Chemical Foundations: Elements, Atoms, and Ions · 2017. 6. 7. · learned physics in J.J. Thompson’s laboratory interested in alpha particles, positively charged particles (made

Chemical Foundations:

Elements, Atoms, and

Ions

Elements

115 different elements

88 of which occur naturally, the rest have

been made in laboratories

vary in abundance

9 elements account for most of the

compounds found in earth’s crust (see table

3.1 page 51)

Element Names and SymbolsNames have many sources: descriptions of

their properties, place the element was

discovered, honoring a famous scientist

Element symbols usually consist of the first

letter or first two letters of the element’s

name

The first letter is always capitalized, the

second is not

Examples: Oxygen (O) or Neon (Ne)

The Law of Constant Composition

a compound always contains elements in

exactly the same proportion by mass

this occurs because the molecules that make

up a compound are all identical (ex. H₂O)

John Dalton (early 1800’s) offered an

explanation for this known as Dalton’s

atomic theory

Dalton’s Atomic Theoryelements are made of tiny particles called atoms

all atoms of a given element are identical

the atoms of a given element are different from those of

any other element

atoms of one element can combine with atoms of other

elements to form compounds...a given compound

always has the same relative numbers and types of

atoms

atoms are indivisible in chemical processes...which means

atoms are not created or destroyed in chemical

reactions...a chemical reaction only changes the way

atoms are grouped together

Dalton’s Atomic Theory

Dalton predicted that atoms of two different

elements could combine in different ways to

form different compounds (examples: NO,

NO₂, N₂O)

When the existence of these substances was

verified, Dalton’s atomic theory was widely

accepted

Compounds

always contain the same relative numbers of atoms of

each element

the types of atoms and the number of each type in each

unit (molecule) of a compound are expressed by the

chemical formula

in a chemical formula the atoms are indicated by the

element symbol and the number of each type of atom is

indicated by a subscript

when only one atom of a given type is present, the

subscript 1 is NOT written

Chemical Formula Example

N₂ON: nitrogen

2: two atoms of nitrogen

O: oxygen

no subscript: one atom of oxygen

The Structure Of The Atom

for most of the 1800’s scientists wondered how an atom

was structured

in the late 1890’s J.J. Thomson conducted experiments

using a cathode ray tube to prove that atoms can emit

tiny negative particles which are now called electrons

He predicted that the atom must also contain positive

particles that balance exactly the negative charges

carried by the electrons, giving the atom a charge of

zero overall

Cathode Ray Tube

Plum Pudding Model

J.J. Thomson and William Thomson (Lord Kelvin, not

related) are credited with this atomic model

The atom is structured like plum pudding (a pudding with

raisins randomly distributed throughout)....think of a

chocolate chip cookie

They believed the atom could be thought of as a uniform

“pudding” of positive charge with enough negative

electrons scattered within to counterbalance the positive

charge

Ernest Rutherford

learned physics in J.J. Thompson’s laboratory

interested in alpha particles, positively charged particles

(made of two protons and two neutrons with a mass

about 7500 times that of an electron but this was not

known then)

1911 he performed an experiment that involved shooting

alpha particles at a thin metal foil surrounded by a

detector screen that flashed every time an alpha particle

hit it

Ernest Rutherford

although most of the alpha particles passed

straight through the foil, some of them were

deflected at large angles, and some were

reflected backwards

he described the results as comparable to

shooting a gun at a piece of paper and

having the bullet bounce back

Rutherford’s Conclusions

the large deflections of the alpha particles was caused by

a center of concentrated positive charge within the atom

that would repel the positively charged alpha particles

Most of the alpha particles passed directly through the foil

because the atom is mostly open space

the deflected alpha particles were those that had a “close

encounter” with the positive center of an atom

those alpha particles that scored a “direct hit” on the

positive center were reflected backwards

Gold Foil Experiment

The Nuclear Atom

Due to the results of his experiment, Rutherford described

the structure of the atom as having a dense center of

positive charge (the nucleus) around which tiny

electrons moved in a space that was otherwise empty

1919 Rutherford concluded that the nucleus contained

protons (particles located in the nucleus)

a proton has the same magnitude of charge as an

electron, but its charge is positive

Protons = +1 charge and Electrons = -1 charge

Expected ResultsPlum Pudding Model

Actual ResultsNuclear Model

Further Work By Rutherford

Reasoned the hydrogen atom has one proton in the central

nucleus and one electron moving around it

Reasoned that other atoms must have nuclei composed of

many protons bound together in some way

1932 he and James Chadwick showed that most nuclei

contain a neutral particle called a neutron that is slightly

more massive than a proton but has no charge

Modern Concept of Atomic Structure

a tiny nucleus (about 10⁻¹³cm in diameter)

electrons (-1 charge) that move around the nucleus at an

average distance of about 10⁻⁸cm from it

if the nucleus of an atom were the size of a grape, the

electrons would be about one mile away on average

the nucleus contains protons (+1 charge) and neutral

neutrons

the neutrons’ function is not obvious, they may help hold

the protons (which repel each other) together to form

the nucleus (see table 3.4 page 63 for relative masses)

Electrons

the space in which the electrons move accounts for most

of the atomic volume

the electrons are the parts of the atom that intermingle

when atoms combine to form molecules

therefore it is the number of electrons an atom possess

that affects the ways in which it can interact with other

atoms

the number of electrons an atom has determines its

chemical behavior

Isotopes

after the discovery of the neutron, Dalton’s statement that

all atoms of the same element were identical, needed to

be changed

all atoms of the same element contain the same number of

protons and electrons (because an atom is neutral), but

they can have differing amounts of neutrons

atoms with the same number of protons but differing

amounts of neutrons are called isotopes

Isotopes

Atomic Number vs. Mass Number

the number of protons in a nucleus is called the atom’s

atomic number

the sum of the number of protons and neutrons in the

nucleus is called the atom’s mass number

this information can be noted as follows

example written in chemistry text The atomic mass given in the

periodic table is an average of the

atomic mass of each of the isotopes

of that element (atomic weight)

You can always determine the number of neutrons present in a given atom by subtracting the

atomic number from the mass number.

Periodic Table of Elements

a chart that shows all of the known elements and gives a

good deal of information about each one

elements are listed in order of increasing atomic number (#

of protons) as well as in specific horizontal and vertical

columns

the elements were first arranged in this way in 1896 by

Dmitri Mendeleev because of the similarities in the

chemical properties of some elements

it is called a periodic table because as we increase the

atomic numbers, every so often (periodically) an

element occurs with properties similar to those of an

earlier (lower-atomic-number) element

Periodic Table of Elements

Elements with similar chemical properties are

within the same family

Families of elements lie in the same vertical

column on the periodic table, each column is

called a group

Groups are referred to by the number over the

column (see pages 68-69 in text and back

inside cover)

Note: Groups 1A-8A do not include the transition metals. See page 68 for group numbers that

include the transition metals.

Special Group Names

Some groups are given special names in

addition to their group number

Column 1 = alkali metals

Column 2 = alkaline earth metals

Column 7 = halogens

Column 8 = noble gases

a large collection of elements that spans

many vertical columns consists of the

transition metals

Most Elements Are Metals

Chemical Properties of Metals:

efficient conduction of heat and electricity

malleability (can be hammered into thin

sheets)

ductility (can be pulled into wires)

a lustrous (shiny) appearance

Nonmetals

elements that appear in the upper righthand

corner (to the right of the “stair-step”heavy

black line) as well as hydrogen

lack those properties that characterize metals

show much more variation in their individual

properties than metals do

Metalloids (Semimetals)

elements that lie close to the “stair-step”

heavy black line

have a mixture of metallic and nonmetallic

properties

silicon, germanium, arsenic, antimony,

tellurium

Note: See table in text page 70

Natural States of the Elements

most elements are quite reactive: their atoms

tend to combine with those of other elements

to form compounds

therefore, we do NOT often find elements in

nature in pure form (uncombined with other

elements)

Exceptions include: gold, platinum, and silver

Noble Metals

gold, platinum, and silver are members of a class of metals

called noble metals because they are relatively

unreactive

the term noble implies a class set apart

Group 8 is also called the noble gases because these

elements are gases that do not react with other

elements and are found in nature in their pure form

(helium, neon, argon, krypton, xenon, and radon)

Diatomic Molecules

some elements can be found in pure form but not as single

atoms but as two atoms of the same element bonded

together to create a molecule

these molecules are called diatomic molecules

examples: N₂ or O₂

Diatomic Molecules

all of the elemental forms of the group 7

(halogens) contain diatomic molecules

(fluorine, chlorine, bromine, and iodine)

hydrogen, nitrogen, and oxygen are also

diatomic in their elemental forms

Natural States of the Elements

At normal temperatures (about 25°C or 77°F) the elements

found in gaseous form are the noble gases of group 8

(as individual atoms) as well as hydrogen, nitrogen,

oxygen, fluorine, and chlorine (as diatomic molecules)

Two elements are liquids in their elemental forms at the

same temperature: bromine (which is diatomic) and

mercury

All other elements are solids in their elemental forms at

25°C

Allotropes

different forms of the same element

for example, solid carbon is found in three forms:

diamond, graphite, and buckminsterfullerene

Ions

an atom is a neutral entity because it has an

equal number of protons with positive

charges and electrons with negative charges

ions (atoms with charges) are produced by

taking a neutral atom and adding or

removing one or more electrons

Cations

a positively charged ion is called a cation

it is produced when one or more electrons are LOST from

a neutral atom

if an atom loses two electrons the ion it creates has a +2

charge (because now there are two more protons with a

positive charge than there are electrons with a negative

charge)

a cation is named using the name of the parent atom

example: an aluminum atom that has lost 3 electrons is

called an aluminum ion or aluminum cation (Al³⁺)

Anions

a negatively charged ion is called an anion

it is produced when a neutral atom GAINS electrons

if a neutral atom gains one electron it now has an overall -

1 charge because it has one more negatively charged

electron than it does positively charged protons

an anion is named by taking the root name of the atom and

changing the ending to -ide

for example, an atom of fluorine can gain one electron and

will be called a fluoride ion or fluoride anion

Ion Charges and the Periodic Table

metals always form positive ions by losing

electrons

nonmetals form negative ions by gaining

electrons

most of the transition metals form cations with

various positive charges

Note: see table on page 79 in text

Compounds That Contain Ions

substances that contain ions can conduct an electric

current only if the ions can move

the current travels by the movement of the charged ions

in solid NaCl (table salt) the ions are tightly held and

cannot move

when the solid melts and becomes a liquid the structure of

the atoms is disrupted and the ions can move

therefore, an electric current can travel through melted salt

when salt is dissolved in water the ions can move around

and are dispersed throughout the water, allowing it to

conduct a current

Ionic Compounds

a compound that results when a metal reacts with a

nonmetal to form cations and anions

metals lose electrons (become cations) and nonmetals

gain electrons (become anions)

the numbers of cations (+) and anions (-)in an ionic

compound must be equal so that they net charge is

zero

ionic compounds have very high melting points and

conduct an electric current when melted or dissolved in

water

ionic and molecular compounds in solution

salt dissolving in water animation