Module 2: Chapter 2 (Atoms, ions and compounds)

• atom consists of a nucleus made up of two types of subatomic particle - protons and neutrons • third subatomic particle is called an electron, arranged outside the nucleus in shells

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, and therefore different masses

• In chemical reactions isotopes retain the same level of reactivity as they have the same number of electrons in the outer shells -> isotopes of the same element would react in a similar way • physical properties change with each isotope

Positive ions, have fewer electrons than protons

Negative ions, have more electrons than protons

• The mass of an atom is not exactly the same as the individual masses of the constituent protons, electrons and neutrons • The strong nuclear force binding the protons and nucleus comes at the expense of a fraction of their mass, mass defect

• this isotope is the international standard for the measurement of atomic masses • mass of a carbon-12 isotope is defined as exactly 12 atomic mass units (12u) • Standard mass for atomic mass is 1U, mass of 1/12th of a carbon-12 isotope • 1u = approx. mass of proton/neutron

Relative Isotopic mass is the mass of an isotope relative to 1/12th of the mass of an atom of carbon-12

Relative atomic mass Ar is the weighted mean mass of an atom of an element relative to 1/12th of the mass of an atom of carbon-12 the weighted mean mass takes into account the percentage abundance and relative isotopic mass of each isotope

• Technique used to experimentally find the percentage abundances • a sample is placed in the mass spectrometer • the sample is vaporised and then ionised to form positive ions • the ions are accelerated, heavier ions move more slowly and are more difficult to deflect than lighter ions, so the ions of each isotope are separated • the ions are detected on a mass spectrum as a mass-to-charge ratio m/z, each ion reaching the detector adds to the signal, so the great the abundance the larger the signal

• Atoms of metals in groups 1,2,3 all lose electrons to form positive ions • Atoms of non-metals in groups 15,16,17 all gain electrons to form negative ions. These ions can me named adding ‘ide’ to the stem

H+, proton H-, hydride

ZN(2+)

Ag(+)

Polyatomic ions contain atoms of more than one element covalently bonded together

OH(-)

NO3(-)

CO3(2-)

SO4(2-)

NH4(+)

HCO3(-)

MnO4(-)

Cr2O7(2-)

PO4(3-)