Atoms and Elements

C H M 1 0 9 S U R O V I E C S P R I N G 2 0 1 4

I. Atoms and Atomic Theory 2    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

A. Fundamental Laws of Matter 3  There are three fundamental laws of matter  Law of conservation of mass  Law of constant composition  Law of multiple proportions

A. Law of conservation of mass 4  In a chemical reaction matter is neither created or destroyed

B. Law of Definite Proportions 5  In 1797 Joseph Proust noticed that elements of a given compound always combined in definite proportions in all samples of a compound.

C. Law of Multiple Proportions 6  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.

D. Atomic Theory 7  John Dalton explained the 3 previous laws along with his Atomic Theory which states:

II. Structure of an atom 8   Atom: basic unit of an element that can enter into chemical combination Atom possesses structure:  Electron   Proton Neutron

A. Electrons 9  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)

Cathode Ray Apparatus 10

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III. Structure of the Atom 12  Since the overall atom is neutral, and we know that they contain electrons they must have a positive particle to balance the atom out.

II. Structure of the Atoms 13

A. Protons and Neutrons – The Nucleus 14   Ernest Rutherford, 1911 Bombardment of gold foil with α particles  Expected to see the particles pass through the foil

Rutherford Backscattering 15 Found that some of the alpha particles were deflected by the foil

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B. Current Atomic Model 16 Most of the atom’s mass and all of its positive charge are contained in a small cone called the nucleus Most of the volume of atoms is empty space which is where the electrons are There are as many electrons as protons

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Rutherford ’s Model of the Atom

atomic radius ~ 100 pm = 1 x 10 -10 m nuclear radius ~ 5 x 10 -3 pm = 5 x 10 -15 m If the atom is a football stadium, then the nucleus is a marble on the 50-yard line.

VI. Proton, Neutron and Electron 18  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

A. Elements 19  What makes elements unique is the number of protons, neutrons and electrons in an atom of the element.

B. Isotopes 20  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.

B. Isotopes 21    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 Mass Number Atomic Number A Z

X

Element Symbol

Do You Understand Isotopes?

14 How many protons, neutrons, and electrons are in C ?

11 How many protons, neutrons, and electrons are in C ?

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C. Molecules and Ions 23  Molecules: aggregate of two or more atoms in a definite arrangement held together by chemical forces H 2 H 2 O NH 3 CH 4  Ion: atom or group of atoms with a net charge due to the loss or gain of electrons.

C. Molecules and Ions  Cation: ion with a positive charge 24  Anion: ion with a negative charge

Do You Understand Ions?

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Polyatomic ions 26

V. Periodic Law and Table 27   Mendeleev constructed a periodic table based on the elements known at the time. He noticed that certain groups of elements had similar properties

A. Ions and Periodic Table 28   Metals tend to lose electrons Nonmetals tend to gain electrons

VI. Atomic Mass 29  Since isotopes have different masses for the same element an atomic mass is a weighted average of the isotopes

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.

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The Mole.

We use a mole to count the amount of material containing 6.022 x 10 23

B. Mole conversion 31  Using conversion factors we can convert between moles and atoms.

C. Moles and Mass 32   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