The Atom

What is Matter?

• anything that has mass and takes up space

• matter is made up of atoms

How Small are Atoms?

• It would take a stack of about 50,000 aluminum atoms to equal the thickness of a sheet of aluminum foil from your kitchen

• If you could enlarge a penny until it was as wide as the US, each of its atoms would be only about 3 cm in diameter – about the size of a ping pong ball

• A human hair is about 1 million carbon atoms wide • A typical human cell contains roughly 1 trillion atoms• A speck of dust might contain 3x1012 (3 trillion) atoms • It would take you around 500 years to count the

number of atoms in a grain of salt

Results

• How many cuts were we able to make?• Do you think we could keep cutting the paper

forever? Why or why not?• You would have to cut the paper in half

around thirty-one (31) times to get to the size of any atom.

What is an Atom?• the smallest unit into

which matter can be divided

• building blocks of all matter

• comprised of a nucleus (at it’s center) and an electron cloud (surrounding the nucleus)

• made up of sub-atomic particles

• smaller than an atom• protons • neutrons• electronsWhat are

Subatomic Particles?

Early PostulatesWhat is everything (Matter)made of?

Greek Mythology –Fire, Air, Water, Earth

Aristotle in “Metaphysics”----- The shape and existence of all things are determined by void and atoms through the differences in shape, position and arrangement of the atoms and the proportion of void in the substance.

Leucippius (Born-50BCE) Democritus (Born –460 BCE)

Postulated an indivisible form of matter

THE ATOM

The Plum Pudding Model

Electrons

(Negativly Charged)

J.J. Thompson- 1904

Field

(Positively Charged)

Watermelon model/ The Chocolate Chip Model

Earnest Rutherford-1911

Discovery of the Nucleus

Rutherford called it a“Central Charge(1920 called it a Proton)

Early Atom Models:What is the

Rutherford Model?

• mostly empty space

• small, positive nucleus

• contained protons and electrons scattered around the outside

Bohr,s Model

Nucleus

1913 – Atomic Model with Fixed Orbits proposed – Modeled after the Solar System

Electrons

Protons

Inner Orbit

Outer Orbit

Nucleus

The Atom has No Charge

Number of Protons =

Number of Electrons

Early Atom Models:What is the Bohr Model?

• electrons move in definite orbits around the nucleus

What are Protons?

• positively charged particles (+)

• help make up the nucleus of the atom

• equal to the atomic number of the atom

• contribute to the atomic mass

• equal to the number of electrons

+

+

+++

+++

-

-

-

-

--

-

-+

What are Neutrons?

• neutral particles; have no electric charge

• help make up the nucleus of the atom

• contribute to the atomic mass

+

+++

+++

-

-

-

-

--

-

-+

What are Electrons?

• Negatively charged particles (-)

• found outside the nucleus of the atom in the electron orbits/levels

• each orbit/level can hold a maximum number of electrons

• 1st orbital = 2 electrons• 2nd orbital = 8 electrons• 3rd orbital = 8 electrons

• move so rapidly around the nucleus that they create an electron cloud

• mass is insignificant when compared to protons and neutrons

• equal to the number of protons• involved in the formation of chemical

bonds

+

+++

+++

-

-

-

-

--

-

-+

- -

What are Valence Electrons?

• found in the outermost energy level of the electron cloud (called the valence shell)

• involved in bonding

• determine the chemical properties of an element

Notice that the electrons are not apart of the nucleus

-

-

-

-

Protons and neutrons are grouped together to form the “center” or nucleus of an atom

How is the Atom’s Center Formed?

Atomic Structure

How are Atoms Described?

• atomic number = number of protons

• in a neutral atom, the # of protons = the # of electrons

• atomic mass= the number of protons + the number of neutrons

What is Atomic Number?

• the number of protons in the nucleus of an atom

+ ++

-

-

-

What would be the atomic number of this atom?

What is the Atomic Mass?

• the total number of protons and neutrons in an atom’s nucleus

• expressed in Atomic Mass Units (amu)– each proton or neutron has

a mass of 1 amu

+ ++-

-

-What would be the atomic

mass of this atom?

+ 3

4

3 protons + 4 neutrons = an atomic mass of 7 amu

Why did we not account for the electrons when calculating the mass

number?

Hydrogen (H) Atom

• Let’s use the Bohr Model to sketch a hydrogen atom.

• notice the one electron in the first orbital

+

-

+

-

= 1

= 0

= 1

P = Atomic # =E = P =N = Atomic Mass – Atomic # =

Oxygen (O) Atom

• notice the two electrons in the first orbital/level and the six in the second

P = Atomic #

________ E = P________N = Atomic Mass – Atomic #

________

Bohr Diagram

Sodium (Na) Atom• notice the two electrons in the first orbital/level, eight in

the second, and one in the third

P = E =N =

Bohr Diagram