Daltons’ Models of Atoms Carbon dioxide, CO 2 Water, H 2 O John Dalton proposed all elements were composed of atoms that were characteristic of that element Chemical reactions involved the rearrangement of these atoms

Thompson’s Model of an atom Thompson proposed the plum pudding model Which describes the atom as consisting of these negatively charged particles embedded randomly in positive matter

Rutherford’s Model of an atom Rutherford model proposed that: Most of the mass of an atom and all of the positive charge, must be located a tiny central region called the nucleus Most of the volume of an atom is empty space The electrons move in circular orbits around the nucleus The force of attraction between the positive protons and negative electrons is electrostatic

Chadwick’s Model of an atom James Chadwick discovered the uncharged particle we know as the neutron. He reasoned the nucleus of atoms must contain neutrons as well as protons

Characteristics of Atoms All atoms are neutral Same number of positive and negatively charged particles Therefore same number of protons and electrons

Characteristics of Atoms The number of protons in the nucleus of an atom is the atomic number The mass number of an atom is the total number of protons and neutrons The mass of an electron is so small it is insignificant C 12 6 Mass number Atomic number How many protons? How many neutrons? How many electrons?

What about Sodium Na Atomic number is 11 – therefore there are 11 protons Mass number is 23 – therefore the number of neutrons 23 – 11 = 12 The number of electrons equals the number of protons So there are 11 electrons

Isotopes All atoms of elements have the same number of protons in the nucleus but the number of neutrons can vary Atoms of the same element with different numbers of neutrons are called isotopes Isotopes have the same atomic number but different mass numbers

Isotopes An isotope can be represented by the symbol for the atom together with the atomic and mass numbers C 12 6 Mass number Atomic number C 13 6 C 14 6 or How many neutrons are present now?

Ions When an atom loses or gains an electron I produces a charged particle known as an ion An ion with fewer electrons that protons is a positive ion or cation An ion with more electrons that protons is a negative ion or anion

Ions We know sodium has: 11 protons12 neutrons 11 electrons We represent this as If we remove an electron from sodium we would be left with an ion that had: 11 protons12 neutrons 10 electrons Sodium now has a positive charge This is represented as Na Na +

Homework Questions Q 3, 7, 9, 12, 21, 23

The Periodic Table Figure 3.2 This form of the periodic table is in common use. Colour is used to distinguish the blocks of elements.

What’s are the limitations of Rutherford’s model The laws of physics indicate that electrons moving in circular orbits should continually emit electromagnetic radiation (or light). As radiation is emitted, electrons should lose energy and spiral into the nucleus, causing the atom to cease to exist. When the element is heated only light of certain specific energies, and not light of every energy, is emitted. When the light is passed through a prism onto a screen an emission spectrum is obtained

Emission Spectrum Calcium Sodium

Refining Rutherford’s Model In 1913 a Danish physicist, Niels Bohr suggested that the laws of physics that scientists used to explain the behaviours of large objects such as cars did not apply to the motion of very small objects such as electrons. He proposed that electrons in atoms: Circled the nucleus without losing energy Could move only in certain fixed orbits of particular energies

Arranging electrons around a nucleus Niels Bohr also stated: The orbit in which an electron moved depended on the energy of the electron Electrons with low energy were in orbits close to the nucleus High-energy electrons were in outer orbits

Heating an element can cause an electron to absorb energy and jump to a higher energy state When the energy returns to a lower energy state it releases a fixed amount of energy which can be seen as light The electron can return in a number of different ways. Each possible path produces light of a particular colour in the emission spectrum Arranging electrons around a nucleus

Successive Ionisation Energies If an atom has six electrons, then each can be removed in turn. The electron that is least strongly attracted to the nucleus will be most easily removed. The amount of energy to remove this electron is the first ionisation energy of the element. Each of the remaining five electrons will have a specific ionisation energy higher than the first ionisation energy.

Ionisation energy – what can it tell us? Figure 2.17 (a) Graph of the ionisation energies of a sodium atom. (The logarithm of ionisation energy is used to provide a more convenient vertical scale.) (b) The closer an electron to the nucleus, the greater the amount of energy required to remove it from the atom.

Shells Each shell is a different energy level Electrons in the same shell – are about the same distance from the nucleus –Have about the same energy

Shells Different shells (energy levels) can hold different numbers of electrons The lowest energy shell closest to the nucleus, known as the first shell can hold two electrons The second shell can hold eight The third shell eighteen Shell Number Maximum number of electrons n2n 2 (where n is the shell number)

Electron Configuration The way in which electrons are arranged around the nucleus is known as the electron configuration of an atom. In a stable atom all electrons are as close to the nucleus as possible. Electrons will occupy inner shells before outer shells The electrons in the outermost shell are called valence electrons. It is valence electrons that are involved in chemical reactions

However … After the second shell the electron shells fill in stages. –The outer most shell can never have more than eight electrons regardless of how many the shell can hold. –Therefore the third shell which can contain 18 electrons will not contain more than eight electrons until there are at least two electrons in the fourth shell. Lets look at some examples Questions: pg 27

Quantum Mechanics Figure 2.19 The electron in a hydrogen atom acts like a cloud of negative charge around the nucleus. The density of the cloud at any point gives an indication of the amount of time an electron spends there. German scientist Erwin Schrodinger proposed that electrons behave as waves around the nucleus. There are no definite orbits or shells for electrons in this model. Electrons are thought to move in regions of space surrounding the nucleus called orbitals.

Shells and Subshells Within an atom there are major energy levels which we call shells Within the shells are energy levels of similar energy called subshells; s,p,d and f In atoms containing more than one electron, the energies of the subshells increase in order s < p < d < f

Subshells are made up of orbitals (the regions in which electrons move) The orbitals within a particular subshell are of equal energy Shells and Subshells

Pauli exclusion principle States that: An atomic orbital may hold a maximum of two electrons, i.e. it may hold 0, 1 or 2 electrons. Therefore the maximum number of electrons in an s-subshell, with its one orbital, is two.

The order of the energies of the subshells is 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d … Why is 3d after 4s?? Pauli exclusion principle

Some examples Sodium Iron Question 14 pg 31 Topic Questions