chemistry

• Isotopes are atoms of the same element with the same atomic number but different mass number (different number of neutrons)

  The number of protons is the same, as this is what defines the element and the chemical properties are thus the same. The different number of neutrons in their nuclei result in different physical properties
• $^AX$An isotope is represented like this where X is the element, A is the mass number and Z (under A) is the atomic number
• Mass number is the number of protons + neutrons
• Atomic number is the number of protons
• Neutron number is the mass no. - atomic no.
• Calculating the average mass number of an element: add everything up and divide by the number of added things (eg. Cl: 3 have mass of 35 so 3 x 35 = 105 and 1 has mass of 37 so 1 x 37 = 37 so 105 + 37 = 142. Then 142 divide by 4 = 35,50)
• Neutral atoms and ions

  When atoms lose or gain electrons during chemical reactions, they form ionsAs a result there are no longer equal numbers of electrons and protons in the atom and it's no longer neutral as a whole. The result is positive ions called cations or negative ions called anions
• Cations - positive ions:

  Metal atoms tend to lose electrons. Those metals in group I, II and III lose the same number of electrons as their group number. Transition metals tend to lose 2 electrons per atom, but sometime lose 1, 3 or even 4 electrons. Because metal atoms lose electrons, there are too many protons in the nucleus to balance the remaining electrons, resulting in a positive ion
• Anions - negative ions:

  Non-metal atoms tend to gain electrons. The non in groups VI and VII gain 2 electrons or 1 electron respectively (they gain {8 - Group number} electrons). Because they have too many electrons to balance the protons in the nucleus, they become negative ions
• The nucleus of an atom contains all the protons and neutrons and is responsible for the mass of the atom
• The nucleus is an extremely small, dense structure
• The rest of the atom's volume contains the electrons
• The movement of electrons is responsible for the size (volume) of the atom
• Electrons are arranged around the nucleus in different energy levels (electron shells)
• Energy levels correspond to the orbits or shells proposed by Bohr in his planetary model
• Electrons do not follow set pathways, but the probability of finding an electron within a certain space about the nucleus can be calculated
• Every energy level contains a particular number of spaces in which electrons are likely to be found in certain configurations
• Each energy level is divided into SUB LEVELS. The number of different sub levels in a particular energy level is the same as the number of that energy level. These sub levels are given the following names: s, p, d and f sublevels occurring in that order
• Orbitals are areas where we can find electrons (can only fit 2 electrons into each orbital)
• 1st energy level - 1 sub level - s

  2nd energy level - 2 sub levels - s and p

  3rd energy level - 3 sub levels - s, p and d

  4th energy level - 4 sub levels - s, p, d and f
• To represent energy levels we use energy level diagrams (Aufbau diagrams) which use blocks and go up from 1s -> 2s -> 2p -> 3s -> 3p -> 4s only know up to here (4p -> 5s -> 5p -> 6s -> 6p -> 7s -> 7p -> 8s -> 8p -> 9s -> 9p -> 10s -> 10p -> 11s -> 11p -> 12s -> 12p -> 13s -> 13p -> 14s -> 14p -> 15s ->)

  
• Electron configurations (s, p notation)

  An alternative way to represent energy levels (no diagram). The little numbers/indices must add up to the atomic number (this example adds up to 14 and the element is silicon)

  eg. $1s^22s^22p^63s^23p^2$
• Energy level of Cl = $1s^2 2s^2 2p^6 3s^2 3p^5$
• Another way to do s, p notation/electron configurations is to use noble gasses and they represent their s, p notation and then you add what is left. In this example Helium is the noble gas that comes before Beryllium so you use that and the s, p notation is 'already there' and then you add what is still needed which was 2s2

  eg. Be = $(He) 2s^2$
• Electron configuration in a short way for Si = $(Ne) 3s^2 3p^4$
• Excited states are when the electron is in a higher energy level than the ground state. Ground state are when all the electrons in orbitals are at the lowest possible energy. Excited state is when the electrons 'jump' and it is then not stable. There are lots of different ways for excited states but only one ground state per element