Chapter 4
Atomic Structure
Pre-Test
1. True/False: Compounds have fixed compositions
2. What is an atom?3. Which of the following units is a unit
of mass?a) mLb) ˚Cc) g d) cm
Pre-Test continued
4. Volume is:a) The straight-line distance between two
pointsb) The quantity of matter in an objectc) The amount of space taken up by an
objectd) A representation of an object or event
Pre-Test continued
5. What is density?6. Which two of the following events can
take place when a liquid absorbs energy?
a) The average kinetic energy of the particles in the liquid increase
b) The temperature decreasesc) The liquid freezesd) The liquid changes to a gas
4.1 Studying Atoms
Studying and understanding atoms can be difficult--we cannot see atoms with our naked eye– So we have to use indirect evidence to
prove our hypotheses
Ancient Greek Atom Models
Democritus-believed that all matter consisted of extremely small particles that could not be divided– Called these particles atoms
Comes from the Greek word atomos, which means uncut or indivisible
– Thought there were different types of atoms with specific sets of propertiesEx: liquid atoms were smooth and round; solid
atoms were rough and prickly
Ancient Greek Atom Models
Aristotle- did not think there was limit to the number of times matter could be divided– Thought that all substances were built up
from only four elements: earth, air, water, and fireThese elements were a combination of four
qualities: hot, cold, dry, and wet
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Dalton’s Atomic Theory
John Dalton born in England in 1766– Teacher, in spare time did scientific
experiments– Was interested in weather, so he studied
the behavior of gasesHe concluded (correctly) that gas consists of
individual particles, based on the way gases exert pressure
Dalton-Evidence for Atoms
Massed elements before he mixed them, and then massed the compound that formed after the reaction took place– Gathered evidence for atoms– Concluded that no matter the size of the
sample of a particular compound, the ratio of the elements that make it up is always the same, therefore the composition of compounds is fixed
Dalton’s Theory
Dalton proposed the following theory:– All matter is made up of individual
particles called atoms, which cannot be divided.
Dalton’s Theory
– Dalton’s theory has four main pts:All elements are composed of atomsAll atoms of the same element have the same
mass, and atoms of different elements have different masses
Compounds contain atoms of more than one element
In a particular compound, atoms of different elements always combine in the same way
Thomson’s Model of the Atom
Thomson found evidence for subatomic particles
Thomson’s experiments provided the first evidence that atoms are made of even smaller particles– Atoms overall are neutral in charge even
though there are charged particles inside the atomProtons (+1), Neutrons (no charge), Electrons
(-1)
Rutherford’s Atomic Theory
In 1899 Rutherford discovered that uranium emits fast moving particles that have a positive charge– Alpha particles
Gold Foil Experiment– A narrow beam of alpha particles at a thin
sheet of gold foil– The screen around the foil was made of a
material that flashed when struck by a fast-moving alpha particle
Gold Foil Experiment continued Marsden (a student of Rutherford’s) noticed
that some of the alpha particles were reflected and some went right through the foil– This lead to the notion of the nucleus of an atom
and it being positively charged– Nucleus- a dense, positively charged mass
located in the center of the atom According to Rutherford’s model, all of an
atom’s positive charge is concentrated in its nucleus
4.2 The Structure of an Atom
Properties of Subatomic Particles– 1920: Rutherford saw evidence for the
existence of 2 subatomic particles and predicted a 3rd
– Protons, Electrons, and Neutrons are subatomic particles
Protons
Proton- a positively charged subatomic particle that is found in the nucleus of an atom
Each proton has a +1 chargeCan be abbreviated: p+
Found in the nucleus of an atom
Electrons
Electron- a negatively charged subatomic particle that is found in the space outside the nucleus
Each electron has a -1 chargeCan be abbreviated: e-
Found in the space outside of the nucleus
Neutrons
Neutron- a neutral subatomic particle that is found in the nucleus of an atom
No chargeHas a mass almost identical to that of a
protonWas proven to exist in 1932 by James
ChadwickCan be abbreviated: n Found in the nucleus of an atom
Properties of Subatomic ParticlesParticle Symbol Relative
ChargeRelative Mass (proton=1)
Actual Mass
Electron e- 1- 1/1836 9.11 x 10-28
Proton p+ 1+ 1 1.674 x 10-24
Neutron n 0 1 1.675 x 10-24
Comparing Subatomic ParticlesProtons, electrons, and neutrons can
be distinguished by mass, charge, and location in an atom
Atomic Number
Atomic Number- equals the number of protons in an atom of a particular element
Atoms of different elements have different numbers of protons
Each positive charge of a proton is balanced by one negative charge of an electron--> therefore for every proton there is an electron– So, the atomic number also tells us how many
electrons are in an atom
Mass Number
Mass number- the sum of the protons and neutrons in a nucleus of an atom
To figure out the number of neutrons in an atom you do the following calculation:– Number of neutrons = Mass number -
Atomic number
Isotopes
Isotopes- atoms of the same element that have different mass numbers
Isotopes of an element have the same atomic numbers, but different mass numbers because they have different numbers of neutrons
4.3 Modern Atomic Theory
Bohr’s Model of the Atom– Niels Bohr a Danish physicist worked with
Rutherford for a while– Bohr expanded on Rutherford’s model by
including information about electrons– Bohr included a description of the
arrangement of electrons in an atom This is now the centerpiece of the modern
atomic model
Bohr’s Model
Energy Levels– e-’s move with constant speed in fixed orbitals
around the nucleus– Each e- has the a specific amount of energy– If an atom gains or loses energy, the energy of
an electron can change– Energy levels- are the possible energies that e-’s
in an atom can havee-’s cannot exist between energy levelsNo two elements have the same set of energy levels
– An e- in an atom can move form one energy level to another when the atom gains or loses energy
Bohr’s Model
Evidence for energy levels– Scientists can measure the energy gained
when e-’s absorb energy and move to a higher energy level
– They can measure the energy released when the e- returns to a lower energy level
Electron Cloud Model
Bohr’s model was improved as new discoveries were/are made
There are energy levels, but they do not move in orbitals like planets– e-’s move in much less predictable ways– Scientists must deal with probability when trying
to predict the location of e-’s– Electron Cloud- a visual model of the most likely
locations for e-’s in an atomThe denser the cloud the more probable it is to find an
e- there
Energy Levels, Orbitals, & ElectronsEnergy Level Number of
OrbitalsMaximum Number of Electrons
1 1 2
2 4 8
3 9 18
4 16 32
Atomic Orbitals
Orbital- a region of space around the nucleus where an electron is likely to be found
Electron Configuration
Electron Configuration- the arrangement of electrons in the orbitals of an atom– The most stable e- configuration is the
one in which the electrons are in orbitals with the lowest possible energies
– Ground state- when all e-’s in an atom have the lowest possible energies