Chemistry Unit – Part 2

Bohr Models Bohr models are used to predict reactivity in elements. Reactivity refers to how likely an element is to form a compound with another element. When looking at Bohr models, we look at its valence electrons (the electrons on the last energy level) to determine reactivity.

Bohr Models There are two parts to a Bohr model. The nucleus and the orbits which contain electrons Nucleus A nucleus is made of protons and neutrons which must be indicated on a Bohr model If an atom were the size of a football stadium, the nucleus would be the size of a marble.

Bohr Models Orbits Electrons travel around the nucleus in specific paths called shells or orbits.

Drawing Bohr Models Draw the nucleus. Write the number of neutrons and the number of protons in the nucleus. (p+ , n0) Draw the first energy level. (First orbit for e- ) Draw the electrons in the energy levels according to the rules below. Make sure you draw the electrons spaced out evenly (possible locations are top, bottom, or side) - If more than 4 electrons, draw in pairs.

Rules for Energy Levels Level 1 (closest to the nucleus) can hold a maximum of 2e. Level 2 can hold a max of 8e. Level 3 can hold a max of 8e. You must fill one level before going on to draw the next level!

Guided Practice In order to draw Bohr models of these elements, you must first determine the number of protons, neutrons, and electrons. 6 C Carbon 12.011

Guided Practice 16 S Sulfur 32.066

Guided Practice 3 Li Lithium 6.941

Guided Practice 10 Ne Neon 20.180

Complete Bohr Periodic Table

Reactivity and Stability Atoms are stable or not depending on their electron structure Noble gases are unreactive Alkaline Earth Metals and Chalcogens are moderately reactive Alkali metals and halogens are very reactive WHY!

Chemical reactivity is related to the number of valence electrons that need to be lost or gained so that the atom has a completely filled outer electron shell. The fewer the number of electrons needed, the greater the reactivity of the element. Lets look at an example:

Draw Bohr models for Sodium Beryllium Carbon Helium

Noble gas – has a full valence shell Noble gas – has a full valence shell. It does not require any further electrons. It is STABLE Alkaline Earth Metals – need to lose 2 electrons Chalcogens – need to gain 2 electrons These are both moderately reactive Once they react, they will have a similar structure to their nearest noble gas

Beryllium after losing 2 electrons Helium

Alkali metals – need to lose 1 electron Halogens – need to gain 1 electron These are both VERY reactive, because they only need to gain/lose ONE more electron to become stable and have a similar structure to their nearest noble gas.

Sodium Fluorine Neon Missing 1 electron gaining 1 electron

So what happens during a chemical reaction? During a chemical reaction, one element combines with another to form a new substance. There must be some movement of electrons so that both atoms can become stable (complete their valence shell)

Who will do what? You have one atom of the elements below. Match up the atoms which you think will react together. Draw a line to connect them. Think about which will lose electrons, or gain. As well as how many! Li Cl Ar Xe Mg O

Chemical reactions There are three activities that can provide an atom with a completely filled valence electron shell. 1. The atom can lose/give up one or more electrons Alkali metals only have 1 electron, and alkaline earth metals have 2. It is simpler for them to give away their extra electrons than find enough to fill their outer shells

2. The atom can gain more electrons Halogens and chalcogens need to gain only 1 or 2 more electrons. It is simpler for them to find the remaining electrons than to lose all of their valence electrons 3. The atom can share electrons with another

Complete the Table In the table indicate if the elements listed are : Reactivity - high, moderate, zero Electrons gained or lost - gained, lost # of electrons - number (0, 1, 2) Family - name the family Answer questions 2 and 3

Elements, Atoms and Molecules Elements and compounds are both PURE SUBSTANCES Some substances are made of one element. This smallest particle of an element is called an atom. It is impossible to break down gold into anything simpler than gold. Some elements (such as oxygen and chlorine) will be made of two atoms bonding together to form a molecule (O2 is the formula for oxygen gas, so it is 2 atoms combined). These elements are called diatomic elements.

***Elements can be formed from atoms OR molecules Most substances, however, are compounds made by combining two or more elements. Water, steel, rubber, nylon, and sugar are all compounds, made from elements that are joined together in precise ways.

Compounds and molecules These are NOT elements – they have different atoms together. A compound has totally different characteristics from the elements that formed the compound.

Compounds and Molecules A compound is the simplest form of a material that contains two or more elements in a fixed proportion (it is made up of different elements). A molecule is the smallest particle of a compound (it is made of atoms). Compounds are pure substances, because all their molecules are the same

Mixtures These are NOT pure substances. They are a combination of different types of molecules and atoms They can be separated Filtration or distillation for example **Compounds CAN also be separated, but it is more difficult and involves electrolysis – adding energy to break apart the molecules

For each, state whether you have : Pure Substance or mixture Element Compound or Neither Atoms or Molecules or Both

1.

2. He He

Complete elements, molecules practice sheets

Chemical Formulas Chemical formulas tell you: The elements that make up a compound The number of each element in the compound For example: H2O. H = symbol for hydrogen. The number following H indicates the number of hydrogen atoms present. O = symbol for oxygen. The number following O indicates the number of oxygen atoms present. When no value is shown, it is treated as 1.

Compound Structure Elements Number of Atoms Total CO2 CuSO4 Carbon 1 3 Oxygen 2 CuSO4 Copper 6 Sulfur 4

Compound Elements Number of Atoms Total Na2CO3 NH4C2H3O2 Sodium 2 6 Carbon 1 oxygen 3 NH4C2H3O2 Nitrogen 12 Hydrogen 7

Placing a number in front of a molecule tells you HOW MANY of that molecule you have! Compound Elements Number of Atoms Total 2 CaCl2 Calcium 2 6 Chlorine 4 3 CH3 Carbon 3 12 Hydrogen 9