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AS Chemistry. Revising Atoms. Learning Objectives Candidates should be able to: Identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses. Deduce the behaviour of beams of protons, neutrons and electrons in electric fields.
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AS Chemistry Revising Atoms
Learning Objectives Candidates should be able to: • Identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses. • Deduce the behaviour of beams of protons, neutrons and electrons in electric fields. • Describe the distribution of mass and charges within an atom. • Deduce the number of protons, neutrons and electrons present in both atoms and ions given proton and nucleon numbers (and charge). • Distinguish between isotopes on the basis of different numbers of neutrons present.
History of the Atom Democritus: Ancient Greek Philosopher-Scientist, ‘a tomos’ – cannot be cut. The problem: he was unable to provide the evidence needed to convince people that atoms really existed.
History of the Atom • In 1808, an English school teacher named John Dalton proposed that atoms could not be divided and that all atoms of a given element were exactly alike. Dalton’s theory is considered the foundation for the modern atomic theory. Dalton’s theory was developed with scientific basis and was accepted by others.
History of the Atom • At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. • Thomson suggested that they could only have come from inside atoms. So Dalton's idea of the indestructible atom had to be revised. • Thomson imagined the electrons as the bits of plum in a plum pudding
History of the Atom In 1872-1937, Rutherfordet al. ran experiments to determine the structure of an atom. When positively charged particles are fired into gold foil, most pass straight through while a few are violently deflected. This implies a dense, positively charged central region containing most of the atomic mass and that the atom is mostly space.
The Gold Foil Experiment • Rutherford expected the alpha particles to go straight through the gold foil. • Instead, some of the alpha particles were deflected, implying a central positively charged region (nucleus).
History of the Atom • In 1913, the Danish scientist Niels Bohr suggested that electrons in an atom move in set paths (energy levels) around the nucleus much like the planets orbit the sun. • Electrons can only be in certain energy levels and must gain energy to move to a higher energy level or lose energy to move to a lower energy level.
History of the Atom • In the 1920’s deBroglie & Shrodinger showed that the “solar system” model of the atom was incorrect. Instead, electrons orbit the nucleus in orbitals. • This is called quantum mechanics. We will look at this in our next lesson.
Even Smaller Particles! • For some time people thought atoms were the smallest particles and that they could not be broken into anything smaller. • We now know that atoms are themselves made from even smaller and simpler particles. • These particles are • Protons • Neutrons • Electrons
Evidence for Sub-atomic particles • J.J. Thompson – discovered presence of electrons and proposed ‘Plum Pudding’ model of the atom. • Rutherford’s ‘Gold foil’ experiment concluded that an atom's mass must be concentrated in a small positively charged nucleus and that most of the atom must be empty space. This space must contain the electrons.
Properties of Sub-atomic Particles • There are two properties of sub-atomic particles that are especially important: • Mass • Electrical charge Element atoms contain equal numbers of protons and electrons and so have no overall charge
Properties of Sub-atomic Particles proton electron neutron
The Nucleus a dense core of protons and neutrons containing nearly all the mass of the atom ‘Shells’ of electrons electrons are really very very tiny so the atom is mostly empty space. How Are the Particles Arranged? • Protons, neutrons and electrons are NOT evenly distributed in atoms. • The protons and neutrons exist in a dense core called the nucleus. • Around the outside are very thinly spread electrons. • These electrons exist in layers called shells.
Proton or Atomic Number • The atom of any particular element always contains the same number of protons. E.g. • Hydrogen atoms always contain 1 proton • Carbon atoms always contain 6 protons • Magnesium atoms always contain 12 protons • The number of protons in an atom is known as its atomic or proton number. • It is the smaller of two numbers shown in most periodic tables 12 C 6
How Many Protons? • Note that any element has a definite and fixed number of protons. • If we change the number of protons in an atom then this changes that atom into a different element. • Changes in the number of particles in the nucleus (protons or neutrons) is very rare. It only takes place in nuclear processes such as radioactive decay, nuclear bombs or nuclear reactors.
Mass or Nucleon Number • The mass of each atom results almost entirely from the number of protons and neutrons that are present. (Remember that electrons have a relatively tiny mass). • The sum of the number of protons and neutrons in an atom is the mass number.
2nd Shell 3rd Shell 4th Shell How Are Electrons Arranged? • Electrons are not evenly spread. • The exist in energy levels known as shells. • The arrangement of electrons in these shells is often called the electron configuration. 1st Shell
1st Shell: 2 electrons 2nd Shell: 8 electrons 3rd Shell: Initially 8 electrons How Many Electrons per Shell? • Each shell has a maximum number of electrons that it can hold. The maximum
1st Shell: Fills this first 2nd Shell: Fill this next 3rd Shell: And so on Which Shells do Electrons go into? • Opposites attract. • Protons are + and electrons are – charged. • Electrons will occupy the shells nearest the nucleus unless these shells are already full.
1st Shell: Fills this first 2nd Shell: Fill this next Working Out Electron Arrangements • How many electrons do the element atoms have? (This will equal the atomic number). • Keeping track of the total used, feed them into the shells working outwards until you have used them all up. Drawing neat diagrams helps you keep track!
How Many Neutrons 1 • It is not strictly true to say that elements consist of one type of atom. • Whilst atoms of a given element always have the same number of protons, they may have different numbers of neutrons. • Atoms that differ in this way are called isotopes. Remember: The number of protons defines the element
How Many Neutrons 2 • 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. • The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density.
13 12 C C 99% 1% 6 6 Isotopes: Carbon • Natural samples of elements are often a mixture of isotopes. About 1% of natural carbon is carbon-13. Protons Electrons Neutrons 6 6 6 6 6 7
1 2 H H 3 H 1 1 1 Protons Electrons Neutrons Protons Electrons Neutrons Protons Electrons Neutrons Hydrogen (Deuterium) (Tritium) Isotopes: Hydrogen Hydrogen exists as 3 isotopes although Hydrogen-1 makes up the vast majority of the naturally occurring element.
Cl 37 Cl 35 17 17 75% 25% 17 Protons Electrons Neutrons Protons Electrons Neutrons 17 17 17 18 20 Isotopes: Chlorine About 75% of natural chlorine is 35Cl the rest is 37Cl.
AS Chemistry Atomic Orbitals
Learning Objectives Candidates should be able to: • Describe the number and relative energies of the s, p, and d orbitals for the principal quantum numbers 1, 2 and 3 and also the 4s and 4p orbitals. • Describe the shapes of s and p orbitals. • State the electronic configuration of atoms given the proton number.
Location of Electrons – The Problem with Bohr’s Model An electron’s exact location cannot be determined. Imagine the moving blades of a fan – If you were asked where any one of the blades was located at a certain instant, you would not be able to give an exact answer – the blades are moving too quickly! It is the same with electrons –the best a scientist can do is calculate the chance of finding an electron in a certain place within an atom
Energy levels and sub-levels Energy levels These are broadly similar to the “shells” used in GCSE Chemistry You need to know about energy levels 1, 2, 3 and 4 at A-level Energy level 1 is lowest in energy and closest to the nucleus
Energy levels and sub-levels Sub-levels The main energy levels contain sub-levels The different main energy levels have different sub-levels in them There are four types: s, p, d, f
