Atoms & Their StructureChapter 2 Section 1

Part 2

Objectives

How have historic experiments led to the development of the modern model of the atom?

How is the modern model of the atom different from previous models?

What information is available in an element block of the periodic table?

Important Vocabulary Atom Atomic theory Law of

conservation of mass/matter

Law of definite proportions

Electron Proton

Electron cloud Isotope Neutron Nucleus Atomic number Mass number Atomic mass unit Atomic mass

Subatomic Particles

In the mid-1880’s, through many experiments, scientists discovered that atoms can be broken down

The smaller parts of atoms are called subatomic particles

The three most important subatomic particles are the electron, proton, & neutron

Other subatomic particles include: quarks, leptons, photons, gravitons, & neutrinos

Discovery of the Proton

Evidence for a positively charged particle was found in 1886 by Eugen Goldstein

He observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays

He called these rays canal rays

The Proton Positively charge particle Resides in the nucleus of an atom Has a 1+ charge Its mass is 1.67 x 10-24 g Symbolized with a p or p+

Discovery of the Electron

Its discovery was by accident and occurred in 1897

J.J. Thomson, an English physicist, was studying current using electrodes, one positive (anode) and the other negative (cathode)

His experiment determined that the cathode ray was negatively charged Cathode-ray tubes, are currently

used in TV sets, computer monitors and radar displays

Cathode-Ray Tube

Thomson’s Results

Thomson confirmed his prediction by seeing how electric and magnetic fields affected the cathode ray

His experiments showed that a cathode ray consists of particles that have mass and a negative charge

He also developed the plum-pudding model of an atom

Discovery of the Nucleus In 1909, Ernest Rutherford

disproved Thomson’s plum-pudding model by shooting a small beam of positively charged particles at a piece of gold foil

Some of the particles in the beam were reflected back

Leading Rutherford to hypothesize that there must be a positively charged mass in the center of the gold atoms

Rutherford’s Experiment

Rutherford’s Atomic Model

Is also known as the nuclear atom

In the nuclear atom: The protons and neutrons

are located in the nucleus The electrons are

distributed around the nucleus and occupy almost all the volume of the atom

This model was better than Thomson’s but it still was incomplete

The Nucleus

It is the dense central portion of the atoms

It contains nearly all the mass of an atom and all of the positive charge Protons and neutrons!

This part of the model of the atom is still considered true today

Bohr’s Model of the Atom

In 1913, Niels Bohr suggested that electrons in an atom move in set paths around the nucleus

“Like planets in orbit” The path defines the electron’s energy

level

1. Electrons can only be in certain energy levels

2. Electrons must gain energy to move to a higher energy level

3. Electrons must lose energy to move to a lower level

Bohr’s Model

Nucleus

Electron

Proton

Robert A Millikan

Was a U.S. physicist He carried out

experiments to find the quantity of charge carried by an electron

Using Thomson’s charge-to-mass ratio of an electron, he calculated the mass of an electron in 1916

Quantum Mechanics Model In 1926 Erwin Schrödinger, an Austrian

physicist, took the Bohr atom model one step further

He used mathematical equations to describe the likelihood of finding an electron in a certain position

Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron

This model can be portrayed as a nucleus surrounded by an electron cloud

Quantum Mechanics Model

Electrons

Are negatively charged particles Charge is 1‒

Symbolized by e or e‒

Mass is 9.11 x 10-28 g or 1/1840 the mass of a hydrogen atom

Discovery of the Neutron

Was discovered by the English scientist James Chadwick in 1932

Irene Joliot-Curie had discovered that when alpha particles hit a sample of beryllium, a beam that could go through almost anything was produced

Using this experiment done by Irene Joliot-Curie, Chadwick concluded that the particles in the beam had no charge

Neutrons

Particles that have no charge Reside in the nucleus Have a mass of 1.67 x 10-24 g Symbolized with a n or n0

Summary of Atoms Are the building blocks of

molecules Smallest part of an element that still

has the element’s properties Unreacted atoms have no overall

charge Atoms have 5 basic parts Atoms are made of protons, neutrons,

& electrons (subatomic particles) The protons & neutrons are housed in

the center of the atom in the nucleus Electrons are moving around outside of

the nucleus within the electron cloud

Modern Atomic Model By 1925, Bohr’s model didn’t explain

electron behavior The new model proposed that electrons

behave like waves on a vibrating string This is known as the “wave-particle

duality of nature” This model was developed by Louis de

Broglie, Albert Einstein & Max Planck