Objective To understand what atoms are and how their ...

Objective – To understand what atoms

are and how their characteristics

determine the periodic table.

Throughout history, scientists have tried to

explain what made up matter

Democritus – Greek philosopher

First to propose “atoms”

Invisible, indestructible, fundamental units of matter

Formulated the first “atomic theory” Elements are made of tiny

particles called atoms.

All atoms of a given element are identical, but different from atoms of any other element.

Compounds are formed when atoms of different elements combine in fixed proportions.

A chemical reaction involves the rearrangement of atoms, not a change in the atoms themselves.

Found that atoms are made of smaller particles

Used cathode ray tube to shoot a beam of electrons that travel in a straight line Put magnets on sides of tube and the

ray bent towards the positive side.

Since opposites attract, electrons must be negative

His model was that electrons floated in a soup of positive particles “plum pudding” or “chocolate chip

cookie” model

Millikan – first measured the electrical charge of an electron Oil drop method – put

a charge on a drop of oil and dropped it between two charged plates. He would adjust the power of the plates to suspend the drop in mid-air, defying gravity

Goldstein – found the proton Used a cathode ray

tube to observe canal rays (protons) traveling in opposite directions of cathode rays and were attracted to the negative end of the magnet.

Found the nucleus of the atom Gold foil experiment – shot

positively charged Helium at gold foil to see if atom was same all the way through

Most particles when straight through, some were deflected

Because H+2 is positively charged and some were deflected, he concluded there must be a positively charged mass in the atom.

Atom is mostly empty space.

Shot alpha particles, He+2, at an atomic nucleus

Found that mass changed, but not the charge.

Had to be a particle – the same mass as a proton with no charge Essential discovery for

the fission of uranium Necessary for nuclear

energy

Determined what keeps electrons in orbit around nucleus

Proposed that electrons have a set amount of energy putting them in different energy levels, orbits, around the nucleus

Electrons can change energy levels; higher levels are further from the nucleus

Atom -a basic unit of matter that consists of

a dense, central nucleus surrounded by a

cloud of negatively charged electrons.

Element - a pure chemical substance

consisting of one type of atom distinguished

by its atomic number

Found on the periodic table

Atomic Number – the number of protons in the

nucleus of an atom

Isotope – atoms of the same element with

different numbers of neutrons

Protons – positively charged particles Found in the nucleus

Neutrons – neutral particles same average mass as protons

Found in the nucleus

Electrons – negatively charged particles Found in orbits around the

nucleus

Very small mass

Mass Number – the total number of protons and neutrons Protons and neutrons have the same mass and are found in the

nucleus of an atom

Electrons are approximately 2000 times smaller than protons and neutrons

The nucleus of an atom is relatively heavy since it holds most of the atom‟s mass Protons and neutrons

Different isotopes have different mass numbers

Atomic Mass = the average mass of all of the isotopes of an element Because electrons are so small, rounding the atomic mass will

give you the average mass number.

Atomic number = protons = electrons

Protons + neutrons = mass number

Mass number – atomic number = neutrons

Mass number – protons = neutrons

A way to organize the 118 known elements

based on increasing atomic number

Also organized based on other trends

To be discussed later

Developed by Dmitri Ivanovich Mendeleev

(with historical help from many others)

Late 1800‟s

First to develop a table that predicted

undiscovered elements based on gaps in size

Also first to recognize other trends in the table.

Atomic Number

Symbol

Name (if included)

Atomic Mass

Na

11

22.99 Sodium

Shorthand for the

box on the

periodic table

56 26Fe

Using your periodic

table…

How many protons

does helium (He)

have?

How many neutrons

are in an atom of

carbon (C)?

How many electrons

are in an atom of

lithium (Li)?

Symbol Mass Number

Atomic Number

Name Symbol Atomic

#

Mass # Protons Neutrons Electrons

Phosphorus - 31

5 6

8436Kr

9 9

When atoms gain or lose electrons Cation – positively

charged, lost one or more electron

Anion – negatively charged, gained one or more electron

Charge can be found on the nuclide

2713Al+3

If charge is positive, the atom lost electrons. If it‟s negative, it gained them.

For the following, how many electrons can be found in the atom?

12

6C+2

35

17Cl-

79

34Se-2

Name Nuclide Atomic # Mass # Protons Neutrons Electrons

Carbon - 14 10

31H

+

6 3 2

9 10 10

Protons and Neutrons in an atom

are found clustered together in

the nucleus

Electrons are found in orbits, or

energy levels, around the nucleus

If electrons move between energy

levels they absorb or emit energy

Moving away from the nucleus requires

energy, moving toward the nucleus

releases energy

Each energy level can only hold a certain number of electrons

Octet rule – each orbital (energy level) is full once 8 electrons are found in it Except the first, it only has two electrons

Example:

Oxygen has an atomic number of 8.

It has 8 protons and 8 electrons.

Two electrons in its first orbital, and 6 in the second

The number of electrons in the outermost orbital are considered valence electrons

Helps determine the reactivity of the element The closer to a full or empty orbital the more reactive the atom

Ie Na only has one valence electron, it is very close to empty. It is highly reactive.

Ions only gain or lose valence electrons

Atoms want full valence orbitals

They want to be like the closest Nobel Gas to them

The last column of the periodic table

This makes them the least reactive.

The number of electrons they are likely to gain or lose is based on emptying or filling an orbital

This is the Oxidation Number of the element

The elements on the left side of the periodic table are more likely to lose electrons, elements on the right side are more likely to gain electrons

The ones in the middle can become either anions or cations

Example: Na will likely be a ____ ion with a charge of ___

Use the Bohr‟s model (planetary model) to

draw atoms

Protons, neutrons are found in the nucleus

Electrons are found the in energy levels

surrounding the nucleus

Don‟t go to the next energy level until the one before

it is full

Example: Be

Atomic #?

Mass #?

Protons?

Neutrons?

Electrons?

4p

5N

-

-

-

-

Determine the number of subatomic particles

for the following ions and atoms, and draw

the Bohr‟s model of the atom

Atom/

Ion

Name Nuclide Atomic

#

Mass # Protons Neutrons Electrons Valence

Electrons

Li

S

Ne

Cl-1

Mg+2

Draw the following

atom/ions

Ca+2

F-

Ar

The Bohr‟s model is a very simple way to

draw atoms

Through technology we‟ve determined where

electrons are arranged within at atom or

molecule

Electron configuration is the arrangement of

electrons in an atom or molecule. They tell

you how many electrons are in each energy

level

Use the periodic table as a map Divide it into four parts

There are three parts of the electron configuration to indicate where the electrons are around the atom The big number: stands for the energy level

The letter (s, p, d, & f): the shape of the orbital s – spherical etc

The exponent: the number of electrons in that orbital

Each orbital gets filled before you move on to the next one.

Example: O 1s2, 2s2, 2p4

H

He

Li

C

N

Na

Fe

Elements are organized by increasing atomic number

Also organized into periods (rows)

Read from left to right

Groups/families (columns) All elements in a family

have similar trends All have the same

number of valence electrons

Metals

Alkali Metals

Alkaline Earth metals

Transition Metals

Inner Transition Metals

Metalloids

Non-Metals

Metalloids

Gasses

The organization of the periodic table is not

just based on atomic number

There are other trends that show up on the

periodic table

Trend #1 – Atomic Radius

Definition – the average distance from the

nucleus to the outermost electron

As you travel to the left of the periodic table,

the elements have a larger radius

As you travel down the periodic table, the

elements have a larger radius

Trend #2 – Ionization energy

Definition: The amount of energy required to remove one electron

As you travel to the right across a period, the ionization energy increases

As you travel up a group, the ionization energy increases

Trend #3 – Electron Affinity

Definition – the amount of energy gained when an electron is added to it

As you travel to the right across a period, the electron affinity increases

As you travel up a group, the electron affinity increases

Trend #4 – Electronegativity Definition – the ability of an element to

attract pairs of electrons in a covalent bond

As you travel to the right across a period, electronegativity increases.

As you travel up a group, the electronegativity increases

Metallic Characteristics Increase with lower valence electrons

and larger atomic radius

Non-metallic characteristics Increase with higher valence electrons

and smaller atomic radius

Place the following elements in increasing

order based on each criteria

C, Na, Sr, Al, Ne

Atomic Radius

Ne, C, Al, Na, Sr

Ionization Energy

Sr, Na, Al, C, Ne

Electron Affinity

Sr, Na, Al, C, Ne

Electronegativity

Ne, Sr, Na, Al, C

Results from a loss of the forces of the nucleus

Strong nuclear force: a super strong force that acts

between protons and neutrons in the nucleus, binding

them together

This attractive force is stronger than the force that repels

„like charges‟ and that attracts „opposite charges‟

Only acts at extremely small distances (10-15m)

When the nucleus becomes unstable, this

nuclear force becomes unbalanced, radioactive

decay occurs

Tends to occur in atoms with large proton to neutron

ratios

When they break down they emit radiation

Types

Alpha α

Beta β

Gamma γ

Alpha α

Most common form of radiation

Alpha radiation consists of fast flying positively charged particles

Combination of protons and neutrons Aka the nucleus of a Helium atom, atomic number 2

Beta β

Medium strength of radiation

Beta radiation consists of fast flying negatively charged particles

Each beta particle is an electron that is ejected by an atomic nucleus

Gamma γ

strongest form of radiation

Occurs when an atom in an excited state releases energy

Extremely short wavelength, much more energetic than visible light

Gamma radiation carries lots of electric charge and no mass

When atoms/elements break down, they

become another element

This process emits radiation

Types

Alpha particle emission

Beta particle emission

Alpha particle emission When an atom breaks down and emits an alpha

particle A 4He nucleus (2 neutrons and 2 protons)

Occurs with massive nuclei that have too large of a neutron to proton ratio

To determine the products of alpha particle emission, you subtract a He nucleus

Example:

23592U 231

90Th + 42He

Just like a math equation: the top numbers have to be equal and the bottom numbers have to be equal. They symbol goes with the atomic number found on the bottom

Beta Particle Emission

When a atom breaks down and emits a beta

particle

Energy converts a proton into a neutron (β+) and emits

a positive charge

OR energy can convert a neutron into a proton (β-) and

emits a negative charge

Example

231

90Th 23191Pa + 0-1e

The time required for half of the atoms in a sample of a radioactive isotope to decay Different isotopes decay at different rates

The longer the half life, the greater the stability

Example Radium-226 has a half life of 1620years

This DOES NOT mean that in 3240 years it will be gone!

This does mean that after another 1620 years, half of the remaining half will be gone, leaving ¼ of the original sample

The half life of the sample continues like this: ½ will remain, ¼ will remain, 1/8 will remain, 1/16 will remain, and so on Each half life cycle leaves 1/(2n) of the original

Half lives are VERY consistent and not affected by environmental conditions

Half-lives can be measured by a radioactive detector and by measuring how much decay occurs per year.

A 100 gram sample of 13C decays to 25 grams in 20.6 seconds. What is its half-life? Original = 100g

Left = 25g

25/100 = ¼

This means it when through 2 half life cycles. 20.6 seconds / 2 = 10.3s

The half life of 258Md is 2,800 years. If there are 33g of the sample left after 1,400 years, how many grams were in the original? Half life = 2,800 years

Time passed = 1,400 years

½ of a half life has passed, so Only ¼ of a sample has decayed, so ¾ is left

33/(3/4) = 44

There are 5.0g of 210Bi left after 30.45 days.

How many grams were in the original sample

if its half-life is 6.09days?

Original = ?

Left = 5.0g

Half life = 6.09days

Time passed = 30.45 days

How many half life cycles?

30.45/6.09 = 5

5 cycles means 1/(25) of the sample is left

1/32 left

5.0g /(1/32) = 160g

Nuclear fusion Taking two atoms and making a new

one plus neutrons

Occurs mostly in lighter atoms

Releases radiant energy as gamma radiation

Found in stars and the hydrogen bomb

Nuclear Fission Splitting atoms into two new ones

Creates two new smaller nuclei and releases neutrons

Generally only occurs in heavier atoms

Releases gamma radiation

Method behind nuclear power and nuclear weapons