CHM 109SuroviecFall 2015

Atoms and Elements

I. Atoms and Atomic Theory

An element is composed of tiny particles called atoms All atoms of the same element have the same chemical properties

Compounds are formed when two or more atoms of different element combine

Chemical reactions involve only separation, combination or rearrangement or atoms

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A. Fundamental Laws of Matter

There are three fundamental laws of matter Law of conservation of mass

Law of constant composition

Law of multiple proportions

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A. Law of conservation of mass

In a chemical reaction matter is neither created or destroyed

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B. Law of Definite Proportions

In 1797 Joseph Proust noticed that elements of a given compound always combined in definite proportions in all samples of a compound.

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C. Law of Multiple Proportions

This law asserts that when 2 elements (A&B) form two different compounds, the masses of element B that combine with 1 gram of element A can be expressed as a ratio of small whole numbers.

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D. Atomic Theory

John Dalton explained the 3 previous laws along with his Atomic Theory which states:

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II. Structure of an atom

Atom: basic unit of an element that can enter into chemical combination

Atom possesses structure: Electron Proton Neutron

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A. Electrons

First evidence for subatomic particles came from the study of the conduction of electricity by gases at low pressures J.J. Thomson, 1897 Rays emitted were called cathode rays Rays are composed of negatively charged particles called electrons

Electrons carry unit negative charge (-1) and have a very small mass (1/2000 the lightest atomic mass)

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Cathode Ray Apparatus10

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III. Structure of the Atom

Since the overall atom is neutral, and we know that they contain electrons they must have a positive particle to balance the atom out.

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II. Structure of the Atoms

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A. Protons and Neutrons – The Nucleus

Ernest Rutherford, 1911Bombardment of gold foil with α particles

Expected to see the particles pass through the foil

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Rutherford Backscattering

Found that some of the alpha particles were deflected by the foil

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B. Current Atomic Model

1. Most of the atom’s mass and all of its positive charge are contained in a small cone called the nucleus

2. Most of the volume of atoms is empty space which is where the electrons are

3. There are as many electrons as protons

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atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Rutherford’s Model of the Atom

If the atom is a football stadium, then the nucleus is a marble on the 50-yard line.

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VI. Proton, Neutron and Electron

Amu = 1/12 the mass of a carbon atom containing 6 protons and 6 neutrons. This makes the proton ~1amu.

The proton and electron have electrical charge

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A. Elements

What makes elements unique is the number of protons, neutrons and electrons in an atom of the element.

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B. Isotopes

All atoms of a given element have same number of protons, but the number of neutrons can change

This means that one element can have different masses.

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B. Isotopes

Atomic number (Z) = number of protons in the nucleus

Mass number (A) = number of protons and neutrons in the nucleus

Isotope = atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

Mass Number

Atomic NumberElement Symbol

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Do You Understand Isotopes?

How many protons, neutrons, and electrons are in C14

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How many protons, neutrons, and electrons are in C11

6 ?

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C. Molecules and Ions

Molecules: aggregate of two or more atoms in a definite arrangement held together by chemical forces

Ion: atom or group of atoms with a net charge due to the loss or gain of electrons.

H2 H2O NH3 CH4

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C. Molecules and Ions

Cation: ion with a positive charge

Anion: ion with a negative charge

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Do You Understand Ions?

How many protons and electrons are in ?Al2713

3+

How many protons and electrons are in ?Se7834

2-

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Polyatomic ions26

V. Periodic Law and Table

Mendeleev constructed a periodic table based on the elements known at the time.

He noticed that certain groups of elements had similar properties

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A. Ions and Periodic Table

Metals tend to lose electronsNonmetals tend to gain electrons

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VI. Atomic Mass

Since isotopes have different masses for the same element an atomic mass is a weighted average of the isotopes

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VII. Molar Mass

We would like to know the number of atoms in a given mass of atoms.

Since atoms are so small we count them by weight.

A. The Mole.We use a mole to

count the amount of material containing 6.022 x 1023

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B. Mole conversion

Using conversion factors we can convert between moles and atoms.

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C. Moles and Mass

The next question is what is the mass of that many atoms.

It has been determined that the mass of 1 mole of atoms is equal to its atomic mass in amu

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