Atomic Structure & Basic Periodic Table

(semester 1)Peterson recreated the file from an old master 2013 so

that NOW it will “open” & “edit” easily.

•The smallest particle of an element that retains the properties of the element is called an atom

• Atoms are extremely small (a copper atom has a diameter of 0.000000128 mm )

•Atoms can be seen using a scanning tunneling microscope

•Gold clusters showing pentagonal atomic arrays revealed by aberration-corrected scanning transmission electron microscopy

Even in the 80’s, we could see the movement of atoms as we compared different scans of a sheet of gold foil. The atom positions changed !

History of the Atom460 BC Democritus

•Developed the idea of “atoms”

• “Matter is composed of empty space through which atoms move.”

384 – 322 BC Aristotle

•One of the most influential philosophers

•Criticized Democritus saying “nothingness of empty space cannot exist”

1808 John Dalton

•“all matter is made up of tiny spheres that are able to bounce around with perfect elasticity” and Dalton called them atoms.

•See next page…

Dalton created the Atomic Theory model:

1. All elements are composed of tiny, indivisible parts called atoms

2. Atoms of the same element are identical *Each element is unique.

3. Atoms of different elements can physically mix or chemically combine to form compounds.

4. Chemical reactions occur when atoms are separated, joined or rearranged.

Much of this theory is still accepted. Some changes have been made to #1 & #2.

1888 Eugen Goldstein

• discovered positive particles in atoms

• Observed rays traveling in the opposite direction of cathode rays

(Rutherford called these particles “protons” in 1920.)

1898 J. J. Thompson

• Passed electric current through a glass filled with gas. He discovered that a beam of negative charges traveled from the cathode (-) to the anode(+)

• Thompson called the smaller, negative charges “electrons”

• Thompson developed

the idea of the “plum

pudding model”

(see next page)

Plum Pudding Model

Atoms are made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron’s charge, like plums surrounded by pudding.

1910 Ernest Rutherford “Gold Foil Experiment”

•Fired helium nuclei at a piece of solid, gold foil which was only a few atoms thick.

•Found that most of them passed through, and to their surprise, about 1 in 10,000 were deflected, bounced straight back.

Rutherford overturned the “plum pudding model”.

Rutherford concluded:

1. Atoms are mostly space

2. Atoms have a solid nucleus at the center which contains most of the mass of the atom.

3. The first time atoms were thought to have a nucleus.

1913 Henry Moseley“The properties of elements are determined by the number of protons in their nucleus.”

“The charge of the nucleus of an atom is equal to the number of protons in the atom.”

The number of protons in an atom is called the atomic number.

“Elements should be put in order of their atomic number, NOT atomic mass.”

Moseley was right. As a result, the problems with Mendeleev’s periodic table were solved.

1913 Niels Bohr

(Studied under Rutherford)

•Electrons were in orbits, like planets orbiting the sun.

•Each orbit is only able to contain a set number of electrons.

•Each orbit has a fixed energy level, and therefore the electrons do not lose energy.

1931 James Chadwick

Discovered the neutron, the neutral subatomic particle in the nucleus which allows protons to be held together tightly.

Mendeleev (1834) Created the FIRST periodic table

• Mendeleev put elements in order of their atomic mass.

•He saw their properties naturally occurring (boiling point, melting point, density, reactivity, etc. ) all falling into a pattern.

•“Periodic” means “to occur in a pattern”

But, a few problems where properties weren’t perfectly in order.

Moseley discovered how to know the number of protons in an atom. The periodic table is NOW in order of atomic number.

Atomic Structure

Subatomic Particles

Particle Charge Mass

Proton + charge 1

Neutron NO charge 1

Electron - charge 0.0005 tiny!(1 p+ mass = 2000 e- mass) (Practically zero compared to protons and neutrons!)

Atomic Structure:

Subatomic Particles (particles that make an atom)

•The nucleus is the tiny positive core of the atom which contains most of the mass of the atom.

•The proton (p+) is the positively charged (+1) particle found in the nucleus of the atom.

•The neutron (no) is the particle with no charge (0) found in the nucleus of the atom.

“ Neutrons are Neutral in the Nucleus” Please remember that Neutrons are NOT negative.

The electron (e-) is the negatively charged (-1) particle found outside of the nucleus.

Atomic mass - the number of protons and neutrons in an atom; the mass of the nucleus

Atomic number – the number of protons in an atom; Identifies the element

The number of electrons = The number of protons in a neutral atom

2

He

4.003

Helium has __________ protons and ________ electrons.

The atomic mass of helium is ________

APE MAN (for neutral atoms)

Atomic Number = # Protons = # Electrons

Mass number -- Atomic Number == # Neutrons(rounded

atomic mass

until taught about isotopes)

Since an atom is electrically neutral, the number of protons equals the number of electrons.

Atomic Number = # Protons = # ElectronsElement Atomic

NumberProtons Electrons

silver ? ? ?

? 82 ? ?

? ? 8 ?

? ? ? 30

Neutrons = Atomic mass - Atomic number (rounded)

Element Atomic Mass

Atomic Number

Neutrons

? 39 19 ?

? 11 ? 6

? ? 18 22

Ions are charged atoms.

•Anions – atom gains an electron, giving it a negative charge

•Cation – atom loses an electron, giving it a positive charge.

I’m so depressed!

Why?I’ve lost an electron!

Yes, I’m positive!Are you sure?

symbol p+ n0 e-

S-2

? 7 10

? 21 18

“Charges are written as superscripts.”

Isotopes are atoms that have the same number of protons but different numbers of neutrons.

All isotopes have the chemical properties of that element.

Isotopes are atoms that have the same number of protons but different numbers of neutrons.Isotope Atomic

MassMass Number

Atomic Number

Protons Neutrons Electrons

Iron-54

Iron-56

Iron-57

In order to identify the various isotopes of an element, chemists add a number after the element’s name. The number added is called the mass number and it represents the sum of the number of protons and neutrons in the nucleus.

Potassium-39 Potassium-40 Potassium-41

Protons 19 19 19

Neutrons 20 21 22

Electrons 19 19 19

Number of neutrons = mass number – atomic number

Atomic mass of an element is the weighted average mass of the isotopes of that element.

The unit is “amu”, atomic mass units.

Where Does “amu” come from?“ Webster’s: a unit of mass for expressing masses of atoms, molecules, or nuclear particles equal to 1⁄12 the mass of a single atom of the most abundant carbon isotope, 12C”

How many protons are in carbon-12? Multiply by “1 amu” for each proton = _____

How many neutrons are in carbon-12? Multiply by “1 amu” for each neutron = _____

How many electrons are in carbon-12? Multiply by “0 amu” for each electron = ______

Add these “amu” masses =_______ 1/12 of this is an “amu”

Example: the atomic mass of chlorine (Cl) is 35.5 amu35.5 amu. Chlorine exists naturally as 75% chlorine-35 and 25% chlorine-37 .

How?How?

75% = 75 / 100 = 0.75

25% = 25/100 = 0.25

0.75 x 35 + 0.25 x 37 = 35.5 amu35.5 amu

Note: Our periodic table has an average atomic mass for chlorine of 35.453 . It was calculated with data of greater precision.

Example

Natural copper (Cu) consists of 2 isotopes…

Copper -63 makes 69% and Copper-65 makes the remaining 31% of copper atoms in the world.

Calculate the average atomic mass.

Step 1:Step 1: convert percents to convert percents to decimals (abundance for each decimals (abundance for each isotope)isotope)

Step 2:Step 2: ADD the ADD the (Mass x (Mass x (abundance)(abundance) decimal)decimal) for all isotopes given = for all isotopes given = answer!answer!1. 69%= 69 / 100 = 0.69 31% = 31/100 = 0.31

2. [ 63 x 0.69 ] + [ 65 x 0.31] = 63.62 amu

Isotopic Symbol

X = element symbol Here X = He (helium)

A = Mass Number Here, the mass number = 4

Z = Atomic Number Here, Z = 2 ( 2 protons in He.)