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The Atom
Scientists discovered that materials around us are made up of millions of tiny particles.
They called these particles atoms.
Atom
Word atom comes from the Greek word for “cannot be split up”
e.g. the smallest piece of copper is called an atom of copper.
Atom Definition
An atom is the smallest particle of an element which still retains the properties of that element.
Each element is made up of atoms of that element e.g. the element gold is made up of atoms of gold
Size of an Atom
Atoms are very small
A full stop . Contains about 1016 atoms of carbon
A special microscope called an electron microscope can be used to see atoms
Dalton’s Theory of Atoms
In 1808 John Dalton proposed a theory on atoms after doing many experiments.
He imagined atoms to be like marbles tiny, round and hard only smaller. He though atoms could not be divided into anything simpler.
Particles inside Atoms
In the 1890’s other Scientists began to study atoms. Found that Dalton was incorrect when he said atoms could not be split up into anything simpler.
They found ting particles in side atoms called sub-atomic particles. Sub atomic means “inside atoms”
Chapter 29 – The AtomChapter 29 – The Atom
The atom
An atom is made up of three subatomic particles:
• Protons• Neutrons• Electrons
The Atom
The electrons which is negatively charged was discovered in 1897
The proton was then discovered it was positively charged
The neutron which has no charge (neutral) was discovered in 1932
Subatomic particles• The nucleus of an atom is made up of protons and
neutrons.• The electrons whizz around the nucleus in paths
called orbits or shells.
Chapter 21 – The AtomChapter 21 – The Atom
Differences in sub-atomic particles
Chapter 21 – The AtomChapter 21 – The Atom
Mass
The mass of these particles is very small.
A new unit of mass called the Atomic Mass Unit (a.m.u) is used to measure their mass.
1 proton = 1 a.m.u.
1 neutron = 1 a.m.u.
1 electron = 1/1840 a.m.u.
Charge
An electron has a negative charge (-1)
A proton has a positive charge (+1)
A neutron has no charge (0 charge)
Structure of an Atom
The Planetary Model
Niels Bohr proposed that electrons move around the nucleus in fixed paths called orbits.
Planetary Model
Review
Complete the table:Name of particle
Where situated in atom
Relative Mass
Relative Charge
Proton
Nucleus 0
Outside the nucleus
- 1 unit
Answer
Name of particle
Where situated in atom
Relative Mass
Relative Charge
Proton Nucleus 1 a.m.u. + 1
Neutron Nucleus 1 a.m.u. 0
Electron Outside the nucleus
1/1840units - 1 unit
Quiz
1. ________ was a famous Scientist who put forward an atomic theory in 1808?
2. List two words to describe Daltons theory of an atom? _______ __________
3. The centre of an atom is called the _____
Answers
1. John Dalton
2. Small and hard
3. Nucleus
Electron Cloud
The movement of the electrons around the nucleus gives rise to an electron cloud
Size of the nucleus
The nucleus of an atom could be imagined as a marble at the centre of Croke Park pitch. The electrons would be like specks of dust at various parts of the stands. Everything in between would be just empty space and the electrons are free to move in this space.
Atomic number and mass number
Each element in the periodic table has its own atomic number and mass number.
Chapter 21 – The AtomChapter 21 – The Atom
Atomic number
Atomic number of an atoms is the number of protons in the nucleus of that atom.
Chapter 21 – The AtomChapter 21 – The Atom
Atomic Number
The atomic number of an atom is equal to the number of protons and the number of electrons in an atom.
e.g. atomic number of copper is 29
Number of protons in copper is 29
Number of electrons in copper is 29
Complete the following table
Element Atomic Number
Number of protons
Number of electrons
Hydrogen
Lithium
Oxygen
Sodium
Magnesium
Periodical Table
Mass numberMass number is the number of protons and
neutrons that an atom at that element possesses.
Mass number = number of protons + number of neutrons
Chapter 21 – The AtomChapter 21 – The Atom
Mass number
Find the Mass number of:
For exampleSo an atom of magnesium has Atomic number = 12• 12 protons• 12 electrons
• Mass number = 2424 – 12 = 12
Number of neutrons = 12
Chapter 21 – The AtomChapter 21 – The Atom
Example
Helium:
Atomic number =
Number of protons =
Number of electrons =
Mass number =
Number of neutrons =
Helium
Atomic number = 2
Number of protons = 2
Number of electrons = 2
Mass number = 4
Number of neutrons = 4 - 2 = 2
Example
Chlorine
Atomic number =
Number of protons =
Number of electrons =
Mass number =
Number of neutrons =
Chlorine
Atomic number = 17
Number of protons = 17
Number of electrons = 17
Mass number = 35
Number of neutrons = 35-17 = 18
Complete the table for the first 20 elements p.g. 207 t/b
Element Atomic Number
Number Protons
Number Electrons
Mass No. Number of Neutrons
Hydrogen
Helium
Lithium
Berillium
Boron
Carbon
Nitrogen
Oxygen
flourine
ElementsElement Atomic
NumberNumber Protons
Number Electrons
Mass No. Number of Neutrons
Hydrogen
Helium
Lithium
Berillium
Boron
Carbon
Nitrogen
Oxygen
flourine
Electronic configuration
Electronic configuration refers to the way electrons are arranged in each shell or orbit.
Chapter 21 – The AtomChapter 21 – The Atom
How electrons are arranged
• Electrons whizz around the nucleus in shells or orbits.
• The first shell can hold up 2 electrons and the other shells can hold up to 8 electrons.
• Electrons must fill a shell before starting a new shell.
Magnesium electronic configuration
As magnesium has 12 electrons, they will be arranged:
2 in the first shell8 in the second shell2 in the third shellor, more simply:
(2, 8, 2)
Chapter 21 – The AtomChapter 21 – The Atom
ReactivityElements react with each other to obtain a full outer
shell of electrons.
Chapter 21 – The AtomChapter 21 – The Atom
IsotopesIsotopes are atoms at the same element that have
the same atomic number but different mass numbers.
In other words, the same numbers of protons but different numbers of neutrons.
Chapter 21 – The AtomChapter 21 – The Atom
Chapter 21 – The AtomChapter 21 – The Atom