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Chapter 2
Atoms and the
Periodic Table
Atoms and the Periodic Table
2
2.1 Atoms First
2.2 Subatomic Particles and Atomic Structure
Discovery of the Electron
Radioactivity
The Proton and the Nuclear Model of the Atom
The Neutron
2.3 Atomic Number, Mass Number, and Isotopes
2.4 Average Atomic Mass
2.5 The Periodic Table
2.6 The Mole and Molar Mass
The Mole
Molar Mass
Interconverting Mass, Moles, and Numbers of Atoms
Atoms First
An atom is the smallest quantity of matter that still retains the properties of matter.
An element is a substance that cannot be broken down into two or more simpler substances by any means.
Examples: gold, oxygen, helium
2.1
Once a single atom has been obtained, dividing it smaller produces subatomic particles.
The nature, number, and arrangement of subatomic particles determine the properties of atoms, which in turn determine the properties of all things material.
Subatomic Particles and Atomic Structure
In the late 1800’s, many scientists were doing research involving radiation, the emission and transmission of energy in the form of waves.
They commonly used a cathode ray tube, which consists of two metal plates sealed inside a glass tube from which most of the air has been evacuated.
2.2
Subatomic Particles and Atomic Structure
When metal plates are connected to a high-voltage source, the negatively charged plate, or cathode, emits an invisible ray.
The cathode ray is drawn to the anode where it passes through a small hole.
Although invisible, the path is revealed when the ray strikes a phosphor-coated surface producing a bright light.
Subatomic Particles and Atomic Structure
Researches discovered that like charges repel each other, and opposite charges attract one another.
J. J. Thomson (1856-1940) noted the rays were repelled by a plate bearing a negative charge, and attracted to a plate bearing a positive charge.
Subatomic Particles and Atomic Structure
This prompted him to propose the rays were actually a stream of negatively charged particles.
These negatively charged particles are called electrons.
By varying the electric field and measuring the degree of deflection of cathode rays, Thomson determined the charge-to-mass ratio of electrons to be 1.76×108 C/g. (C is coulomb, the derived SI unit of electric charge.)
Subatomic Particles and Atomic Structure
R. A. Millikan (1868-1953) determined the charge on an electron by examining the motion of tiny oil drops.
The charge was determined to be -1.6022×10-19 C.
Subatomic Particles and Atomic Structure
Knowing the charge, he was then able to use Thomson’s charge-to-mass ratio to determine the mass of an electron.
Subatomic Particles and Atomic Structure
Wilhelm Rontgen (1845-1923) discovered X-rays. They were not deflected by magnetic or electric fields, so they could not consist of charged particles.
Antoine Becquerel (1852-1908) discovered radioactivity, the spontaneous emission of radiation.
Radioactive substances, such as uranium, can produce three types of radiation.
Subatomic Particles and Atomic Structure
Alpha (α) rays consist of positively charged particles, called α particles.
Beta (β) rays, or β particles, are electrons so they are deflected away from the negatively charged plate.
Gamma (γ) rays, like X-rays, have no charge and are unaffected by external electric or magnetic fields.
Subatomic Particles and Atomic Structure
Ernest Rutherford used α particles to prove the structure of atoms.
The majority of particles penetrated the gold foil undeflected.
Sometimes, α particles were deflected at a large angle.
Sometimes, α particles bounced back in the direction from which they had come.
Rutherford proposed a new model for the atom:
Positive charge is concentrated in the nucleus.
The nucleus accounts for most of an atom’s mass and is an extremely dense central core within the atom.
A typical atomic radius is about 100 pm
A typical nucleus has a radius of about 5×10–3 pm
1 pm = 1×10–12 m
Subatomic Particles and Atomic Structure
Protons are positively charged particles found in the nucleus.
Neutrons are electronically neutral particles found in the nucleus.
Neutrons are slightly larger than protons.
Subatomic Particles and Atomic Structure
Atomic Number, Mass Number, and Isotopes
Elemental symbol
All atoms can be identified by the number of protons and neutrons they contain.
The atomic number (Z) is the number of protons in the nucleus.
Atoms are neutral, so it’s also the number of electrons.
Protons determine the identity of an element. For example, nitrogen’s atomic number is 7, so every nitrogen has 7 protons.
The mass number (A) is the total number of protons and neutrons.
Protons and neutrons are collectively referred to as nucleons.
Mass number
(number of protons + neutrons)
Atomic number
(number of protons)
2.3
Atomic Number, Mass Number, and Isotopes
Most elements have two or more isotopes, atoms that have the same atomic number (Z) but different mass numbers (A).
1 proton
0 neutrons
1 proton
1 neutron
1 proton
2 neutrons
Isotopes of the same element exhibit similar chemical properties, forming the same types of compounds and displaying similar reactivities.
Average Atomic Mass
2.4
Atomic mass is the mass of an atom in atomic mass units (amu).
1 amu = 1/12 the mass of a carbon-12 atom
The average atomic mass on the periodic table represents the average mass of the naturally occurring mixture of isotopes.
Average mass (C) = (0.9893)(12.00000 amu) + (0.0107)(13.003355 amu)
Isotope Isotopic mass (amu) Natural abundance (%)
12C 12.00000 98.93
13C 13.003355 1.07
= 12.01 amu
The Periodic Table
The periodic table is a chart in which elements having similar chemical and physical properties are grouped together.
2.5
The Periodic Table
Elements are arranged in periods, horizontal rows, in order of increasing atomic number.
The Periodic Table
Elements can be categorized as metals, nonmetals, or metalloids.
Metals are good conductors
of heat and electricity.
Nonmetals are poor
conductors of heat or
electricity.
Metalloids have
intermediate properties.
The Periodic Table
A vertical column is known as a group.
The Periodic Table
Group 1A elements (Li, Na, K, Rb, Cs, Fr) are called alkali metals.
The Periodic Table
Group 2A elements (Be, Mg, Ca, Sr, Ba, Ra) are called alkaline earth metals.
The Periodic Table
Group 6A elements (O, S, Se, Te, Po) are called chalcogens.
The Periodic Table
Group 7A elements (F, Cl, Br, I, At) are called halogens.
The Periodic Table
Group 8A elements (He, Ne, Ar, Kr, Xe, Rn) are called the noble gases.
The Periodic Table
Groups 1B and 3B-8B are called the transition elements or transition metals.
The Mole and Molar Mass
The mole is defined as the amount of a substance that contains as many elementary entities as there are atoms in exactly 12 g of carbon-12.
This experimentally determined number is called Avogadro’s number (NA).
We normally round this to 6.022×1023.
1 mole = 6.022×1023, just like 1 dozen = 12 or 1 gross = 144.
NA = 6.0221415 x 1023
2.6
The Mole
One mole each of some familiar substances:
Aluminum
Copper
Water
Salt (Sodium Chloride)
Sugar (Sucrose)
Helium (in balloon)
Molar Mass
The molar mass of a substance is the mass in grams of one mole of the substance.
By definition, the mass of a mole of carbon-12 is exactly 12 g.
Mass of 1 carbon-12 atom: exactly 12 amu
Mass of 1 mole of carbon-12: exactly 12 g
Although molar mass specifies the mass of one mole, making the units (g), we usually express molar masses in units of grams per mole (g/mol) to facilitate cancellation of units in calculations.
Interconverting Mass, Moles, and Number of Atoms
Molar mass is the conversion factor from mass to moles, and vice versa.
Avogadro’s constant converts from moles to atoms.
Chapter Summary: Key Points
2
Atoms
Elements
The Atomic Theory
Discovery of the Electron
Radioactivity
The Proton and the Nucleus
Nuclear Model of the Atom
The Neutron
Atomic Number
Mass Number
Average Atomic Mass
The Periodic Table
Molar Mass

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