CH. 4 ATOMIC STRUCTURECH. 4 ATOMIC STRUCTURE

4.1 The Structure of an Atom4.1 The Structure of an Atom

GREEK MODELS OF ATOMSGREEK MODELS OF ATOMS� Philosopher Democritus believed that all matter

consisted of extremely small particles that cannot be divided.

� He called these particles “atoms” from the Greek word “atomos” which means uncut or indivisible

� He believed there were different types of atoms ex: liquids had round and smooth atoms and solids had rough, prickly atoms

� Aristotle did not think there was a limit to the number of times matter could be divided.

DALTONDALTON’’S ATOMIC THEORYS ATOMIC THEORY

EVIDENCE FOR ATOMSEVIDENCE FOR ATOMS

� Dalton determined that all compounds have a fixed composition.

� For example, a 100 gram sample of magnesium combines with 65.8 grams of oxygen.

� A 10 gram sample of mangnesium combines with 6.58 grams of oxygen.

� The ratio remains constant.

DALTONDALTON’’S THEORYS THEORY

� All matter is made up of individual

particles called atoms, which cannot be

divided.

4 MAIN POINTS OF 4 MAIN POINTS OF

DALTONDALTON’’S THEORYS THEORY� All elements are composed of atoms.

� All 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.

THOMSONTHOMSON’’S MODEL OF S MODEL OF

THE ATOMTHE ATOM

� Some materials gain the ability to attract or

repel other materials, this is referred to as a

positive or negative electric charge.

� Object with opposite charges attract, or pull

together.

� J.J. Thomson used an electric current to

learn more about atoms.

ThomsonThomson’’s Experimentss Experiments

DESCRIPTION OF DESCRIPTION OF

THOMSONS EXPERIMENTSTHOMSONS EXPERIMENTS

� He used a glass tube, with 2 metal disks on

each end with opposite charges. Each disk

had wire that connected it to an electric

current. When connected, a glowing beam

appeared in the space between the disks.

� He hypothesized that the beam was made up

of charged particles that interact with the air

in the tube causing it to glow.

DESCRIPTION (CONTDESCRIPTION (CONT’’D)D)

� Thomson also concluded that the particles in the beam had a negative charge because they were attracted to the positive plate. (Remember opposites attract)

� He hypothesized that these particles came from inside atoms. The following supported this:

– No matter what metal he used the particles produced were identical

– The mass of the particles was about 1/200 the mass of a hydrogen atom, the lightest atom of all.

THOMSONTHOMSON’’S MODELS MODEL

� An atom does not have a – or + charge.

� How can an atom contain – particles and

still be neutral?

� In Thomson’s model, the atom contains

negatively charged particles evenly

scattered throughout a positively charged

mass of matter.

THOMSONTHOMSON’’S MODEL S MODEL (CHOCOLATE CHIP ICE CREAM)(CHOCOLATE CHIP ICE CREAM)

� Chocolate Chips=negative

particles

� Vanilla Ice

Cream=positively charged

mass of matter

� By spreading out the

chocolate chips, their –

charges balance out the +

charges.

RUTHERFORDRUTHERFORD’’S ATOMIC THEORYS ATOMIC THEORY� Discovered that Uranium emits fast moving

particles that have a + charge.

� He named these particles alpha particles.

� Through the gold foil experiment, he concluded that the + charge of an atom is not spread out like Thomson had proposed.

� Rutherford determined this + charge is concentrated in a small, central area he called the nucleus

� The nucleus is a dense, positively charged mass located in the center of the atom.

4.2 The Structure of an Atom4.2 The Structure of an Atom

Properties of Subatomic Properties of Subatomic

ParticlesParticles

� The 3 types of subatomic particles are:

1. Proton

2. Electron

3. Neutron

ProtonsProtons

� A positively charged subatomic particle that

is found in the nucleus of an atom.

� Each proton is assigned a charge of +1.

� Some nuclei contain more than 100 protons.

ElectronsElectrons

� Greek word means “Amber”

� A negatively charged subatomic particle

that is found in the space outside the

nucleus.

� Each electron has a charge of -1

NeutronsNeutrons

� A neutral subatomic particle that is found in

the nucleus of an atom.

� It has a mass almost exactly equal to that of

a proton.

Comparing Subatomic Comparing Subatomic

ParticlesParticles� Protons, electrons, and neutrons can be

distinguished by mass, charge, and location in an atom.

� Protons and Neutrons are similar in mass.

� Electrons have a charge that is equal in size, but opposite of the charge of a proton.

� Neutrons have no charge.

� Protons and Neutrons are located in the nucleus, electrons are found outside the nucleus.

Atomic NumberAtomic Number� Atomic Number of an element is equal to the

number of protons of that element.

� For example, Hydrogen has 1 proton therefore the atomic number is 1.

� Atoms of different elements have different numbers of protons.

� The numbers are used to identify elements since no two are alike.

� Each + charge is balanced by a – charge, so the atomic number is also equal to the # of electrons in an atom.

� Each atom of Hydrogen, also has 1 electron.

Mass NumberMass Number

� The Mass Number is the sum of the protons

and neutrons in the nucleus of an atom.

� Example

Aluminum has 13 protons

14 neutrons

27 Mass Number

o Number of neutrons= Mass #- Atomic #

IsotopesIsotopes

� All atoms do have the same number of protons and electrons.

� However, they do not have the same number of neutrons.

� Isotopes are atoms of the same element that have different numbers of neutrons.

� Isotopes have the same atomic # but different mass #’s because they have different numbers of neutrons.

Examples of an IsotopeExamples of an Isotope

� Oxygen has 8 protons.

� Some atoms of oxygen have 8 neutrons and a mass # of 16.

� Some oxygen atoms have 9 neutrons and a mass # of 17.

� These isotopes are referred to as Oxygen –16 and –17.

� Both can react to from H2O or combine with iron to form rust.

4.3 Modern Atomic Theory4.3 Modern Atomic Theory

BohrBohr’’s Model of the Atoms Model of the Atom

� Bohr’s model of an atom is similar to a

solar system with planets revolving around

a sun.

� He agreed with Rutherford’s model of a

nucleus surrounded by a large volume of

space.

� Bohr’s model is focused on the electrons.

Energy LevelsEnergy Levels

� Bohr’s model suggests that electrons move with constant speed in fixed orbits around the nucleus.

� Each electron has a specific amount of energy.

� If it gains or loses energy, the electron energy can change.

� These possible energies that electrons can have are referred to as energy levels.

� No two elements have the same set of energy levels.

Energy Levels (contEnergy Levels (cont’’d)d)

� An electron can move from one energy

level to another when it loses or gains

energy.

� It is possible for it to move more than 1

energy level.

� The size of the jump determines the amount

of energy gained or lost.

Electron Cloud ModelElectron Cloud Model

� An Electron Cloud is a visual model of the most

likely locations for the electrons in an atom.

� Scientists use this to describe how electrons move

around the nucleus.

� Think of an airplane propeller, when the plane

isn’t moving you can count the blades present,

however, when it starts to move the blades are still

there, they are just hard to locate at any specific

time.

Atomic Atomic OrbitalsOrbitals� The electron cloud represents all the orbitals

in an atom.

� An orbital is a region of space around the

nucleus where an electron is likely to be

found.

Atomic Atomic Orbitals Orbitals (cont(cont’’d)d)

� The lowest energy level has only one orbital

� Higher energy levels have more than one orbital.

32164

1893

842

211

Maximum #

of Electrons

# of OrbitalsEnergy

Level

Electron ConfigurationsElectron Configurations� Electron Configurations are arrangement of

electrons in the orbitals of the atom.

� The most stable configuration is the one in which the electrons are in orbitals with the lowest possible energies.

� When this happens, the atom is said to be in its “ground state”

� If the electrons move to higher energy levels this is called the “excited state”

� The excited state is less stable, and eventually the electron will lose energy and return to ground state