Chapter: 2 Atomic StructureTitle: Lesson 1 Models of the Atom, Isotopes and Mass SpectraLearning Objectives:

• Understand how models of the atom have evolved over time

• Recap atomic structure in terms of the sub atomic particles

Dalton’s Atomic Theory

1. Elements are made up of atoms2. Atoms of each element are identical.

Atoms of different elements are different.

3. Compounds are formed when atoms combine. Each compound has a specific number and kinds of atom.

4. Chemical reactions are rearrangement of atoms. Atoms are not created or destroyed.

5. Atoms react in specific ratios.

Experiments to determine what an atom was

• J. J. Thomson - used Cathode ray tubes• He discovered that different metals

produced a stream of negatively charged particles when a high voltage was applied across electrodes.

Thomson’s Experiment

Voltage source

+-

Thomson’s Experiment

Voltage source

+-

Thomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s Experiment

• By adding an electric field

Voltage source

Thomson’s Experiment

By adding an electric field

+

-

Voltage source

Thomson’s Experiment

By adding an electric field

+

-

Voltage source

Thomson’s Experiment

By adding an electric field

+

-

Voltage source

Thomson’s Experiment

By adding an electric field

+

-

Voltage source

Thomson’s Experiment

By adding an electric field

+

-

Voltage source

Thomson’s Experiment

By adding an electric field he found that the moving pieces were negative

+

-

• The particles were the same regardless of the metal, so he suggested they were part of all atoms.

• As atoms had no net charge, he proposed that the negative charges were scattered within a large sphere of positive charge.

Thomson’s Theory

The evolution of ‘The plum pudding model’

• The atom was imagined to be a sphere of positive charge with negatively charged electrons dotted around inside it - like plums in a pudding.

Rutherford's Scattering Experiment

What he did:•Used uranium to produce alpha particles

•Aimed alpha particles at gold foil by drilling hole in lead block

•Since the mass is evenly distributed in gold atoms alpha particles should go straight through.

•Used gold foil because it could be made atoms thin

A scientist called Ernest Rutherford designed an experiment to test the plum pudding model.

Lead block

Uranium

Gold Foil

Fluorescent Screen

Task: look at the picture and deduce what happened in the experiment What conclusions could Rutherford make about the structure of the atom?

Lead block

Uranium

Gold Foil

Florescent Screen

What Rutherford expected…

Rutherford thought the mass was evenly distributed in the atom

What Rutherford got…

How Rutherford explained it…

+

• Atom is mostly empty• Small dense, positive

piece at the center• Alpha particles

are deflected by it if they get closeenough

+

Conclusions

• Some of the alpha particles emerged from the foil at different angles, and some even came straight back.

• The positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom.

• As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.

The Rutherford Scattering Experiment

Alpha particles (positive charge)

Thin gold foil

Some particles passed through, some were deflected

backwards

Conclusion – atom is made up of a small central nucleus surrounded by electrons orbiting in shells

Modern View

• The atom is mostly empty space

• Two regions• Nucleus - protons and

neutrons• Electron cloud - region

where you have a chance of finding an electron

Protons, electrons & neutronsParticle Charge Atomic

MassWorking it out

Protons +1 1 BottomElectrons -1 1/1840 BottomNeutrons 0 1 Top - Bottom

• Mass number – an atom of one particular element has a different mass from another element.

• Atomic number – the number of protons tells us which element an atom is.

Atomic Structure

• Lithium

• The total number of protons & neutrons = 7 i.e. Mass number (A)

• The number of protons = 3 i.e Atomic number (Z)• An atom is neutral, so the number of electrons = 3 also.• A –Z = 4 = number of neutrons.

Examples

• NitrogenTwice as heavy asLithium

• The total number of protons & neutrons = 14 i.e. Mass number (A)

• The number of protons = 7 i.e Atomic number (Z)• An atom is neutral, so the number of electrons = 7 also.• A –Z = 7 = number of neutrons.

Examples

Charge on the ionsMetals lose electrons to form positive ions while non-metals gain electrons to form negative ions.

Positive ions = Cations Negative ions = Anions

1 2 3 4 5 6 7 8/0

1+ 2+ 3+ N/A 3- 2- 1- N/A

Na+ Al3+ N/AN/A N3- O2- F-Mg2+

Group

Charge

Example

The number of electrons gained or lost by an atom is related to the group in which the element is found.

The loss or gain of electrons has a direct effect on its atomic structure.

Sodium Ion

Chloride Ion

What are isotopes?

atomic number is the same

mass number is different

The reactivity of different isotopes of an element is identical because they have the same number of electrons.

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

The different masses of the atoms means that physical properties of isotopes are slightly different.

carbon-12 carbon-13

Q. Word means ‘equal place’… Why is that appropriate?

Isotopes of chlorineAbout 75% of naturally-occurring chlorine is chlorine-35 (35Cl) and 25% is chlorine-37 (37Cl).

17 protons

18 neutrons

17 electrons

17 protons

20 neutrons

17 electrons

• Hydrogen

• Same number of protons/electrons so same chemical properties.

• All hydrogen atoms – same number of protons.

Example

IsotopesIsotope

Mass Number

12 13 14

Atomic Number

6 6 6

Number of neutrons

6 7 8

An isotope will be radioactive if its nuclei are unstable. Large atomic nuclei, with more than 83 protons and their associated complement of neutrons, are inherently unstable. Uranium and plutonium are examples of such elements.

Complete the Test Yourself Questions

• Page 59

• Questions 1-4

• Check your answers on page 559

What is relative atomic mass?The relative atomic mass (Ar) of an element is the mass of one of its atoms relative to 1/12 the mass of one atom of carbon-12.

Most elements have more than one isotope. The Ar of the element is the average mass of the isotopes taking into account the abundance of each isotope. This is why the Ar of an element is frequently not a whole number.

Carbon-12 was chosen as the standard in 1961. This was because it was abundant, and easy to transport and store as it was a solid.

average mass of an atom × 12mass of one atom of carbon-12

relative atomic mass(Ar)

=

Working out relative atomic mass

Using mass spectra to calculate ArThe mass spectrum of an element indicates the mass and abundance of each isotope present. For example, the mass spectrum of boron indicates two isotopes are present:

How can this be used to calculate the Ar of boron?

0

100

20

40

60

80

abun

danc

e (%

)

0 2 4 6 8 10 12m/z

10B (20%)

11B (80%)

Calculating ArMost elements have more than one isotope. The relative atomic mass of the element is the average mass of the isotopes taking into account the abundance of each isotope.

Example: what is the Ar of boron?In a sample of boron, 20% of the atoms are 10Br and 80% are 11Br.If there are 100 atoms, then 20 atoms would be 10Br and 80 atoms would be 11Br.The relative atomic mass is calculated as follows:

Ar of Br = (20 × 10) + (80 × 11)100

Ar of Br = 10.8

Calculating Ar of magnesium

Example: What is the Ar of magnesium?

In a sample of magnesium, 79.0% of the magnesium atoms are 24Mg, 10.0% are 25Mg and 11.0% are 26Mg.

2. Add these values, and divide by 100

(1896 + 250 + 286) / 100

Ar of Mg = 24.3

24 × 79.0

25 × 10.0

26 × 11.0

1. Calculate mass × abundance of each isotope

Ar calculations

Complete the test yourself questions

• Page 61• Questions 5-7• Check your answers on page 559