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 Symbols● Names 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 Theory● elements 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: nitrogen2: two atoms of nitrogenO: oxygenno 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