Chapter 6

Introduction to Atoms

Ch 6 Sec 1

Development of Atomic Theory

Vocabulary

• Atom• Electron• Nucleus• Electron cloud

Vocabulary

• Atom– Smallest unit of an element that still maintains the

properties of that element• Electron– Subatomic particle– Negative charge

Vocabulary

• Nucleus– Atom’s central region– has protons and neutrons

• Electron Cloud– Region around the nucleus where electrons are

found

Beginning of Atomic Theory

• Democritus – Greek philosopher 440 BCE– Thought you cut any thing till at some point

you could cut it down into anything simplier• Atomos

– Not able to be divided

Dalton’s Atomic Theory• By 1700s science was in full swing• John Dalton– Wanted to know why compounds always

form in set ratios– Set forth 3 key theories • All substances made of atoms. Atoms cannot be

created or destroyed• Atoms of same elements are identical• Atoms join with other atoms to make new

substances

Thomson’s Discovery of Electrons

• In 1837 JJ Thomson discovered the electron– Proved that the atom isn’t the smallest

particle• Found negatively charged particle– Electron

• Thought electrons scattered throughout the atom– Plum pudding model

Rutherford’s Atomic “Shooting Gallery”

• In 1909 Ernest Rutherford wanted to tested Thomson’s subatomic particle idea

• Aimed positive particles at a thin sheet of gold foil– Most positive particles passed through but

some bounced off to side and some bounced back

Nucleus and the Electrons

• Rutherford’s experiment proved that there’s a nucleus– Dense center of the atom– Positive charge

Copy these diagrams

Nucleus & Electrons

• Bohr’s electrons levels– Electron’s move in definite energy levels

outside the nucleus.– Electrons only in those energy levels, never

in between

Nucleus & Electrons

• Modern Atomic Theory– Electrons still in energy levels– But can move from level to level

Ch 6 Sec 2: The Atom

Vocabulary

• Proton• Atomic mass unit• Neutron• Atomic number• Isotope• Mass number• Atomic mass

Vocabulary

Define on your own!

Parts of the Atom

• There are 3 main subatomic particles– Proton • Positive charge

– Neutron• No charge, neutral

– Electron• Negative charge

Parts of an Atom

• The number of subatomic particles determine how the atoms behaves– Change the # of protons = change the

element– Change the # of neutrons = change the

weight of the atom– Change the # of electrons = change how the

atom may form chemical bonds

Parts of an Atom

• Only protons and neutrons have measurable mass

• Mass measured in “amu” units – Atomic mass unit

Parts of an Atom

• Electrons– Compared to protons & neutrons, electrons

have extremely small mass– Negative charge of one electron cancels the

positive charge of one proton– Ion • An atom with unequal # of electrons vs protons• Very important to chemical bonding!

Atoms and Elements

• Role of Neutrons– Act as a buffer to the positive charges of the

protons• Otherwise the nucleus would tear apart

– Nucleus does not have to equal number of neutrons and protons• Generally more neutrons than protons inside an

atom’s nucleus

Atoms and Elements

• Atomic number of an element = number of protons– Every oxygen atom has 8 protons– If you change the number of protons, you

change the element!

TEST ALERT…..

# of protons = Atomic Number

Isotopes

• Can vary the number of neutrons• Isotopes– Atoms of the same elements, but with

different number of protons

Isotopes

• Radioactive atoms – Have unstable nucleus and decay at fast

rates• Nearly all isotopes of the same elements

have similar chemical and physical properties

Isotopes

• Difference in isotopes– Mass number = sum of protons and neutrons•MN = P + N

–Carbon isotope• 6 protons• 10 neutrons• mass number = 6 + 10 = 16

Isotopes

• Naming Isotopes– Write the name of the element, followed by

a hyphen and the mass numberoxygen-18Mass = protons + neutrons18 = 8 protons + ____ neutrons18 = 8 protons + 10 neutrons

Isotopes

• Atomic mass– Weighted average of all the isotope mass

numbers naturally occurring– Most elements found in nature contain a

mix of 2 or more isotopes

Forces in Atoms

• Gravitational force– Pulls particles together

• Electromagnetic Force– Particles with same charge repeal each other– Reason for neutrons in a nucleus

Forces in Atoms

• Strong Force– Helps to keep protons near each other

• Weak Force– In radioactive isotopes• Plays role when neutrons change to protons or

electrons– Changing is nuclear reaction