There are two types of notation for atomic number and mass number. Despite the method of notation, the larger number is always the mass number & the smaller number is always the atomic number.
Why is the sum of protons and neutrons called the mass number? Because electrons have so little
mass, they can be disregarded when considering the mass of an atom.
Neutrons are the most massive subatomic particle.
Protons are almost as massive as neutrons.
Electrons are the least massive.
When atoms share electrons chemical bonds are formed. Chemical bonds can be either:
1. Ionic
2. Covalent
But if the nucleus of an atom is altered, it is a nuclear reaction.
Chemical reaction = change to the electron situation. Nuclear reaction = change to the nucleus situation. One example of a nuclear reaction is nuclear fission. Nuclear fission occurs when a single
nucleus breaks apart into two smaller nuclei.
Another type of nuclear reaction is nuclear fusion. Nuclear fusion results from combining two nuclei
into one, larger nucleus.

The polonium (Po) atom above has decayed by nuclear fission. It has split into lead (Pb) and helium
(He). How many neutrons are in the polonium atom? (218 - 84 = 134)
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Atoms ‘start’ with the same number of protons & electrons. Equal numbers of (+) charge & (-) charge means an atom has a net charge of zero. An atom in which the electrons and protons are not equal is an ion, and it has a total net (+) or (-) charge.
A chemical bond exists when two or more atoms share electrons and form a molecule.
There are two types of chemical bonds:
1. Covalent Bond – electrons shared equally.
2. Ionic Bond – electrons shared unequally.
What does it mean to ‘share’ an electron? It means that an electron will spend some time around one atomic nucleus, and then some amount of time around the other atomic nucleus.
In watery solution, sodium chloride shows that it is made of sodium ions (Na+) and chloride ions (Cl–).
Which statement explains the formation of these ions? Answer: The sodium atom loses an electron and the chlorine atom gains an electron.
The most common ionic form of copper is Cu+2. How do copper atoms get a +2 charge when they become ions? Answer: They lose 2 electrons.
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Without having to do too much thinking we can simply refer to the Periodic table to see how many electrons are in the valence. All elements in column I have 1 electron in their outer-most shell. All elements in column II have 2 electrons in their outer-most shell. Elements in column VI have 6 electrons in their valence… etc.

The Noble Gases all have a full valence, meaning they have 8 electrons in the ‘valence’. Eight is the most any atom can have. In nature, atoms will attempt to mimic the Noble Gases. They want to have eight electrons in their outer shell, or valence.
For example: Sodium (Na) wants to be like the noble gas neon (Ne). Chlorine (Cl) wants to be like the noble gas argon (Ar). By gaining or losing an electron, the sodium ion looks like neon and the chloride ion looks like argon.
When making Bohr models here are your rules:
The first energy shell has 2 electrons maximum. Each subsequent shell has a maximum of 8.
The row number designates how many energy shells.
The column number designates how many electrons are in the outer shell.
Here is a Bohr model with all 3 energy shells full of electrons. What element is it?

Only the last statement is true.
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Remember that: Elements are on the periodic table. Compounds are combinations of two or more elements. Mixtures are combinations of two or more compounds.
Is salt (NaCl) an element, compound or mixture? (Compound)
Is sodium an element, compound or mixture? (Element)
Speaking of salt, in which of these scenarios do you think salt would dissolve fastest in water?
Cool water, not stirred
Cool water, stirred
Hot water, not stirred
Hot water, stirred
(Heating while stirring would dissolve salt the fastest.)
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Ball & Stick models are a common way to represent molecules/compounds.
Notice the double-bond? Notice the single-bonds?

When zone 4 reacts or shares electrons with zone 1, then ionic bonds are formed. If any zone reacts with itself, then covalent bonds are formed.
Naming compounds with covalent bonds is easy! All you have to know is some Greek!
Mono means "one"
Di means "two"
Tri means "three"
Tetra means "four"
Penta means "five"
Then you simply say the prefix before the name of the element in the compound…
H
2
S is dihydrogen sulfide or (dihydrogen monosulfide)
CO is carbon monoxide
HI is hydrogen monoiodide
N
2
O
3 is dinitrogen trioxide
NF
5
is nitrogen pentafluoride
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“Eureka!” said Archimedes. What is the volume of the bolt?
We can tell the volume of the bolt by how much the water level in the cylinder changes. The volume is 12 mL because 42 mL – 30 mL is 12 mL.
When reading temperature you are allowed to guess at the last number. We can see that the temperature is 12 °C and we can see that there are 2 more decimal places beyond the 12.Therefore we know that the temperature is at least
12.2 °C. But if we look closely we can see that the temperature goes beyond 12.2 °C and we can guess at that number… it looks like it’s about 12.25 °C to me.

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For unit conversion, start by figuring out what your starting unit is, then figure out what your ending unit is. For example: If I want you to convert kilometers to yards, then I will give you this:
In the following instance you’re converting kilometers, so start with kilometers.
You want to know yards, so finish with yards.
Notice the pattern? Kilometers were on top, and then it was on the bottom. Meters were on the top, and then it was on the bottom. When the answer you’re looking for (yards) is finally on top, you can stop.
I will never give you more information than you need, nor will I give you less information than you need.
So make sure to use all of the information somewhere in the calculation.