Atoms: The Building Block of Matter 

3-1 The Atom: From Philosophical Idea to Scientific Theory

From Philosophical Idea to

Scientific Theory

The first idea of matter was simply that all matter was infinitely divisible. Ex. Folding a piece of paper Ex. You could continue to cut a piece of

copper into smaller and smaller pieces forever. The “Particle Theory” of matter was first

supported by the Greeks and others scientists (Democritus) around 400 B.C.

From Philosophical Idea to Scientific Theory

He proposed that all matter that makes up the world is composed of small, indivisible particles.

Democritus called the building block of matter, atomos, or the atom. Interestingly, Aristotle did not agree with Democritus

because there was no evidence to support these claims.

Ex. You cannot continue to cut a piece of copper into smaller pieces, eventually you get to copper atoms which cannot be divided any farther.

Foundations of Atomic Theory

One of the biggest speculations at the beginning of the 1700’s was on whether elements always combine in the same ratios when forming compounds. Chemical Reaction – the transformation of one

substance into new substances.Law of Conservation of Matter (Mass) –

states that matter is neither created nor destroyed during a physical or chem. Rxn.

Foundations of Atomic Theory

Law of Definite Proportions – a Chem. cmpd. Contains the same elements in exactly the same ratio regardless of the source or sample size. H2O – taken from a stream.

H2O – taken from a paper cup.

NaCl always contains 39.34% Na by mass; 60.66% Cl by mass.

Foundations of Atomic Theory

Law of Multiple Proportions - if 2 or more different cmpds. are composed of the same 2 elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. In CO2 : 1 g C combines with 2.66 g O

Dalton’s Atomic Theory

I like this guy because he was a schoolteacher. Dalton proposed an explanation for the laws listed

above. He believed that elements are composed of atoms,

and that only whole numbers of atoms can combine to form cmpds.

Ex. Water could never a formula H2.124O

Look at Dalton’s Postulates on pg. 66.

Modern Atomic Theory

Dalton turned Democritus’s idea into a scientific theory that could be tested by experiment.

Some parts of Dalton’s theory have actually been proved NOT to be true. We will discuss these later.

Atoms: The Building Block of Matter

3-2 The Structure of the Atom

Although Dalton thought the atom to be indivisible, it is actually composed of other subatomic particles.

Subatomic Particles protons, neutrons, electrons.

Discovery of the Electron

Electron was discovered through experiments using cathode-ray tubes.

A stream of charged particles flows from the cathode to the anode in a cathode ray tube, causing the fluorescent material inside the tube to glow.

Discovery of the Electron

The negative electrode is the cathode. The positive electrode is the anode. Cathode rays were deflected by magnetic

fields. The ray was deflected away from a negative

field and toward a positive field. Particles that compose cathode rays are

(-)vely charged.

Discovery of the Electron

Thomson’s Plum Pudding Model of the atom. An atom contains a

specific number of electrons which are in pool of positive charge.

Ex. Like the raisins in plum pudding (or the chocolate chips in a cookie)

He knew only that there was positive charge, NOT that there were positive particles.

Discovery of the Atomic Nucleus

Rutherford’s Gold Foil Experiment: If Thomson’s Model was correct, the alpha

particles should have passed directly through the foil with only slight deflections. Most of the particles acted this way, but some were deflected at wide-angles. The wide angle deflection could only have been caused if there was a powerful force in the atom. He reasoned their must be a small, dense center containing most of the mass of the atom – nucleus.

The Gold Foil Experiment (figure 3-14)

http://micro.magnet.fsu.edu/electromag/java/rutherford/ Most particles passed

through gold without a problem

1 in 8000 alpha particles deflected

These were sent in ALL directions including straight back!

What does this mean?

Most of the atoms positive charge, as well as the mass is in the middle, called the nucleus.

Most pass through the empty space but occasionally one gets close enough to the positive nucleus to deflect it.

Composition of the Atomic Nucleus Nucleus contains positive protons, neutral

neutrons. The nucleus has a net positive charge. Atoms are electrically neutral because the positive

nucleus is surrounded by a sea of negative electrons.

The number of protons in an atom determines the atom’s identity.

See Table 3-1 pg. 74

Question: If an atom contains positive particles, what keeps the atom together? Don’t like charges repel each other?

Strong Nuclear Force Binding Energy Size of the atom

Atoms: The Building Block of Matter

3-3 Counting Atoms

The Structure of the Atom

The atom has a positively changed central core Contains Protons and Neutrons Protons are positive, equal and opposite to

electrons Neutrons do not carry a charge and are slightly

more massive

1 proton has the mass of about 2000 electrons

Electrons move in space around the nucleus

Rutherford visualized it as a mini solar system.

Atomic Numbers

Henry Moseley found that atoms contain unique positive charge in their nucleus.

The number of protons is called the atomic number.

The atomic number indicates protons.

Chlorine has 17 protons = atomic number

Practice

How many protons and electrons are in a magnesium atom?

What is the name of the element that has atoms that contain 11 protons.

Ions

When an atom gains or loses electrons it acquires a charge Fewer electrons means positive charge More electrons means negative charge

Charge of ion = # protons - # electrons

Sample

Write the chemical symbol for the ion with 9 protons and 10 electrons

Answer F-

What is the symbol of the ion with 13 protons and 10 electrons?

Answer Al3+

7 Protons and 10 electrons? N 3-

Isotopes

Dalton said all atoms of an element are the same.

Not quite true, ISOTOPES have a different number of neutrons

In nature, elements are almost always found as a mixture of isotopes

Isotopes are usually in the same percentages.

To identify isotopes more specifically Use the Mass Number Mass Number = (# protons) + (# neutrons)

To identify an isotope chemists write the mass number behind the element symbol for example Cl-37 indicates that this chlorine has 20 neutrons, it is written in symbol form as 37

17Cl

Cl-35 has 18 neutrons and is written as 35

17Cl

The Mass of an Atom

Measured by Atomic Mass Units (AMU)

The atomic mass is approximately the same as the sum of protons and neutrons

This is not very precise so Scientists define it more precisely.

1 amu is equal to 1/12 the mass of a carbon-12 atom.

= 1.66 x 10 –24 grams Carbon 12 is the only element with an AMU

equal to protons and neutrons, because of isotopes

The average mass of an element’s atoms is called the atomic mass.

AM = (mass isotope x abundance)+(mass isotope x abundance)+...

Fundamental Subatomic ParticlesParticleParticle LocationLocation Charge Charge

(C)(C)Mass (g)Mass (g) Mass Mass

(AMU)(AMU)

ProtonProton Inside Inside nucleusnucleus

+1.602 x 10+1.602 x 10--

1919

1.673 x 101.673 x 10-24-24

11

Neutron Neutron Inside Inside nucleusnucleus

00 1.675 x 10-1.675 x 10-2424 11

ElectronElectron Outside Outside nucleusnucleus

-1.602 x 10-1.602 x 10--

1919

9.109 x 10 9.109 x 10 ––

2828 00

Relative Mass to Numbers of Atoms

Relative Mass to Numbers of Atoms Avogadro’s number – is the number of

particles in exactly 1 mole of a substance. 6.02 x 1023

1 mole of carbon = 6.02 x 1023 atoms 2 moles of silver = 1.204 x 1024 atoms 1 mole of water = 6.02 x 1023 water molecules

2 moles of marshmallows = 1.204 x 1024 marshmallows.

Relative Mass to Numbers of Atoms

Here’s where it could get tricky…

1 mole of water (H2O) = 6.02 x 1023 molecules

How many atoms are in 1 mole water?

H2O is composed of 2 H and 1 O atoms = 3 total atoms.

3 x (6.02 x 1023) = 1.81 x 1024 atoms in water

Relative Mass to Numbers of Atoms Molar Mass – the mass of one mole of a

substance. The amount of a substance that contains

Avagadro’s number of particles. Usually written with unit g/mol. Molar masses are on the periodic table.

Numerically they are the same as atomic mass. Ex. Molar mass of He = 4.00 g/mol Molar mass of Al = 26.98 g/mol

Relative Mass to Numbers of Atoms Now is when it gets fun! Gram/mole

Conversions…Ready?   Be sure to look at the chart on pg.82. This is

one of the most important ideas which we cover this year. The only way to understand this is to practice, practice, practice.

Practice Problems pgs. 82-85