atomic structure and the periodic table

• an atom is the smallest part of an element that can exist on its own
• the radius of an atom is 0.1nm
• the atomic number is the number of protons in an atom (and the number of electrons)
• the mass number is the sum of protons and neutrons in the atom
• the overall charge of an atom is neutral
• atoms have a neutral change because the number of protons and electrons are equal so the charges cancel each other out.
• electrons orbit the nucleus in energy levels
• maximum number of electrons each shell can hold:

  2,8,8,8
• reactants are the starting materials in a chemical reaction
• the products are the new substances produced by a chemical reaction
• an ion is a charged atom that has either lost of gained electrons
• a positively charged ion is called a cation
• a negatively charged ion is called an anion
• an aqueous solution is a solution in water
• a pure substance is a substance that is made up of one element or compound
• a soluble substance can dissolve in a liquid
• an insoluble substance cannot be dissolved in water
• a catalyst is a substance that increases the rate of a chemical reaction without being used up in the reaction
• an unstable atom doesn't have a full outermost shell of electrons
• an element is a pure substance, made up of one type of atom, that cannot be broken down into simpler substances by chemical means
• a molecule is two or more atoms chemically bonded together
• a diatomic molecule is two atoms of the same element covalently bonded together
• examples of diatomic molecules:

  hydrogen, nitrogen, fluorine, oxygen, iodine, chlorine, bromine
• a polyatomic molecule is three or more atoms covalently bonded together
• a compound is two or more different elements chemically bonded together in fixed proportions
• a mixture is a substance that contains two or more substances that are not chemically combined
• isotopes are atoms of the same element with different numbers of neutrons
• Dmitri Mendeleev created the periodic table
• group 1 elements are alkali metals
• as you go down group 1

  the reactivity increases
• why does the reactivity increase as you go down group 1?

  as more shells increase, the atom gets larger. so the outermost electron gets further from the nucleus and the attraction between the nucleus and the outermost electron gets weaker. this means the electron is more easily lost, hence the increase in reactivity.
• why do alkali metals become more reactive as you go down the group?

  as you go down group 1, the atoms radius gets bigger so the outermost electron gets further away from the nucleus. so, as this positive nucleus is the only thing holding the outermost electron in place, the increased distance weakens the attractive force.
• elements in the middle of the periodic table are called transition metals
• group 7 elements are halogens
• as you go down group 7

  the reactivity decreases and melting/boling point increases
• why does the reactivity decrease as you go down group 7?

  as you go down group 7, the number of shells increase, so the outermost electron gets further away from the positive nucleus. so the attractive force (that's needed to pull in an extra electron from another atom) gets weaker. this makes it harder to gain another electron.
• displacement reaction

  when a less reactive metal is displaced by its compound by a more reactive metal
• group 0 elements

  noble gases
• noble gases as you go down the group:

  boiling point increases
• 1. John Daltons theory (1800s)


  John Dalton described atoms as solid spheres and said the different sized spheres make up the different elements.