Chapter 4

4.1 Atoms

Democritus (460 BC – 370 BC)

first suggested the idea of atoms

Indivisible and indestructible

Atoms The first model

of the atoms was Dalton’s

“All mater is made up of individual particles , which are indivisible”

Dalton’s Atomic Theory1. All matter is made of atoms.

Atoms are indivisible and indestructible.

Dalton’s Atomic Theory

2. All atoms of a given element are identical in mass and properties

Dalton’s Atomic Theory

3. Compounds are formed by a combination of two or more different kinds of atoms.

Dalton’s Atomic Theory

4. A chemical reaction is a rearrangement of atoms

Thomson’s Model Discovered electrons

Often called the “Plum-Pudding” Model

No mention of amount of electrons or their arrangement around the nucleus

Revised Dalton’s theory to account for subatomic particles

Rutherford Model

Discovered nucleusAll of an atom’s positive

charge is concentrated in its nucleus

Electrons surround a dense nucleus

Rest of the atom is empty space

Rutherford Model

Known as the nuclear model The protons are located in the

nucleus The electrons are around the

nucleus The electrons occupy most of

the volume of the nucleus

The Atom The smallest part of an element VERY SMALL

Atomic Structure

Atoms can be broken downProtonsNeutronsElectrons

Every Element is different based on the number of each (individual personality)

Protons (p+) Positively Charged

Each has a “+1” charge

Electrons (e-) Negatively charged

Each has a “-1” charge

Neutrons (n0) No charge or “neutral”

Mass = mass of proton

The Atomic Nucleus

The central core of an atomMade of p+ and n0

Most of the mass, little volume

Nucleus has a positive charge

The Atomic Nucleus

Electrons orbit around nucleus like planets in the solar system Called the “electron cloud”Very little mass, lots of

volume

How do we know the number of each elements p+ , e- , n0 Periodic Table is arranged by the

element’s numbers

1 1.008

H Nuclear Symbol

Hydrogen Name of Element

Atomic Number

Mass Number (round to the nearest whole number)

Atomic Number Amount of protons from one

element to the next

Ex: Oxygen atomic number = 8 because it

has 8 protons

Atomic Number Since all elements start off

as neutral ….

The number of protons = number of electrons!

Mass Number Mass Number = protons + neutrons

Composition of an Element

Use atomic number and mass number to determine composition

# p+ = atomic # # e- = atomic #

# n0 = mass # – atomic #

What can change in an atom

Protons: can never change

Electrons: if the number changes, then an ion is formed

Neutrons: If the number changes, then an isotope is formed

IF the proton number changes… Then you have an entirely

different atom

If the neutron number changes… Called an Isotope Mass number changes

If an atom gains electrons, then… The atom becomes negatively charged

If an atom loses an electron, then…

It becomes positively charged

Isotopes of Elements

Protons never change, but the number of neutrons may vary

Isotopes

Isotopes of the same element are the same except for # of n0

# of n0 vary so mass number changes

Isotopes Carbon-12, Carbon-14, Carbon-16 How many protons in each version of

carbon? How many neutrons in each version

of carbon?

Hydrogen

Hydrogen has three known isotopes

Hydrogen-1 (one proton, no neutrons) Hydrogen-2 (one proton, 1 neutron) Hydrogen-3 (one proton, 2 neutron)

4.3 Bohr’s Model

Electrons arranged in circular paths around nucleus

Orbit like planets n = energy level Only a certain amount of

electrons can fit in each energy level

Bohr’s Model

Electrons are located in energy levels with a fixed amount of energy

Energy Levels

Each energy level can only hold 2 electrons

Each energy level has “X” number of orbitals that can hold 2 electrons each

Pauli Exclusion PrincipleEach orbital holds 2 electrons that

spin in opposite directions

Energy Levels

How many electrons fit in the 1st, 2nd, 3rd and 4th energy levels?

Energy Level Number of

OrbitalsMaximum number

of Electrons

1 1 2

2 4 8

3 9 18

4 16 32

Hund’s Rule

When electrons occupy orbitals, one electron enters each orbital until all orbitals contain their max amount

Hund’s Rule

Partially filled orbitals are much more stable than empty orbitals

Example:

Carbon has 6e-

has 2e- in first orbital

has 4e- in second orbital

Orbitals simplified

Each energy level can hold 8 electrons except the first which holds 2

Fill in each level until