History of the Atom & the Periodic Table

Parts of the Atom

Protons = positive charge

Neutrons = no charge

Electrons = negative charge

Nucleus = both protons and neutrons with positive charge

The Nucleus

Nucleus = protons + neutrons The nucleus occupies only a small

fraction of the atom However, it contains almost all the

mass of the atom One proton or neutron is almost

2000 times greater than the mass of an electron

Is There Anything Smaller than an Atom?

YES!! It is called a quark

How Atomic Models have Changed

Model of the atom has changed over the years 6 very important changes have taken

place! 1. Democritus 2. John Dalton 3. J.J. Thompson 4. Ernest Rutherford 5. Niels Bohr 6. Electron Cloud Model

Democritus

400 B.C. Idea proposed that atoms make up all

objects Aristotle disagreed with Democritus'

theory Aristotle’s theory that matter was the

same throughout was accepted

Thought atoms looks like a solid sphere

John Dalton

1800s Proved atoms existed Proposed:

1. All substances are made of atoms and cannot be destroyed

2. Atoms of the same element are exactly alike

3. Atoms join other atoms to make substances (like water)

Proved atoms existed by using a cathode tube

*****Not in notes!

*****Not in notes!

*****Not in notes!

Joseph John “JJ” Thompson 1904 Thompson did not know how

electrons were arranged He thought they were mixed

throughout atom Thought it was a positive sphere with

negative electrons on sphere, scattered around

He thought it looked like a ball of chocolate chip cookies!

Ernest Rutherford

1911 A student of JJ Thompson! Created the Gold Foil Experiment From the experiment, he learned:

1. The atom is mostly empty space 2. The nucleus is very densely packed 3. The nucleus is positively charged

Niels Bohr

Hypothesized that electrons travel in levels around the nucleus

Levels are called orbitals

Modern Electron Model

Known as the electron cloud model Electrons travel in “areas” instead of

levels Areas are called clouds

Match the Model with the Correct Theory…

1. Modern Theory

2. Rutherford Model

3. Thompson Model

The Periodic Table

I. 1st Periodic Table was developed by Dmitri Mendeleev

II. Elements are organized by increasing atomic number (in other words, increase number of protons in the nucleus)

III. Properties in lighter elements could be shown to repeat in heavier elements

Organization of Periodic TableI. Groups (also known as families)

A. Vertical columnsB. Organized by similar propertiesC. Have same # of electrons in outer energy levelsD. Each level can have a maximum # of electronsE. Each row ends when an outer energy level is filledF. You can use electron dot diagrams to represent

outer energy level electrons (known as valance electrons)

II. Periods A. Horizontal rowsB. Organized by increasing # of protons and neutronsC. Classified as metals, nonmetals, and metalloids

Periods…Vertical or horizontal?

HORIZONTAL!!

Groups…Vertical or horizontal?

VERTICAL!!

Valance Electrons (outer electrons) Valance electrons are electrons in the outer

most layer In general, the number of valance electrons

of an element is equal to the group number *Groups 3-12 are

skipped and do not follow the same

pattern Group 18 (Noble Gases)

Stable because outer electron level is full

Atomic Number vs. Mass NumberI. Each element has same number of

protons, but different number of neutrons

II. In a neutral atom, protons (+) and electrons (-) are equal… so they balance out to zero

I. Atomic # – number of protons in an atom (also identifies the element)

II. Mass # – sum of protons and neutrons in the nucleus (identifies mass of the nucleus)

Using Periodic Table, Complete this chart:

Element Atomic # # of Protons / Electrons

# of Neutrons

Mass #

Helium – 4 2 2 2 4

Carbon – 12

Carbon – 14

Oxygen – 16

Isotopes

I. Isotopes – atoms of the same element with different numbers of neutrons

A. Different isotopes have different properties

B. Name of the element, followed by mass # identifies the isotope

1. Ex) Uranium-235 and Uranium-238