Chemical Foundations: Elements, Atoms, and Ions
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