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