The Atom & Periodic Table

How were atoms discovered?What are atoms made of?How is the Periodic Table arranged?

Atoms and the Periodic Table

•Atom “Atomos” (Greek) – smallest piece of matter that still retains those properties

•Matter – anything that has mass and volume, what all material is made up of

•Model – uses familiar ideas to explain unfamiliar facts observed in nature

Models of the Atom1) Greek Model•1) Search for a description of matter

began with a Greek philosopher Democritus more than 2400 years ago▫He thought that matter was divided over

and over – would eventually result in something so small that it couldn’t be divided any longer

▫He named this smallest particle of matter the Atom.

2) Dalton’s Model•Early 1800’s English Chemist John Dalton

did several experiments that led to the acceptance of the atom

•Dalton’s Atomic Theory – 1803▫All elements are composed of Atoms, they

are indivisible and indestructible particles.▫Atoms of same elements are exactly alike.▫Compounds are formed by the joining of

Atoms or two or more elements▫*This theory became one of the foundations

of chemistry

3) JJ Thompson▫Electrons – negatively charged particles▫If there were negative particles he

concluded there had to be positive particles to balance it out

▫Plum Pudding Model

▫ Negatively charged ‘plums’

▫ Positively charged ‘pudding’

4)Ernest Rutherford

•Gold Foil Experiment – (led to downfall of Plum Pudding Model)

•Discovered the proton by shooting small alpha particles through a thin sheet of gold, some were reflected

•Gives us the “Planetary Model” where particles surround the small, dense positively charged center

5) Niels Bohr

•1913, Danish Scientist – thought the negatively charged electrons have a definite orbit around the nucleus – much like the planets revolve around the sun

•Led to the development of …▫Electron Cloud Theory: electrons are

everywhere but nowhere around the nucleus

•Antonius Van den Broek, 1913 – The atomic number in nuclear charge

•Henry Moseley, 1913 – English Physicist who proved Rutherford’s theory through experiments and refined the field of X-ray diffraction

•James Chadwick, 1932 – discovered the neutron

•Cyclotron – allowed for the creation of new elements heavier than uranium

6) Wave model•Today’s atomic model is based on wave

mechanics- electrons don’t move in an orbital but bounce around in a distinct area▫The location of the electron depends on

how much energy it has•Theory of today says that each atom has a

small positively charged nucleus surrounded by a large region in which there are enough electrons to make the atom neutral – a happy atom!

Parts of an AtomParticle Location Charge Mass

Proton Nucleus + 1.67 x 10-27

Neutron Nucleus 0 1.67 x 10-27

Electron Energy level/ shell/ orbital or cloud

- 9.11 x 10-31

Sub Atomic Particles

•Proton: ▫ positively charged particle▫All protons stay the same and cannot leave▫Protons determine the identity of the atom

•Neutron:▫Electrically neutral particle, provides mass▫Number of these can change – gives us

isotopes

Sub Atomic Particles•Electrons:

▫In a neutral atom ( no charge) the number of negatively charged particles is equal to the number of positively charged particles

▫Electron Cloud – space in which electrons are likely to be found, move so fast and are so small we can’t see them.

▫Ions occur when there is an imbalance between protons and electrons, e- are the one that can move

Quarks – sub, sub-atomic

•There are six types of quarks, or flavors- ▫Up, down, strange, charm, bottom and top▫They combine to form hadrons – the most

stable of these are protons and neutrons

Forces within an Atom•Electromagnetic force- can attract or repel

particles (depending on their charge), this is what keeps them in their orbital

•Strong force – opposes the EM force of repulsion between protons▫Strongest but has limited range, only when

protons are very close together•Weak Force – responsible for a process

known as radioactive decay▫Decay – a neutron in the nucleus changes

into a proton and an electron•Gravity- weakest force in nature

Atomic Identity•Atomic Number

▫The number of protons in the nucleus of an atom▫This number stays the same among all atoms of the

same element▫Similar to your SSN

•Atomic Mass▫Sum of the protons and neutrons in the nucleus of

an atom, deals with one atom at a time▫Similar to your weight, it can change

•Average Atomic Mass▫Average of all isotopes of an element in existence,

Deals with more than one atom▫AMU – mass unit or amu (6 with 23 zeros equal 1

gram)

Isotopes vs. IonsIsotopes Ions

▫ Isotopes are atoms with a different mass due to different number of neutrons.

▫ Average Atomic Mass is an average of all occurring isotopes (this is why there is a decimal on some)

• An Ion is an electrically charged atom

• Ions deal with the charge or oxidation number

• How? Gain/Lose e-

•Atomic # (AN or Z)

•Symbol•Name

•Atomic Mass (AMU)

•Pos/Neg number at top tells the charge – deals with ions

Ooxygen

8

15.99

-2

Quiz…

•What is the symbol for Lead?•What is the atomic number of Potassium?•What is the atomic mass of a Carbon

atom?•What is the average atomic mass of

Aluminum? Why isn’t it exactly double the number?

Hydrogen isotopes• Protium Deuterium

Tritium

Atomic number (Z) = number of protons in nucleus

Mass number (AMU) = number of protons + number of neutrons = atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in the nucleus

XAZ

H11 H (D)21 H (T)3

1

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

2.3

ExamplesElement

AN Avg.Atomic Mass

Atomic Mass

Protons

Neutrons

Electrons

Oxygen

Lithium

15

38

Ex) CarbonParticle Location Charge Number

proton Nucleus + 6

Neutron Nucleus 0 6

electron Cloud/shell - 6

AMU 12

Octet Rule•The outer shell of an atom can hold only a

certain amount of electrons•1st shell holds up to 2 e-•2nd shell holds up to 8 e-•3rd shell holds up to 8 e-•4th shell holds up to 18 e-•5th shell holds up to 32 e-•Since we only study the first few rows, we

say the atom can hold up to 8 – hence, Octet Rule

Electron Orbitals – s p d f orbitals

Compounds and Molecules

Diatomic moleculesBinary Compounds: Covalent and Ionic BondsPolyatomic CompoundsNaming and Balancing

A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds, electrons are key to this

H2 H2O NH3 CH4

Diatomic Molecules: contains only two atoms, short list of elements that buddy up – not stable enough alone

H2, N2, O2, Br2, Cl2, I2, F2

A polyatomic molecule contains more than two atoms

O3, H2O, NH3, CH4

2.5

An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.

anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Na 11 protons11 electrons

Na+ 11 protons10 electrons

Cl 17 protons17 electrons

Cl-17 protons18 electrons

2.5

2.6

A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance

An empirical formula shows the simplest whole-number ratio of the atoms in a substance

H2OH2O

molecular empirical

C6H12O6 CH2O

O3 O

N2H4 NH2

2.6