th the mass of an atom of carbon-12

• used for giant structures • the weighted mean mass of a formula unit compared with 1/12th the mass of an atom of carbon-12

• used for simple molecules • the weighted mean mass of a molecule compared with 1/12th the mass of an atom of carbon-12

Ar = (23 x 10) + (100 x 11) = ans / 123 = 10.8

• it finds out about molecules e.g. > identify an unknown compound > find relative abundance of each isotope of an element > determine structural information about molecules

• by measuring the mass-to-charge ratio of ions > m/z = relative mass of ion / relative charge of ion • sample placed in mass spectrometer • sample vaporised and ionised to form +1 ions • ions accelerate, heavier ions move slower than lighter ones • ions detected and current produced to form a mass spectrum