Aristotle

Early Greek Theories• 440 B.C. - Democritus thought matter could

not be divided indefinitely. He called nature’s basic particle the “atom”, from the Greek word meaning “indivisible”.

• 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air.

Democritus

• Aristotle was wrong. However, his theory persisted for 2000 years.

Foundations of the Atom•In the late 1700s chemists believed the moderndefinition of an element as a substance that cannot be further broken down by ordinary chemical means.

•It was also clear that elements combine to formcompounds that have different physical and chemical properties than those of the elementsthat form them.

Na + Cl → NaCl Different properties

•Chemists at this time could not agree whether elements always combined in the same ratio when forming a particular compound.

•When a substance or substances are transformed into one or more new substances, it is caused by a chemical reaction.

Forming lead iodide

•In the 1790s, scientists began to analyze chemical reactions. This lead to the discovery of several basic laws.

•The law of conservation of mass (matter (mass) is neither created nor destroyed during ordinary physical or chemical changes).•Antoine Lavoisier, a French chemist, verified this law by experimentation in 1789.

The Law ofConservationOf Mass

•In 1799, Joseph Proust, a French chemist, showed that a given compound always contains exactly the same proportion of elements by mass.•The principle of the constant composition of compounds, originally called “Proust’s law”, is now known as the law of definite proportions.

•The law of definite proportions (a chemical compound contains the same proportions by mass regardless of the size of the sample or source of the compound.

•For example, sodium chloride (table salt) always consists of 39.34% by mass of the element sodium, Na, and 60.66% by mass of the element chlorine, Cl.

•Proust’s discovery stimulated John Dalton, an English schoolteacher, to think about atoms as the particles that might compose elements. •Dalton discovered another principle that convinced him of the existence of atoms.•He noted, for example, that carbon and oxygen form two different compounds that contain different relative amounts of carbon and oxygen (CO and CO2).•This led to the law of multiple proportions in 1803.

•The law of multiple proportions (if two or more different compounds are composed of the same two 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.

•For example, the elements carbon and oxygen form two compounds, carbon dioxide and carbon monoxide. CO molecules are always composed of one C atom and one O atom. CO2 molecules are always one C atom and two O atoms.

Law of Multiple Proportions

Dalton’s Atomic Theory•In 1808, John Dalton proposed an explanation for the three laws (Atomic Theory).

•All matter is composed of extremely small particles called atoms.•Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass and other properties.•Atoms cannot be subdivided, created, or destroyed.•Atoms of different elements combine in simple whole-number ratios to form chemical compounds.•In chemical reactions, atoms are combined, separated, or rearranged.

Modern Atomic Theory

•Aspects of Dalton’s theory have been proven incorrect (we know now that atoms are divisible and a given element can have atoms with different masses).

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Atom•An atom is the smallest particle of an element that retains the chemical properties of that element.•Atoms consist of two regions. The nucleus is at the center of the atom and contains at least one positively charged particle called a proton and one or more neutral particles called neutrons.

electron

neutron

proton

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•Surrounding the nucleus is a region occupied by negatively charged particles called electrons (this region is very large compared to the nucleus.•Protons, neutrons, and electrons are usually referred to as subatomic particles.

electron

neutron

proton

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Discovery of the Electron

•The discovery of the electron occurred in the late 1800s when scientists were investigating the relationship between electricity and matter. The experiments were carried out in cathode-ray tubes.

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Cathode-Ray Tube

A simple cathode-raytube. Particles passthrough the tube fromthe cathode, the metaldisk connected to thenegative terminal ofthe voltage source,to the anode, themetal disk connectedto the positive terminal.

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•The experiments carried out by Joseph John Thomson in 1897, showed the particles that composed the cathode ray were negatively charged.•Thomson also thought that atoms must contain some positive charge.

•Thomson suggested that an atom consisted of a cloud of positive charge with the negative electrons embedded in it. This model is often called the “plum-pudding” model.•Watch video clip.

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•In 1909 experiments conducted by Robert A. Millikan showed that atoms were electrically neutral and must contain a positive charge to balance the negative electrons. They also showed that atoms must contain other particles that account for most of their mass.•Watch video clip.

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Discovery of the Atomic Nucleus

•In 1911 Ernest Rutherford, Hans Geiger, and Ernest Marsden bombarded a thin piece of gold foil with fast-moving alpha particles. Some particles were redirected back toward the source. •Rutherford concluded that this was caused by a densely packed bundle of matter with a positive charge (nucleus).

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Most particles passed through. So, atoms are mostly empty.

Some positive -particles deflected or bounced back!

Thus, a “nucleus” is positive & holds most of an atom’s mass.

Radioactive

substance

path of invisible -particles

Lead block

Zinc sulfide screen

Thin gold foil

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Composition of the Atomic Nucleus

•The nuclei by atoms of different elements differ in the number of protons they contain (the number of protons in an atom’s nucleus determine the atom’s identity.

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When two protons are extremely close to each other, there is a strong attraction between them. A similar attraction exists when neutrons are very close to each other or when protons and neutrons are very close together.These short-range proton-neutron, proton-proton, and neutron-neutron forces hold the nuclear particles together and are referred to as nuclear forces.

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Nuclear Forces

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The Sizes of Atoms

•The sizes of atoms are expressed in picometers ( 1 pm = 1 x 10-12 m).•To get an idea of how small a picometer is, consider that 1 cm is the same fractional part of 103 km (about 600 miles) as 100 pm is of 1 cm.•Atomic radii range from about 40 to 270 pm.

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Atomic Number•Atoms of different elements have different numbers of protons. Atoms of the same element all have the same number of protons. •The atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

•An element’s atomic number is indicated above its symbol on the periodic table.

•Elements are placed in order of increasing atomic number.•The atomic number identifies an element.

Isotopes•Isotopes are atoms of the same element that have different masses.•The isotopes of a particular element all have the same number of protons and electrons, but different numbers of neutrons.

Isotopes ofHydrogen

Mass Number•The mass number is the total number of protons and neutrons in the nucleus of an isotope.

•This isotope of oxygen has 8 protons and 8 neutrons in the nucleus. Its mass number is 16.

Designating Isotopes•There are two methods for specifying isotopes. •In the first method, the mass number is written with a hyphen after the name of the element (uranium-235).•The second method shows the composition of a nucleus as the isotope’s nuclear symbol ( ).U235

92

Two different isotopes of carbon.

U23592

•The superscript indicates the mass number and the subscript indicates the atomic number.

•The number of neutrons is found by subtracting the atomic number from the mass number (mass # - atomic # = # of neutrons).

•Example: 235 (protons + neutrons) – 92 (protons) = 143 neutrons.

U23592

Uranium•For a neutral atom, the number of protons equals the number of electrons.

Example:How many protons, electrons, and neutrons are there in an atom of chlorine-37?

Given: name and mass number of chlorine-37

Remember: atomic number = number of protons = number of electronsFrom the periodic table, the atomic number of chlorine is 17.Mass # - atomic # = # of neutrons 37 – 17 = 20

An atom of chlorine-37 is made up of 17 protons, 17 electrons, and 20 neutrons.

•The charge on an ion (an atom with a positive or negative charge) indicates an imbalance between protons and electrons.•Too many electrons produces a negative charge, too few, a positive charge.

•Nuclide is a general term for any isotope of any element.•Below are three hydrogen nuclides.

Relative Atomic Mass•Masses of atoms expressed in grams are very small. For example, an atom of oxygen-16 has a mass of 2.657 x 10-23 g.•For simplicity, scientists use the atomic mass unit (amu) to express the mass of atoms. 1 amu is 1/12 the mass of a carbon-12 atom.•The atomic mass of any nuclide is determined by comparing it with the mass of the carbon-12 atom.

•Remember scientists use standards of measurement that are constant and are the same everywhere.•In order to set up a relative scale of atomic mass, one atom was arbitrarily chosen as the standard and assigned a value.•The carbon-12 atom was arbitrarily assigned a mass of exactly 12 atomic mass units, or 12 amu.•Remember one atomic mass unit, or 1 amu, is exactly 1/12 the mass of a carbon-12 atom.

Average Atomic Masses of the Elements

•Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element.•Calculating the average atomic mass depends on both the mass and the relative abundance of each of the element’s isotopes.

A weighted average is an average that adjusts for the frequency of individual values.The following is an example of how to calculate a weighted average.

Suppose you have a box containing two sizes of marbles. If 25% of the marbles have masses of 2.00 g each and 75% have masses of 3.00 g each, how is the weighted average calculated?

To determine the weighted average, multiply the mass of each marble by the decimal fraction representing its percentage in the mixture. Then add the products.

25% = 0.25 75% = 0.75

(2.00 g x 0.25) + (3.00 g x 0.75) = 2.75 g

Notice the weighted average is closer to 3.00 g because a larger percentage of the marbles have a mass of 3.00 g.

Calculating Average Atomic Mass

•Copper consists of 69.15% copper-63, which has an atomic mass of 62.929601 amu, and 30.85% copper-65, which has an atomic mass of 64.927794 amu.

•The average atomic mass of copper can be calculated by multiplying the atomic mass of each isotope by its relative abundance (expressed in decimal form) and adding the results.

(0.6915 x 62.929601 amu) + (0.3085 x 64.927794 amu) = 63.55 amuThe calculated average atomic mass of naturally occurring copper is 63.55 amu.