Module 2.1 Atoms and Reactions

Greek philosopher Democritus developed the first idea of the atom Suggested you could divide matter only a certain number of times - “átomos” meaning “indivisible”

Atoms are tiny particles that make up elements Atoms cannot be divided All atoms of a given element are the same Atoms of one element are different from those of every other element Dalton used his own symbols to represent atoms Dalton developed the first table of atomic masses

He discovers electrons Discovered “cathode rays” had a negative charge, could be deflected by a magnet and an electric field and had a v v small mass Cathode rays were electrons The idea that an atom could not be split any further (proposed by the ancient Greeks and Dalton) had been disproved

Proposed that atoms were made up of negative electrons moving around a “sea” of positive charge - “plum pudding model” He thought that the overall negative charge = the overall positive charge - an atom is neutral with no overall charge

Gold leaf experiment: Directed a-particles towards a sheet of very thin gold foil. They measured any deflection of the particles. What the results showed: Most particles weren’t deflected at all A small percentage of particles deflected at large angles Few particles were deflected back towards the source His new model (1911): The positive charge of an atom and most of the mass are concentrated in the nucleus, in the centre Negative electrons orbit the nucleus - just like how planets orbit the sun The + and - charges must balance Rutherford had disproved the plum pudding model - the expected result was that the a-particles would pass through undisturbed but the observed results were that a few particles were deflected - indicating a small concentrated positive charge (the nucleus)

Bohr thought electrons could only follow certain paths otherwise they would spiral into the nucleus - “planetary atom” - electrons orbited the central nuclear “sun” in “shells” Bohr’s model helped explain some periodic properties e.g. spectra lines seen in the emission spectra, the energy of electrons at different distances from the nucleus Henry Moseley discovered the link between x-ray frequencies and an element’s atomic number - however, he couldn’t explain this at the time

Discovers the proton Could explain Moseley’s finding that an atom’s atomic number was linked to x-ray frequencies We now know that atomic no. = no. of protons in the atom

Louis de Broglie suggested that particles could have the nature of both a wave and a particle Erwin Schrödinger suggested that an electron had wave-like properties in an atom. He also introduced the idea of atomic orbitals

Discovers neutrons Observed a new type of radiation emitted from some elements Uncharged particles - approx. same mass as a proton These were neutrons

Now thought that protons and neutrons are made up of even smaller particles called quarks

Protons and neutrons in the nucleus Nucleus = centre of atom Electrons orbit the nucleus in “shells” Nucleus = absolutely tiny Nucleus = extremely dense, accounts for almost all of the atom’s mass Most of an atom is empty space

Proton: Mass = 1 Charge = +1 Neutron: Mass = 1 Charge = 0 Electron: Mass = 1/2000 Charge = -1

Different masses Same no. of protons Same no. of electrons Different number of neutrons

Proton number

Protons + neutrons

Add together the relative atomic masses of each atom making up a molecule

Add together the relative atomic masses of each atom making up the formula unit

Identify an unknown compound Find the relative abundance of each isotope of an element Determine structural information about molecules

Relative or percentage abundance on the y-axis and mass-to-charge ratio on the x-axis

m/z m= mass z= the charge of the ion (usually 1)

Lose electrons Form positive ions with the electron configuration of the previous noble gas in the periodic table

Gain electrons Form negative ions with the electron configuration of the next noble gas in the periodic table

It requires too much energy to transfer the outer shell electrons to form ions

NH4+