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Elements, Atoms & Ions

Chapter 4(except 4.3, 4.5)

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4.1 Elements• Over 112 known: 88 are found in nature,

rest are man-made• Abundance is the percentage found in

nature Oxygen is most abundant element (by mass)

in the crust of the earth and in the human body

Iron is the most abundant element (by mass) throughout the entire earth

4.2 Symbols for Elements• Each element has a unique symbol• The symbol of an element may be one

letter or two if two letters, the second is lower case

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4.4 Formulas DescribeCompounds

• A compound is a distinct substance that is composed of atoms of two or more elements.

– Compounds always contain the same relative numbers of atoms of each element.

– Compounds always contain the same relative masses of each element

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• A formula describes a compound by indicating the number and type of each atom present in the simplest unit of the compound

– Example: H2O, not H4O2

C6H12O6 not C3H6O3

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• Rules for writing formulas (pg 90) each element is represented by

its letter symbol the number of atoms of each

element is written to the right of the element as a subscript

When only one atom of a type is given, the subscript 1 is not written

polyatomic groups are placed in parentheses (if more than one)

Examples:

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4.6 The Modern Concept of Atomic Structure

• Atoms are composed of three main parts: protons, neutrons and electrons.

• The nucleus contains protons (+) and neutrons (0).

• The electrons (-) are arranged in energy levels (orbitals) outside the nucleus.

• In a neutral atom,

# protons = # electrons

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• The number of protons in the nucleus is the atomic number

• The number of protons plus neutrons in the nucleus is the mass number.

• Size of atom (see fig 4.9)

– Diameter of nucleus ~ 10-13 cm– Diameter of overall atom ~ 10-8 cm– Diameter of an atom is about 100,000

times the diameter of the nucleus!

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• Masses of parts of an atomElectron = 1 (relative mass)Proton = 1836 Neutron = 1839

-- most of the mass of an atom is located in the nucleus; the mass of the electrons is insignificant

• In summary:• Atoms are very small, although the

overall diameter is very large compared to the diameter of the nucleus. Atoms are mostly empty space!

• Almost all the mass is located in the very tiny nucleus, making it very dense.

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4.7 Isotopes

• All atoms of an element have the same number of protons

• Atoms of an element with different numbers of neutrons are called isotopes

• All isotopes of an element are chemically identical and behave the same in chemical reactions

• Isotopes of an element have different masses

• Isotopes are identified by their mass numbers

mass number = protons + neutronsmass number = protons + neutrons

See examples on page 96See examples on page 96

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4.8 Introduction to the Periodic

Table• See class handout for this

section.

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4.9 Natural States of the Elements

• Elements are usually not found in pure form in nature (on earth).

• Matter around us consists mostly of compounds and mixtures.

• Most elements are reactive and form compounds with other elements.

Ex: Na + Cl NaCl

• Exceptions: the noble gases (Group 8A) and the noble metals (gold, silver, platinum).

Noble gases exist as single atoms.

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• Some gases exist as diatomic molecules at normal temperatures:

(around 25o C)

H2

O2 and N2 are components of air

The halogens -- F2, Cl2, Br2, I2

However, hydrogen and the halogens are rarely present on earth in elemental form because they easily form compounds with other elements.

Ex: Na + Cl NaCl

Mg + 2Br MgBr2

2H2 + O2 2H2O

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• Only two elements are liquids at normal temperatures: bromine (Br2) and mercury (Hg) metal.

• All other elements are solids at normal temperatures (around 25o C).

(cesium and gallium melt at about 30o C)

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• Allotropes Forms of a solid nonmetallic

element with different physical properties.

The different physical properties arise from the different arrangements of the atoms in the solid.

Allotropes of carbon include:• diamond (hard)• graphite (slippery)• buckminsterfullerene

–large, soccer-ball-shaped molecules

• See fig 4.18

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4.10 Ions

• A neutral atom has zero net charge because the number of its protons (+) equals the number of its electrons (-).

• Adding or removing electrons from an atom creates an ion – an atom with a net positive or negative charge.

• When one or more electrons are lost by a neutral atom, an ion with a positive charge is formed, called an cation.

Examples:

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A cation is named using the parent name of the atom:

Na+ is the sodium ion (or sodium cation)

Mg2+ is the magnesium ion (or magnesium cation)

• When one or more electrons are gained by a neutral atom, an ion with a negative charge is formed, called an anion.

• Examples:

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An anion is named by taking the root name of the atom and adding the suffix –ide.

Cl- is the chloride ion

F- is the fluoride ion

O2- is the oxide ion

• Ions are never formed by adding or removing protons to a nucleus.

• Isolated atoms do not form ions on their own.

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• Ions form when metallic elements react with nonmetallic elements. The metal atoms lose one or more electrons, which are in turn gained by the atoms of the nonmetal.

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Ion Charges and the Periodic Table(See fig 4.19)

• Group 1A metals form 1+ ions.• Group 2A metals form 2+ ions.• Group 3A metals form 3+ ions.• Transition metals form cations with

various charges. Examples:Fe2+ and Fe3+

Cu+ and Cu2+

• Group 4A metals form cations with various charges: Pb2+ and Pb4+

Sn2+ and Sn4+

• Group 5A nonmetals form 3- ions.• Group 6A nonmetals form 2- ions.• Group 7A nonmetals form 1- ions