The Atom

Matter Proton Neutron Electron Nucleus Atomic Number Mass Number Isotope Electron Shell Ion

Key Terms

Parts of an Atom

Atoms consist of:◦ Positively charged, heavy protons in the nucleus.◦ Heavy neutrons with no charge in the nucleus.◦ Negatively charged, light electrons orbiting the

nucleus.

Parts of an Atom

Atoms of different elements have different numbers of protons.

He

Li

Pb

Ag

Atomic Symbols

To save time when describing elements chemists use symbols instead of full words.

◦ Eg. ◦ Sodium = Na◦ Potassium = K◦ Mercury = Hg◦ Lead = Pb

Atomic Symbols

Hurry up, I’m a busy man.

The Elements We classify atoms by the number of protons

that they have in their nucleus. In nature we find atoms with up to 92

protons, therefore there are 92 different types of atom.

We call these ‘the elements’.

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Atomic Number (symbol = Z)

8

16O

Mass Number (symbol = A)

The number of protons that an atom has in its nucleus is represented by its atomic number which may be written in either of the ways above.

The atomic number is the number of protons an atom has (symbol Z).

The mass number is the number of protons PLUS the number of neutrons (symbol A).

Helium

= Z = 2 + = A = 4

Atomic Number and Mass Number

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The Periodic Table

Isotopes

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Isotopes Sometimes an atom can have more or less

neutrons than another atom of the same element.

This is known as an isotope (iso = same, tope = place)

‘Where do these go?’

‘Same place.’

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Isotopic Symbol To distinguish between isotopes of an

element, physicists often write the mass number as part of the symbol.

Uranium-235

Isotopes How would you write the isotopic symbol for

the following isotopes?

◦ Carbon with seven neutrons◦ Carbon with eight neutrons◦ Deuterium (Hydrogen with one neutron)◦ Tritium (Hydrogen with two neutrons)◦ Uranium 238◦ Uranium 235

What would happen if... An atom suddenly lost two protons and two

neutrons?◦ What would happen to the mass number?◦ What would happen to the atomic number?

One of the neutrons in an atom suddenly turned into a proton?◦ What would happen to the mass number?◦ What would happen to the atomic number?

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8

16O

2, 8, 18, 32, 502n2

(n = shell number)

Electron Shells

Electron Shells

An atom can have a MAXIMUM of:◦ 2 electrons in the first shell◦ 8 in the second◦ 18 in the third◦ 32 in the fourth

HOWEVER, an outer shell is considered FULL if it has EIGHT electrons.

The exceptions to this are hydrogen, helium, lithium, beryllium and boron. Their outer shell is full if it has TWO electrons.

Electron Shells

Ions

F -

Ions

Na

K

S

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2-

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Atoms are most stable when they have a full outer shell.

This means that atoms other than those in group eight need to gain or lose electrons.

Ions

Drawing Shell Diagrams

The period (row) that the element is in gives you the number of shells.

The group (column) that it is in gives you the number of outer shell electrons.

Drawing Shell Diagrams

Outer Shell Electrons Draw electron shell diagrams for these

atoms. How many electrons are in their outer

shells?

H Li Cl AlO B Ca K

What do they need to do to get a full outer shell?

‘Plum Pudding Model’ By Kurzon (Own work) available at http://upload.wikimedia.org/wikipedia/commons/2/26/Plum_pudding_model.svg via Wikimedia Commons

‘J.J. Thompson’ By Not Mentioned (First World War.com) available at http://upload.wikimedia.org/wikipedia/commons/c/c1/J.J_Thomson.jpg, via Wikimedia Commons

‘Thompson Cathode Ray Exp’ By Kurzon (Own work) available at http://upload.wikimedia.org/wikipedia/commons/0/08/Thomson_cathode_ray_exp.gif, via Wikimedia Commons

‘John Dalton by Charles Turner’ by Charles Turner available at http://upload.wikimedia.org/wikipedia/commons/d/d4/John_Dalton_by_Charles_Turner.jpg, via Wikimedia Commons

Image Credits

‘Ernest Rutherford LOC’ By George Grantham Bain Collection (Library of Congress) available at http://upload.wikimedia.org/wikipedia/commons/6/6e/Ernest_Rutherford_LOC.jpg, via Wikimedia Commons

‘Geiger-Marsden Experiment Expectation and Result’ By Kurzon (Own work) available at http://commons.wikimedia.org/wiki/File:Geiger-Marsden_experiment_expectation_and_result.svg, via Wikimedia Commons

‘Atom Diagram’ By Fastfission at en.wikipedia available at http://upload.wikimedia.org/wikipedia/commons/d/d8/Atom_diagram.png, from Wikimedia Commons

‘Niels Bohr’ By The American Institute of Physics credits the photo [1] to AB Lagrelius & Westphal, which is the Swedish company used by the Nobel Foundation for most photos of its book series Les Prix Nobel. (Niels Bohr's Nobel Prize biography, from 1922) available at http://upload.wikimedia.org/wikipedia/commons/6/6d/Niels_Bohr.jpg, via Wikimedia Commons

Image Credits

‘Erwin Schrodinger’ available at http://commons.wikimedia.org/wiki/File:Erwin_Schr%C3%B6dinger.jpg via Wikimedia commons.

‘D3M0’ By Dhatfield (Own work) available at http://upload.wikimedia.org/wikipedia/commons/5/57/D3M0.png, via Wikimedia Commons

‘Periodic Table (polyatomic)’ By DePiep (Own work) available at http://upload.wikimedia.org/wikipedia/commons/9/98/Periodic_table_%28polyatomic%29.svg, via Wikimedia Commons

‘Build an Atom’ by Kelly Lancaster et al available at https://phet.colorado.edu/en/simulation/build-an-atom courtesy of PhET Interactive Simulations at the University of Colorado.

Image Credits

‘Blausen 0342 ElectronEnergyLevels’ By BruceBlaus (Own work) available at http://upload.wikimedia.org/wikipedia/commons/2/2c/Blausen_0342_ElectronEnergyLevels.png, via Wikimedia Commons

‘Blausen 0476 HeliumAtom’ By BruceBlaus (Own work) available at http://upload.wikimedia.org/wikipedia/commons/d/de/Blausen_0476_HeliumAtom.png, via Wikimedia Commons

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