Atomic Models

INTRODUCTION Scientists have learned to use models to show things that are too big or too small to be shown easily.

INTRODUCTION In the case of atoms, scientists use large models to explain something that is very small. Models of the atom were used to explain data or facts that were gathered experimentally, so these models are also theories.

The Greeks philosopher Democritus was the first person to say that the universe was made of empty space and tiny bits of stuff that he called “a-tomos”. He believed that atoms could not be divided. Democritus – 530 BC INTRODUCTION

Democritus wrote that atoms moved about in a void, bouncing off of one another or interlocking to form clusters. He believed that all the changes that happened in the world were a result of atoms changing their movements or the way they were packed together.

INTRODUCTION The Greeks ideas about atoms would remain virtually unchanged for more than 2,300 years, when an Englishman by the name of John Dalton put forth his ideas about atoms and what they looked like in 1808.

INTRODUCTION Dalton proposed what came to be known as the billiard ball model, because he said that atoms were like solid little balls.

INTRODUCTION Dalton’s model explained that these balls had hooks on them that held them together.

INTRODUCTION Dalton also stated that all elements are composed of indivisible particles, that atoms of the same element are the same, that atoms of different elements are different from each other, and that compounds consisted of atoms of different elements combined together.

INTRODUCTION In 1811 Amedeo Avogadro recognized that atoms and molecules were different. He realized that molecules were made up of atoms.

INTRODUCTION Avogadro also figured out a way to calculate the mass of atoms, the amu. amu stands for atomic mass units, which is the measurement we still use today.

INTRODUCTION In 1897 JJ Thomson and his team of British physicists discovered the electron and that it is a negatively charged particle. Eureka!!! I have found the electron!!!

INTRODUCTION He proposed that atoms were constructed like a bowl of pudding.

INTRODUCTION There was a central mass of positive material in which the negative particles were scattered about like raisins, just like they are in plum pudding. His model became known as the plum pudding model.

INTRODUCTION In 1911 Earnest Rutherford discovered that atoms have a solid positive core at their center. He called these particles protons.

INTRODUCTION During a famous experiment called the gold foil experiment, he fired alpha particles at a piece of gold foil.

INTRODUCTION When the particles didn’t all go through as he predicted he knew that something must be getting in the way. He deduced that there must be a large center mass that the particles ran into. “I’m positive.” Proton

INTRODUCTION Rutherford also said that atoms were mostly empty space surrounded by negative electrons. These electrons would orbit the nucleus, but eventually would spiral inward and collapse on the nucleus.

INTRODUCTION Not too long after Rutherford’s model, in 1913, Niels Bohr released a model which came to be known as the planetary model, because it looked like planets rotating around a sun.

INTRODUCTION In this model the electrons orbited the nucleus in stable orbits. Bohr proposed that the electrons could change orbits if they had enough energy.

INTRODUCTION In 1924 Erwin Schrödinger suggested a new model where electrons orbit the nucleus in a cloud.

INTRODUCTION This electron cloud model, also called the quantum model shows the locations where you would most likely find the electrons in an atom. The cloud is densest where the electron is most likely to be.

INTRODUCTION In 1932 James Chadwick proved that neutrons exist. Until that point most scientists still believed that the nucleus of the atom was a positively charged proton. The problem was that they could not account for the additional mass that was present. Chadwick’s discovery of the new particle solved this dilemma.

INTRODUCTION In 1963, Murray Gell-Mann proposed the quark theory.

INTRODUCTION Protons and neutrons, he said, were made up of smaller fundamental particles that he called quarks.

INTRODUCTION The current model of the atom is sometimes called the quantum mechanical wave model.

INTRODUCTION The current model is based on quantum theory. It supposes that electrons exist in probability clouds called orbitals. It also states that: Electrons do NOT orbit the nucleus in a circular pattern. Electrons do NOT orbit the nucleus in a circular pattern.

OBJECTIVES Identify atoms as the smallest particles of an element. Describe how the theory of atomic structure has changed over time.

IN QUESTION If you had an unknown substance that you needed to identify, what property of the substance would be the most useful in helping you to figure out what it was? density

NOTES Democritus – 530 BC everything made of small parts called a-tomos atoms are indivisible John Dalton billiard ball model atoms - small, solid balls with hooks each element made of its own kind of atom

NOTES Amadeo Avogadro – 1811 – Atoms and molecules are different – Calculated the mass of atoms – amu (atomic mass units)

NOTES J.J. Thomson plum pudding model discovered electrons Ernest Rutherford discovered protons atoms are mostly empty space electron orbits not stable

NOTES Niels Bohr planetary model electron orbits regular and stable Erwin Schrödinger electron cloud model electrons located somewhere in a cloud around the nucleus

NOTES James Chadwick discovered neutrons Murray Gell-Mann – 1963 protons and neutrons made of smaller particles smaller particles named quarks

NOTES current model quantum mechanical wave model – atoms located in cloud like regions – electrons NOT in circular orbits – cannot know where electrons are at any given time with certainty

OUT QUESTION How much time passed between Democritus’ observations of the atom and the development of the billiard ball model by John Dalton? = 2338 years