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 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 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 Atom

Differences in sub-atomic particles Chapter 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 NeutronNucleus 1 a.m.u. 0 ElectronOutside 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 Atom

Atomic number Atomic number of an atoms is the number of protons in the nucleus of that atom. Chapter 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 ElementAtomic Number Number of protons Number of electrons Hydrogen Lithium Oxygen Sodium Magnesium

Periodical Table

Mass number Mass 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 Atom

Mass number

Find the Mass number of:

For example So an atom of magnesium has Atomic number = protons 12 electrons Mass number = – 12 = 12 Number of neutrons = 12 Chapter 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 = = 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 = = 18

Complete the table for the first 20 elements p.g. 207 t/b ElementAtomic Number Number Protons Number Electrons Mass No.Number of Neutrons Hydrogen Helium Lithium Berillium Boron Carbon Nitrogen Oxygen flourine

Elements ElementAtomic Number Number 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 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 shell  8 in the second shell  2 in the third shell or, more simply: (2, 8, 2) Chapter 21 – The Atom

Reactivity Elements react with each other to obtain a full outer shell of electrons. Chapter 21 – The Atom

Isotopes Isotopes 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 Atom