2.1 The Atom

Assessment Objectives State the position of protons, neutrons and electrons in the atom State the relative masses and relative charges of protons, neutrons and electrons Define the terms mass number (A), atomic number (Z) and isotopes of an element Deduce the symbol for an isotope given its mass number and atomic number Calculate the number of protons, neutrons and electrons in atoms and ions from the mass number, atomic number and charge Compare the properties of the isotopes of an element Discuss the uses of radioisotopes

Reference Textbook on Moodle (Topic 2 Atomic Structure) Powerpoint presentation on Moodle Workbook

Key terms Atom Nucleus Proton Neutron Electron Atomic Mass (Mass Number) Atomic Number Isotope Ion

The Periodic Table Contains all the ELEMENTS that we believe exist in the universe. All the matter in the universe is made up of combinations of these ELEMENTS They are different types of atoms. So what is an atom???????

The Atom

Assessment Objective State the position of protons, neutrons and electrons in the atom.

Atomic Structure An atom is made of 3 types of particle. PROTONS (negatively Charged) NEUTRONS (no charge) ELECTRONS In the nucleus of the atom (Moving around outside the nucleus. Negatively charged)

Extension Why do we believe atoms exist?? Historical Perspective

Philosophy 400 B.C. - Democritus thought matter could not be divided indefinitely. This led to the idea of atoms in a void 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle was wrong. However, his theory persisted for 2000 years. Democritus Aristotle

Experimentation John Dalton proposed a modern atomic model based on experimentation not on pure reason. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions Dalton’s “Billiard ball” model ( ) Atoms are solid and indivisible.

Thompson 1897 – Thompson discovered the electron (a small negatively charges particle) He said that atoms were a positive sphere of matter – the PUDDING. Electrons were embedded in this matter – the PLUMS

Ernest Rutherford Ernest fired radiation into a thin strip of gold foil and analysed where the particles ended up. Video

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

The structure of the atom ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”)

Niels Bohr Danish physicist Niels Bohr realised that the secret of atomic structure lay in its discreteness, that energy could only be absorbed or emitted at certain values. Bohr realised that the electrons could only be at specific energy levels (or states) around the atom. The Bohr Model

Quantum Mechanical Model Dense NUCLEUS (composed of PROTONS and NEUTRONS) surrounded by a cloud of ELECTRONS.

Assessment Objective State the relative masses and relative charges of protons, neutrons and electrons.

Sub-Atomic Particles The 3 sub-atomic particles all have different properties. You need to learn these properties. Name of Particle MassChargePosition Proton Neutron Electron 1 1/1840 (Negligible) outside nucleus 1 Positive (+) nucleus Neutral (0) Negative (-) nucleus

How do we know the charge on an electron? Milikan’s Oil Drop Experiment Video

Assessment Objective Define the terms mass number (A), atomic number (Z) and isotopes of an element Compare the properties of the isotopes of an element.

The structure of the atom ParticleRelative MassRelative Charge Proton11 Neutron10 Electron0 MASS NUMBER (A) = number of protons + number of neutrons SYMBOL ATOMIC NUMBER (Z) = number of protons For a NEUTRAL ATOM, the number of electrons = the number of protons

Atoms and Elements Elements are different types of atoms. Elements differ from each other by the number of protons. Hydrogen has one proton while helium has two

Exercise Work out the number of protons, neutrons and electrons: 23 Na Fe Sn F 9 27 Al 13

Isotopes Even though the atomic number (protons) defines the element, the number of neutrons can change. So the same elements can have different number of neutrons. These are called ISOTOPES

Isotope Properties Isotopes are virtually identical in their chemical reactions. (There may be slight differences in speeds of reaction). This is because they have the same number of protons and the same number of electrons. Because they have different numbers of neutrons, their masses are different. The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density.

Isotopes An isotope is an atom with a different number of neutrons: Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. Notice that the mass number is different. How many neutrons does each isotope have? A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

Isotopes: Carbon Natural samples of elements are often a mixture of isotopes. About 1% of natural carbon is carbon-13. Protons Electrons Neutrons C % C %

Isotopes: Hydrogen Hydrogen exists as 3 isotopes although Hydrogen-1 makes up the vast majority of the naturally occurring element. H 1 1 H 2 1 H 3 1 Protons Electrons Neutrons Hydrogen Protons Electrons Neutrons (Deuterium) Protons Electrons Neutrons (Tritium)

Isotopes: Chlorine About 75% of natural chlorine is 35 Cl the rest is 37 Cl. Cl % 17 Protons Electrons Neutrons Protons Electrons Neutrons Cl %

Assessment Objective Calculate the number of protons, neutrons and electrons in atoms and ions from the mass number, atomic number and charge.

Ions We can also add or take away the number of electrons in an atom Again it does not change the type of atom (element) BUT it does change the CHARGE of the atom We call these IONS

Exercise Use your data booklet and find these elements. Hydrogen Lithium Sodium Calculate the number of protons neutron and electron. If we take away one electron what will be the total charge of the atom.

Exercise Calculate the number of protons neutron and electron in the following elements, isotopes and ions. ElementProtonsNeutronsElectrons 6 12 C 6 14 C Cl - O 2- B 3- Ca

Assessment Objective Deduce the symbol for an isotope given its mass number and atomic number.

Exercise Use your periodic table to determine these elements given the following information. ElementProtonsNeutronsElectrons

Mass Number Calculate the mass number of the atoms shown. AtomProtonsNeutronsMass Number Helium22 Copper2935 Cobalt2732 Iodine5374 Germanium MASS NUMBER = Number of protons + number of neutrons Activity

How Many Neutrons Calculate the number of neutrons in these atoms. AtomMass Number Atomic Number Number of Neutrons Helium42 Fluorine199 Strontium8838 Zirconium9140 Uranium23892 Number of Neutrons = mass number - atomic number Activity

How Many Electrons The atoms of any element have an equal number of protons and electrons. AtomProtonsNeutronsElectrons Helium222 Copper Cobalt Iodine Germanium and so overall charge equals zero

Assessment Objective Discuss the uses of radioisotopes

Using Isotopes

Homework Learn the symbols for the first 20 elements in the periodic table. Workbook Questions

Plenary How many types of sub-atomic particle are there? Name them Which particle has a negative charge? Which particle weighs almost nothing? The atomic number tells us the number of What is the charge on an atom? Explain electron 3 proton, neutron, electron protons electron Same number of protons (+) and electrons (-) neutral

Further Reading or Viewing On Moodle Millikan Oil Drop Experiment Proton Discovery Rutherford's Experiment Nuclear Atom Isotopes