The Beginning of the Atomic Theory Democritus was

The Beginning of the Atomic Theory • Democritus was a Greek philosopher who was among the first to suggest the existence of atoms. 460 BC – 370 BC

Atomic Theory • Democritus was responsible for giving the atom its name (Atmos). • In his atomic theory he stated that matter is made up of tiny indivisible particles called atoms (literally, that which cannot be cut).

Aristotle • Another Greek philosopher who disagreed with Democritus’s ideas. • Matter is composed of 4 elements, Earth, water, air, and fire. • Most people thought he was right.

Greek Philosophers

Atom • An atom is the smallest particle an element can be divided and still be the same substance.

The Atomic Theory (late 1700 s) • Scientist learned that elements combine in certain proportions based on mass to form compounds. • Hydrogen and oxygen always combine in the same proportion to A molecule is the smallest a form water. compound can be broken down to and still retains the properties of the compound.

John Dalton • A British chemist wanted to know why. He experimented with substances and his results suggested that elements combined in certain proportions because they are made of single atoms.

Dalton’s Atomic Theory • Atoms are small hard particles that created, divided or destroyed. • Atoms of the same element are exactly alike and atoms of different elements are different. • Atoms can join with other atoms to make new substances.

Dalton’s Atomic Theory • Elements are composed of atoms and atoms of different elements have different masses.

Dalton’s Atomic Theory • Compounds are composed of molecules. • Molecules are composed of atoms in definite proportions.

John Dalton

Size of Atoms • If you could line up 100, 000 copper atoms in a single file, they would be about 1 cm long. • Elements can only be seen with electron microscopes Electron microscope: it uses an electron beam (as opposed to light) to provide magnification that is markedly superior to that of an optical microscope.

J. J. Thomson • One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles. • Electrons, protons, and neutrons. • There are other types of particles (quarks).

J. J. Thomson • In 1897, J. J. Thomson used to cathode ray tube to discover the presence of a negatively charged particle (the electron).

J J Thomson The cathode ray will always be attracted by the positive magnetic and deflected by the negative.

JJ Thomson • After learning atoms contained electrons, Thomson proposed a new model of the atom, the plum pudding model. Electrons float in materials that are positively charged.

Conclusions About Electrons • Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons. • Electrons have so little mass that atoms must contain other particles that account for most of the mass. The mass of an electron is almost 1, 000 times smaller than the mass of a proton.

J J Thomson

Ernest Rutherford (1909) • Beamed positively charged particles at gold foil. • Hypothesis: particles will pass through.

Ernest Rutherford • Most particles passes through but some were deflected & some bounced back.

Rutherford Model • Discovered the nucleus in the center of the atom.

Conclusions of Rutherford’s Experiments • The nucleus is small. • The nucleus is dense. • The nucleus is positively charged.

Rutherford Atomic Model • The atom is mostly empty space. • The positive charge and most of the mass is found in a small area in the center called the nucleus. • The electrons around the nucleus occupy most of the volume.

Earnest Rutherford

Niels Bohr (1913) • Proposed electrons move around the nucleus in certain paths (energy levels) similar to the way planets orbit the sun. Planetary Model

Bohr Model • The energy level an electron normally occupies is called its ground state. But it can move to a higher-energy, less-stable level, or shell, by absorbing energy. This higher-energy, less-stable state is called the electron’s excited state.

Bohr Model • After it’s done being excited, the electron can return to its original ground state by releasing the energy it has absorbed.

Bohr Model • 1 st energy level near the nucleus holds 2 electrons • All other energy levels contain more electrons.

Niels Bohr

Discovery of the Neutron • James Chadwick (1932) noticed a difference in atomic number and atomic weight of atoms.

Discovery of the Neutron • Helium should have an atomic mass of 2 but had a mass of 4.

Discovery of the Neutron • There were 2 extra particles in a helium atom

Discovery of the Neutron • Chadwick proved there was a new kind of particle in the nucleus about as heavy as a proton but with no charge.

Modern Atomic Theory • Electrons don’t travel in paths and their exact paths can’t be predicted. • Regions where electrons are likely to be are called electron clouds

Recent Developments • Protons and neutrons consist of smaller particles called quarks. • up, down, strange, charmed, top, bottom.

$Recent Developments • Quarks have fractional charges of +⅔ and -⅓ unlike the +1$

Recent Developments • Quarks have fractional charges of +⅔ and -⅓ unlike the +1 charge of a proton and the -1 charge of the electron

Development of Atomic Theory 1. 2. 3. 4. 5. Greeks believed the atom was indivisible. Dalton “atomic theory of matter. ” Thompson discovered the electron. Rutherford discovered the nucleus. Bohr “Atoms travel in specific energy levels. 6. Electron cloud model and Quarks.

Development of Atomic Theory