4.8 Development of a Modern Atomic Theory

(Sec 7.1 pg 204)

• Studying something you cannot see is difficult, but not necessarily impossible in science. You use measurements and observations to make educated guesses (hypotheses), but you need a lot of data.

• As more data became available about the atom, scientists' ideas about the atom (and elements) kept changing.

• By the 1700s scientists knew that each element from the periodic table could not be separated into simpler elements.

– 1808 – John Dalton develops the 1st modern theory of the atom, which stated:• All matter is made of atoms.• Each element has its own kind of atom, with its own unique

mass.• All atoms of an element are identical.• Compounds are mixtures of atoms from different elements.• Atoms cannot be created, destroyed or sub-divided.

• Dalton's model suggested that atoms were different from molecules. An atom is a single unit (composed on only one element). A molecule is a group of atoms.

WATER MOLECULE

– 1830 – Michael Faraday showed that positive and negative charges are contained in atoms. He then revised Dalton’s theory:• Matter must contain positive and negative charges.• Opposite charges attract, like charges repel.• Atoms combine to form compounds because of their attraction

to each other.

• Faraday's new model explained the presence of charges in matter, and how atoms group into compounds and molecules.

• However, it did not explain where these charges came from, or how they got into the atom.

– 1898 – J.J. Thomson showed that negatively charged particles were contained in all kinds of atoms. He suggested that particles called electrons were stuck throughout a positively charged atom. He called his model the “Raisin-Bun” model. Thompson proposed:• Atoms contain electrons• Electrons have very little mass and have a negative charge.• The rest of the atom is positive. • Electrons are stuck throughout the atom randomly.• Electrons can be added or removed from atoms to give them

positive or negative charge.

• Thompson's new model explained how charge was transferred between atoms, and why they might have these charges.

• However, this model did not fully explain how positive and negative charges stayed separate.

– In 1911 – Ernest Rutherford conducted a famous experiment that showed that a positively-charged beam often passed through a very thin sheet of gold (only a few atoms thick) without interference, but once in a while the beam bounced off as if it hit something solid (see Fig.7 pg.206).

– His revision to the atom model is sometimes called the ‘nuclear model’:• Atoms are made of 3 particles: electrons (negative), protons

(positive), and neutron (neutral).• The nucleus contains all of the positive charge and most of the

mass. • The nucleus contains the protons and neutrons.• The nucleus is tiny compared to the rest of the atom.• The electrons orbit the nucleus like planets around the sun.• The mass of the electron is 1/1800th the mass of the proton.• The size of the atom is determined by the furthest electron

from the nucleus. • There is only empty space between the nucleus and the

electrons.

• Rutherford knew his model was incomplete, but without more data he could not figure out what was missing.