Lecture 3, Fall CHAPTER 2. Atoms, molecules, and ions.

2.1. The Atomic Theory of Matter. Democritus (~ 400 BC) thought that matter was made of atoms. Aristotle did not. Democritus ( BC): Democritus' ideas were ignored for the next 2000 years

The Atomic Theory of Matter John Dalton ( ): Elements are made of tiny particles called atoms atoms of different elements can combine to form compounds atoms are not created nor destroyed in chemical processes all atoms of one element are identical John Dalton

The Atomic Theory of Matter Dalton’s Atomic Theory explained.. … The Law of Constant Composition … The Law of Conservation of Mass … The Law of Multiple Proportions [in one compound, number and kind of atoms is constant] [total mass before reaction = total mass after reaction] [If two elements form more than one compound, the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers]

The Law of Multiple Proportions : 12 g carbon + 16 g oxygen =>CO 12 g carbon + 32 g oxygen =>CO 2 32 g : 16 g=2 : 1 The Atomic Theory of Matter HW: 8

2.2. The Discovery of Atomic Structure. Cathode Rays and Electrons. Discovery of the electron. J.J. Thomson ( ): discovered the electron atoms of all elements contain electrons electron: negatively charged particles of very small mass

Cathode Rays and Electrons. Discovery of the electron. Cathode ray tube J.J Thomson's original Cathode Ray Tube

TV tubes are cathode-ray tubes. The beam of electrons is accelerated towards the +ve cathode, which has a hole in it through which the beam passes and strikes a target. electrically charged plates

J. J. Thompson and the electron: J. J. Thompson investigated these beams, and found they were repelled by –ve charge, attracted by a +ve charge, and deflected by a magnetic field. He concluded these particles were negatively charged electrons. He was able only to calculate a charge to mass ratio from his experiments.

"smeared-out" positive charge J.J. Thomson's Model: “Plum Pudding Model” electrons embedded in +ve charge like pieces of fruit in a fruit cake

Determination of mass of the electron: Millikan’s oil-drop experiment (1909)

Determination of mass of the electron: Robert Millikan performed his famous oil-drop experiment in which he measured the charge on the electron, from which its mass could then be calculated. Mass of electron = 9.10 x g

Radioactivity and the structure of the atom. In 1896 Henri Becquerel discovered that a Uranium compound spontaneously emitted high-energy radiation, = radioactivity. Henri Becquerel

Becquerel left some urananite wrapped in a piece of photo- graphic paper, and found (right) that it had been ‘fogged’ by the radioactivity Becquerel’s original piece of photographic paper fogged by uraninite

Rutherford and the discovery of the nucleus: Rutherford investigated the nature of radioactivity. Three types of radiation were emitted, α, β, and γ particles. Experiments showed that with electrically charged plates, α rays were deflected towards the -ve plate, γ rays were not affected, and β rays were attracted towards the +ve plate. Earnest Rutherford

Rutherford analyzed particles emitted by radioactive substances: It turns out that α particles are positive He nuclei, β particles are –ve electrons, and γ particles are neutral photons.

Rutherford and the deflection of alpha particles by gold foil. Rutherford examined beams of α particles emitted by radioactive substances. Nearly all of the beams passed straight through a sheet of gold foil a few thousand atoms thick (next slide). His undergraduate student Marsden found that a few were scattered at large angles. From this Rutherford proposed the nuclear model of the atom.

Rutherford’s gold foil experiment:

First page of Rutherford’s paper in the Philospohical Magazine (1911) outlining his ideas on the structure of the atom

Rutherford’s gold foil experiment and the nuclear model of the atom. only one in 20,000 alpha particles is deflected, so atom Is mostly empty space all the protons and neutrons are concentrated in the nucleus electrons orbit the much heavier nucleus Alpha particle is a helium nucleus most alpha particles go straight through

2.3 The Modern View of Atomic Structure. The basic building blocks of the atom are the proton, neutron, and electron. The charge on the electron is x C (Coulombs), and on the proton is x C. The neutron is neutral. Nucleus has 6 protons and 6 neutrons 6 electrons orbit the nucleus An atom – the carbon atom

Every atom has an equal number of protons and electrons = atomic number (Z). Protons and neutrons reside together in the nucleus. Protons are held together by the strong nuclear force, which is much stronger than electrostatic forces, but acts over only very small distances. ISOTOPES: These have same Z (same number of protons) but different numbers of neutrons in the nucleus.

The amu (atomic mass unit). Grams are too big to be convenient for describing the masses of nuclei. We use the amu, which is x g. (This comes from 1/6.022 x which is 1/Avogadro’s number). ParticleChargeMass (amu) Proton Neutron Electron x 10 -4

Atoms are very small. Have diameters between 100 and 500 pm (pm = picometers = m ). Can also be expressed as Angstroms (Å), 1-5 Å (an Å = meters) meters (1 nanometer) or about inches atom