2 Matter Everything is made of matter. Matter has mass and occupies volumeMatter is made of atomsAtoms are the smallest unit of an element, which is a type of matter.
3 What is an atom? An atom is the building block of all matter. It is the smallest unit of a substanceIt has taken many yearsto get to our currentidea of the atom.Since atoms can’t be seen,scientists create models.A model is a representationof something else.
4 The History of the Atomic Model The first atomic model was created in 400 B.C. and was created by Democritus.He stated that atoms were the smallest parts of substances, were indivisible and indestructible, and called them atmos meaning not to be cut.Democratus had no proof, only beliefHis theory was disputed by Aristotle whose theory that matter was uniform throughout was accepted for the next 2,000 years.
6 Out of the Dark Ages – Dalton’s Model In 1803, John Dalton created a model of the atom which was also a solid sphere.There were four parts to Dalton’s TheoryEach element was composed of identical atoms.Atoms of different elements can physically or chemically combine in simple whole number compounds.Atoms of one element can never be changed into another.Chemical reactions occur when atoms are separated, joined, or rearranged
7 Thompson’s Plum Pudding Model J.J. Thompson was the first to suggest that atoms are made of smaller particles.He passed an electric current through a gas and noticed it gave off negatively charged particles.Thompson concluded that the negatively charged particles came from within the atom.Since the gas was not charged there must also be a positively charged part of the atom.Thompson had discovered the electron.
8 Thompson’s Plum Pudding Model From his discovery of the electron, Thompson created this model of the atomAtoms are composed of negatively charged particles called electrons in a positively charged “pudding” material.
10 Rutherford's Gold Foil Experiment In this experiment, a narrow stream of positively charged alpha particles were aimed at a piece of gold foil.An alpha particle is a helium atom without electrons.Most of the particles passed through the foil with no deflection, while some bounced back.Why would this be a surprise?
11 Rutherford's Atomic Model The theory was that the positively charged particles would pass through and be deflected by the like charge of the positive pudding.Since this did not happen, Rutherford theorized that most of the atom was empty space with most of the mass and the positive charge at the center, called the nucleus.Rutherford later discovered and named the positively charged particle in the nucleus the proton.
12 The Rutherford Atomic Model The Rutherford Atomic Model had the positive charge located in the center of the atom in a place called the nucleus.The electrons were distributed in the empty space of the atom.
13 Bohr’s Planetary Model Rutherford’s atomic model could not explain the chemical properties of elements.In an attempt to explain this, Niels Bohr theorized that the electrons orbited the nucleus in specific circular paths.Each electron moves around the nucleus with fixed energy called an energy level.To move to a higher energy level, an electron must absorb a specific amount of energy called a quantum
14 Bohr’s Planetary Model Once an electron gains or loses the quantum of energy it may move to a new orbit or energy level.This model was accurate for hydrogen, but did not explain some of the behaviors seen in atoms of more than one electron.
15 Electron Cloud ModelBy 1926, the electron cloud model had been created.An electron cloud is the area around the nucleus where its electrons are most likely to be found.The position of electrons in the cloud can only be described by a probability.
16 Neutron - The Final Particle The final subatomic particle was discovered inJames Chadwick discovered a particle with no charge but nearly equal mass to the proton.The neutron is located in the nucleus with the protons.A neutron is a combination ofa proton and an electron