Essay Atom and Mass

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Introduction and Review:
Chapter 1: Matter – Its Properties and Measurement (Review on your own) Chapter 2: Atoms and Atomic Theory Chapter 3: Chemical Compounds

What in the world isn’t Chemistry
• Matter is the physical material of the universe. • On the microscopic level, matter consists of atoms and molecules. • As we know, molecules may consist of the same type of atoms or different types of atoms.

We encounter chemistry in our everyday lives

Lavoisier, 1743-1794 Conservation of Matter - A chemical equation must be balanced for mass.

Dalton (1766 – 1844) Atomic Theory - A chemical equation must have the same number of atoms of the same kind on both sides.

Dalton’s Atomic Theory
 All matter is made of small, indestructible particles called “atoms”.  All atoms of a given element are identical and atoms differ for different elements.  Compounds are formed when atoms of more than one element combine  Atoms of different elements combine in simple, whole number ratios (e.g. 1:1 as in AB; or 1:2 as in AB2)

Consequences of Dalton’s Theory
Combinations of elements are in ratios of small whole numbers (molecules)
Carbon monoxide (CO): 1.33 g of oxygen combines with 1.0 g of carbon or O/C = 1.33 Carbon dioxide (CO2): the ratio of O/C is 2.667 2.667/1.33 = 2.0 which is the ratio of oxygen in CO2/oxygen in CO

What are Atoms?
Atoms are the basic unit of an element that can enter into chemical combination. Dalton – Atoms are small and indivisible – not quite!

Atomic Structure
J.J. Thompson’s Experiment (1898 – 1903) Z/m of the electron = 1.76 × 108 C/g

Milliken’s Oil Drop Experiment (1909) charge of the electron = -1.6022 x 10-19 C Therefore: mass electron = (e- charge)/(e- charge-to-mass ratio) = 9.10 x 10-28 g (9.10939 × 10-28 g)

Atomic Structure
Nuclear Model of the Atom (Rutherford: 1910)

Nucleus • nucleus contains protons and neutrons • nuclear radius is 20,000 times smaller than the atomic radius! (5 cm vs 1 km)

The Modern View of Atomic Structure

The Modern View of Atomic Structure

Particle Proton Neutron Electron

mass 1.67 x 10-24 g 1.67 x 10-24 g 9.11 x 10-28 g

charge 1.60 x 10-19 C No charge -1.60 x 10-19 C

Mass of atom is mostly from nucleus, volume of atom is mostly from electron cloud.

Scale of Atoms
Useful units: 1 u (atomic mass unit) = 1.66054 x 10-24 kg 1 pm (picometer) = 1 x 10-12 m 1 Å (Angstrom) = 1 x 10-10 m = 100 pm = 1 x 108 cm The heaviest atom has a mass of only 4.5 x 10-22 g and a diameter of only 4 x 10-10 m. Typical C-C bond length 1.54 Å (1.5 x 10-10 m)

Isotopes, Atomic Numbers, and Mass Numbers • Atomic number Z = number of protons determines the number of electrons thus the element identity • Atomic Mass - Σ of protons and neutrons • By convention, for element X, we write ZAX. • Isotopes have the same Z but different A (variable number of neutrons) • No two elements have the same value of Z.

Isotopes
1

H 1
Hydrogen

2

H 1
Deuterium

3

H 1
Tritium

How many electrons does each isotope have?

In nature, elements occur as a mixture of isotopes

Atomic Weights
• 1H weighs 1.6735 x 10-24 g and 16O 2.6560 x 10-23 g. • We define: mass of 12C = 12 u. Where 12 u is an exact number (with an infinite number of zeros past the decimal). The mass of every other element in atomic mass units is determined by measuring its mass relative to the mass of 12C. • Using atomic mass units: 1 u = 1.66054 x 10-24 g 1 g = 6.02214 x 1023 u Remember that we must include the isotopes when calculating atomic weights. In nature C: 98.892 % 12C + 1.108 % 13C. • Average mass of C: (0.98892)(12 ) + (0.0108)(13.00335) = 12.011 amu. • What is the mass of a single C atom??

The Periodic Table

• Based on the reactions and properties of the elements and their compounds • Reflects the underlying electronic structure of the elements

(1834 – 1907)

The Periodic Table
• Columns in the periodic table are called groups
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