Atoms are not the smallest thing Growing evidence for the divisibility of the indivisible

Electrostatics and electricity The phenomenon of static electricity was known since ancient times. Certain “charged” objects repel and others attract The phenomenon of static electricity was known since ancient times. Certain “charged” objects repel and others attract The discovery of the voltaic cell allowed the harnessing of electrical current from chemical activity The discovery of the voltaic cell allowed the harnessing of electrical current from chemical activity Electrical generation was achieved in 1825 Electrical generation was achieved in 1825 Atoms are neutral. If indivisible, with what are the electrical charges associated? Atoms are neutral. If indivisible, with what are the electrical charges associated?

Faraday’s prescience “Although we know nothing of what an atom is, we cannot resist forming some idea of a small particle; and though we are in equal ignorance of electricity, there is an immensity of facts which justify us in believing that the atoms of matter are associated with electrical powers to which they owe their most striking qualities, and amongst them their chemical affinity.” “Although we know nothing of what an atom is, we cannot resist forming some idea of a small particle; and though we are in equal ignorance of electricity, there is an immensity of facts which justify us in believing that the atoms of matter are associated with electrical powers to which they owe their most striking qualities, and amongst them their chemical affinity.”

Ray of hope Discovery of cathode rays by Julius Plucker Discovery of cathode rays by Julius Plucker Application of a large voltage across an evacuated tube causes a current to flow. The current flow is accompanied by radiation from the excited gas molecules Application of a large voltage across an evacuated tube causes a current to flow. The current flow is accompanied by radiation from the excited gas molecules How does the neutral and indivisible atom create a charge? How does the neutral and indivisible atom create a charge?

Cathode rays are negatively charged particles 1897.J. Thomson demonstrates that cathode rays consist of negatively charged particles, which have a mass very much less than that of the atom. The first sighting of the electron.

The “oil drop” experiment gave a measurement of electron charge

The Thomson model of the atom "I regard the atom as containing a large number of smaller bodies which I will call corpuscles, these corpuscles are equal to each other.... In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpuscles behave like negative ions, yet when they are like negative ions, yet when they are assembled in a neutral atom the negative assembled in a neutral atom the negative effect is balanced by something which effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges of the corpuscles…”

A hierarchy of discovery Investigations of cathode rays led to the discovery of X-rays in 1895 (Wilhelm Roentgen) Investigations of cathode rays led to the discovery of X-rays in 1895 (Wilhelm Roentgen) This new discovery inspired search for other types of invisible emission by substances This new discovery inspired search for other types of invisible emission by substances The world would never be the same again… The world would never be the same again…

Radioactivity Henri Becquerel discovered radioactivity, which suggested that some atoms were capable of decomposing to give smaller particles. Henri Becquerel discovered radioactivity, which suggested that some atoms were capable of decomposing to give smaller particles Frederick Soddy and William Ramsey demonstrated that uranium decayed to give helium. Direct proof that atoms were divisible. Frederick Soddy and William Ramsey demonstrated that uranium decayed to give helium. Direct proof that atoms were divisible.

1909 Ernest Rutherford’s gold foil experiment with alpha particles and the discovery of the nucleus.

The nucleus Tiny Tiny Incredibly dense – contains all the mass of the atom Incredibly dense – contains all the mass of the atom Positively charged Positively charged Contains protons (charged) and neutrons (neutral) – not discovered until much later Contains protons (charged) and neutrons (neutral) – not discovered until much later

Summary of Atom Pictures Dalton: Indivisible atom Dalton: Indivisible atom Thomson: Electrons Thomson: Electrons Rutherford: Nucleus Rutherford: Nucleus

Atoms are mostly nothing

Comparison of subatomic particles Particle MassCharge gramsamucoulombse Electron 9.1x x x Proton 1.67x x Neutron 1.67x

Atoms are neutral: # electrons = # protons The number of protons in the nucleus of an atom determines the atomic number (Z) and indicates the element's identity. For a neutral atom, the atomic number also describes the number of electrons around the nucleus. The number of protons in the nucleus of an atom determines the atomic number (Z) and indicates the element's identity. For a neutral atom, the atomic number also describes the number of electrons around the nucleus.

Variations on the number of neutrons in the nucleus give rise to different isotopes of the same element.

Element notation: Atomic number and mass number Mass number – protons + neutrons Atomi number – protons Element symbol

Isotopes and relative atomic masses It is important to be able to calculate the average mass of a mixture of isotopes of a given element It is important to be able to calculate the average mass of a mixture of isotopes of a given element We need the distribution of the isotopes and their mass numbers We need the distribution of the isotopes and their mass numbers For chlorine there are isotopes with mass numbers 35 and 37, with relative abundances of 75.8 % and 24.2 % respectively For chlorine there are isotopes with mass numbers 35 and 37, with relative abundances of 75.8 % and 24.2 % respectively Average atomic mass = Average atomic mass =

What of the electrons? We now understand the atom to contain a tiny positively charged massive nucleus surrounded by a comparatively vast empty space containing the electrons We now understand the atom to contain a tiny positively charged massive nucleus surrounded by a comparatively vast empty space containing the electrons When atoms combine the electrons must interact When atoms combine the electrons must interact We need to understand the arrangement of electrons in the atom We need to understand the arrangement of electrons in the atom