The Structure of Matter Atomic Models Subatomic Particles Forces Within the Atom

Part One: Atomic models past and present

Atomic Models Throughout History Greek (~400 BC) Dalton (1800) Thomson (1897) Rutherford (1908) Bohr (1913) Wave Model

Greek (~400 BC) Atomos: indivisible Democritus said the smallest piece of matter was an atom Theory wasn’t accepted for 2100 years

Dalton (1800) All elements are composed of atoms. Atoms are indivisible and indestructible Atoms of the same element are exactly alike Atoms of different elements are different Compounds are formed by the joining of atoms of two or more elements

Thomson (1897) Discovered negatively charged particles The atom was divisible! Particles discovered are electrons “Plum Pudding Model” Atom consists of positively charged material with negative charges spread evenly throughout

Here’s JJ!

Rutherford (1908) Gold Foil Experiment Positive particles shot at gold foil occasionally bounced back! Proposed dense, positively charged center called the nucleus

Thomson’s theory

Rutherford’s reality

Rutherford’s atom

Bohr (1913) Electrons move in definite orbits around the nucleus Places each electron in a specific energy level

The Bohr Atom

Wave Model Modern model based on wave mechanics Nucleus is surrounded by electrons Electrons do not move in orbits We can determine the probable location of an electron based on the amount of energy the electron has This probable location is called an orbital

Part Two: Subatomic particles

Three main subatomic particles Proton Neutron Electron

Protons Positively charged Found in nucleus Has a mass of 1 amu The number of protons determines the identity of the atom The atomic number tells the number of protons

Elements are made of atoms Hydrogen-1proton Helium-2 protons Lithium-3 protons The number of protons determines the identity of the element!! (atomic number)

What is that other particle found in the nucleus? It’s a neutron! Neutrons have no charge (they’re neutral) Neutrons have a mass of 1 amu Proton + neutron = mass number

Electrons Electrons hang out in “orbitals” outside the nucleus of the atom Electrons have almost no mass The electron cloud is the majority of the volume of the atom ELECTRONS ARE NEGATIVELY CHARGED!

More electron info! In a neutral atom, the number of electrons is equal to the number of protons When an atom gains or loses electrons, it is called an ion.

The ionization process Now I’m a happy ion! I’m POSITIVE! And my outer shell is full, without that troublesome extra electron.

This chloride ion has added an electron. It is a NEGATIVE ion! (note: the inner 2 electrons are not shown) Now I’m a happy ion!!!

Sodium chloride-everybody’s happy! One lost…… One gained. (positive ion) (negative ion)

Quick Review: The three subatomic particles are proton, neutron and electron Protons are positively charged, electrons are negative, neutrons are neutral When an electron is lost or gained, the atom becomes an ion.

Isotopes Atoms with the same number of protons but different numbers of neutrons are called isotopes. Isotopes are designated by the symbol and the mass number: H-1: 1 proton, no neutrons H-2: 1 proton, one neutron H-3: 1 proton, two neutrons

It’s the Isotopes!

What symbols represent ions? Elements on the left side of the periodic table will lose electrons to become positive ions Examples: Ca2+, Na+,Fe3+,Fe2+,Al3+ Elements on the right side of the periodic table will gain electrons to become negative ions Examples: F-, O2-, P3-, Cl-

How big is the nucleus? If an atom was as big as a football field, the electrons would move over the entire field, while the nucleus would be the size of a lima bean in the middle of the field.

If an atom was an elephant, the nucleus would be as big as a flea on the elephant

How massive is the nucleus? Even though the nucleus is VERY small, it is VERY massive. Even though it’s the size of the flea, it would weigh as much as the elephant. And though the electrons take up as much space as the elephant, they would only weigh as much as the flea.

End of Part 2

Part Three: Forces Within The Atom

Forces that govern the behavior of subatomic particles: Strong Weak Electromagnetic Gravity

Strong force Opposes electro- magnetic force of repulsion between protons “Glues” protons together to form the nucleus Greatest of the forces, but has very limited range

Weak force Responsible for radioactive decay in the atom

Electromagnetic force Repels positively charged protons within the nucleus. Do you remember the force which holds them together so the atom doesn’t explode? (Yes- strong force!)

Strong force overcomes electromagnetic repulsion Electromagnetic force causes proton repulsion Strong force overcomes electromagnetic repulsion

Gravity Weakest of the forces Force of attraction between all objects Effect seen easily only for very large objects

Uses of the forces

THE END