On the periodic table of elements, a group is a column, and a period is a row. The way I remember is that I picture three periods side by side, forming an ellipsis (…). (Plural: ‘ellipses.’) This ellipsis has a horizontal structure, it spans from left to right, in the same way periods on the periodic table are rows, or horizontal blocks of element squares.
A sulfide is a compound containing sulfur and a metal. Group 1 and group 2 elements react in the same way with sulfur as they do with oxygen, forming sulfides (‘S’ being the symbol of sulfur and ‘M’ representing any metal in that group, group 1 elements combine with sulfur to form compounds of the formula M2S; group 2 elements react with sulfur to form compounds of the formula MS) as they do oxides (group 2 elements react with oxygen to create compounds of the formula MO; group 1 elements create compounds of the formula M2O).
What allows us to know these formulas with security? Sulfides (like oxides) are compounds in which the two elements are ionically bonded, meaning that the metal and the sulfur/ oxygen have oppoiste charges. These opposite charges draw them together. Charges are caused by an atom having either more or less electrons than it has protons. When it has the same number of protons and electrons, the positive and negative charges balance out, and the atom is neutral (no charge). When one or more electrons go away, there are more protons than electrons, resulting in a positive charge. Atoms with a charge (whether positive or negative) are called ions, so atoms with more protons than electrons are positive ions, called cations.
But electrons can also be added to a neutral atom, causing it to have more electrons than protons. This will give it a negative charge, making it a negative ion, called an anion.
I remember that cations are positive ions by thinking that cats have paws and associating this with the word pawsitive.
Anyway, all the elements in group 1 (the group farthest to the left, on the periodic table) can form cations with a +1 charge (meaning the number of protons in the atom is one more than the number of electrons in the atom). Since oxygen forms anions with a charge of -2, two atoms of any group 1 element are needed to supply a charge (+2) that is equal to the charge of one oxygen. So what happens is that when bonding, 2 atoms of any group 1 element will be bonded with 1 oxygen atom. Sulfur has a charge of -2 also, which is why two atoms of any group 1 element are needed to balance out the charge of one sulfur atom. The subscripts (the small numbers following elements) in the general formulas I provided represent the number of atoms of the element whose symbol they follow are present. For example, in M2O, there is 1 oxygen atom (the subscript ‘1’ is always there when is no apparent subscript) and 2 M atoms. (Remember that the ‘M’ represents any group 1 metal.)
The group 2 elements, on the other hand, form ions with a +2 charge, which is why only one atom of a given element in that group is needed to balance out the charge of either the sulfur or oxygen.
Units are arbitrary. Seconds are units of time—they split time into measurable pieces that we can count to keep track of how much time has passed. Seconds were created by us, like a foot was created, like a kilogram was created. People decided that a certain amount of stuff (like the amount of time in what we now know as a second) equaled a new unit (the second), a unit which would be used for practical purposes (in this example, keeping time).
The amount of time which would make up a second was arbitrarily chosen. Sorry, but I don’t know how to explain it any better.
As Hank tells us, there are only 7 units (in the International System of Units), and from these units, all others are derived. I’d feel a little guilty about “stealing” all this information, but I’m not stealing—I’m giving credit and I already told you the point in doing this, which is for me to gather information in a certain place, that certain place being this blog/ website. Going over information I receive from elsewhere also helps solidify the information in my memory. These posts are just collections of information. I literally summarize what I have learned.
Speed = length/ time. Since the standard unit for length is the meter (m) and the standard unit for time is the second (s), speed = m/ s.
Acceleration = speed/ time. Since speed is m/s, and this is being divided by time, acceleration = m/s/s, which is the same as m/ s2.
Newtons (N) are units of force. Force = ma, or mass x acceleration. Mass’s standard unit is the kilogram (kg), and we already know acceleration is m/ s2, so force = kg x m/ s2.
Joules (J) are units of work. Work = force x distance. You know force, and distance’s standard unit is meters, as you know. So work = m x kg x m/ s2, which can be written in a more simple way (i.e. it can be simplified) as kg x m2/ s2.
Last but not least, Hank says power = work / time. Work is kg x m2/ s2 and we can divide that by s, the second being the standard unit of time. Watts are units of power. Power = kg x m2/ s2 / s, which can be simplified to kg x m2/ s3.
Then Hank talks about the difference between measured numbers and exact numbers. Exact numbers are numbers that we know all the decimal place values of. Decimal places are places to the right of the decimal point. A couple of books is 2 books: 2.000000000… where the zeros stretch to infinity because there aren’t between 2 and 3 books, there is no fraction of a book in addition to the 2 whole books; there are 2 whole books and that is all. By contrast, it isn’t possible to know all the decimal place values of measured numbers, and it’d be impractical to know them all. People stop measuring at a certain point and round, and if they stop measuring at 2 decimal places, then that’s how precise the measurement is; if that measurement were measured to 3 decimal places it would be more precise/ accurate—makes sense.
When doing math calculations, we want to keep significant figures in mind. If adding/ subtracting two numbers, the result should have the same number of decimal places as the starting number with the fewest decimal places. The starting numbers are the numbers that interacted (either by addition or subtraction) to produce the result. In chemical reactions, the initial stuff are known as the reactants; the ending stuff are known as the products.
If multiplying/ dividing two numbers, the result should have the same number of significant figures that the initial number with the least amount of sig figs had. Figuring out how many sig figs a number has is another story. Hank didn’t tell us this, but I remember from that chemistry book I’ve been reading (and other sources) that for one thing, digits which are not zero are significant. “Leading zeros”—zeros which come before all non-zero digits—are not significant. “Trailing zeros” in a number without a decimal point at least somewhere are not significant, either; however, zeros that are at the end of a number which contains a decimal point (doesn’t matter where the decimal point is) are significant. Zeros sandwiched between non-zero digits are significant.
Last is scientific notation. Scientific notation is a way of expressing a number that makes it very clear which digits are significant in a number. Numbers expressed in scientific notation are in the form: A x 10B, where A is a number with only one digit to the left of the decimal point. There is no restriction that I know of on the number of non-zero digits to the right of the decimal point.
‘B’ can be an either positive or negative number and tells us how many times we are multiplying A by 10. A short way to find the “expanded” form of a number being represented in scientific notation is to move the decimal place to the right or the left (depending on B’s sign) the number of spaces indicated by the magnitude of B. A magnitude is the absolute value. (The magnitude of -7 is 7; the magnitude of 236 is 236.)
Say your number is 0.026,792. You want to express this in scientific notation. First, which numbers are significant? The 2, 6, 7, 9, and 2 (the zeros are leading zeros, thus not significant). These sig figs form A.
A = 2.6792
Only the 2 is on the left of the decimal point, because only one digit can go there. The non-significant digits are not a part of A. But A is not all there is—remember the “x 10B” part.
‘B’ indicates how many spaces and in what direction the decimal point in the number A should move to arrive at the number in “regular form.” Comparing A to the “regular form” of the number (0.026,792), I see that A’s decimal point needs to move 2 places left.
Because the decimal point must move left, B must be negative; because the decimal point must move 2 places, B’s magnitude must be 2.
B = -2
So 0.026,792 expressed in scientific notation is 2.6792 x 10-2.
Have a great day. J