Midterm Study Guide Project

Significant Figures and Cation/Anion

By Austin Lim

Sheppard Chem 4th period

Significant Figures

To start off Significant Figures are each of the digits of a number that are used to express it to the required degree of accuracy, starting from the first nonzero digit.

Rules of Significant Numbers

1. The numbers 1-9 are always significant.

2. Zero is significant only under the following conditions

  • When the zero is between two significant figures (Ex:102, 3005)
  • Zero is also significant if it follows a decimal point after a significant figure. (Ex: 7.0)
  • When it follows after a decimal point and a significant figure. (Ex: 4.750)
3. However Zero is not a significant number under the following conditions
  • When zero is used as a placeholder preceding a decimal point. (Ex: 0.75)
  • Zero is not significant after a decimal, but before a significant figure. (Ex:0.0077)

If more help is needed here is a link to a video that can help : https://www.khanacademy.org/math/arithmetic/decimals/significant_figures_tutorial/v/significant-figures

Cation and Anions

How do we determine whether an element will become a cation or an anion when it becomes an ion?

First we need to know what they are...

Ion: Formed when a neutral atom gains or loses electron(s).

Cation: Formed when a neutral atom loses an electron(s) and becomes positively charged.

Anion: Formed when a neutral atom gains an electron(s) and becomes negatively charged.

Ions from Metal Elements

Generally metals lose electrons to form cations with a positive charge equal to the group number. This connects with the octet rule because the metals 'revert' to the octet for the previous row in the periodic table.

Ex: Potassium (K) is in group 1 and has one valence electron and forms a K^1+ (plus 1 charged) cation.

Ions from Nonmental Elements

Generally nonmetal atoms gain electrons to form ions with a negative charge. This connects into the octet rule for their row in the periodic table. To get the charge, the charge equals the group number subtracted by 8. (charge = group number - 8)

Ex: Fluorine (F) is in group 17 and has 7 valence electrons . To get the charge you would subtract 8 from 7 which will give you - 1 . So Fluorine forms a F^1- (negative 1 charged) anion.

If more help is needed this video may help:



"Understand the Difference Between a Cation and an Anion." About.com Education. N.p., n.d. Web. 06 Jan. 2016.

"Significant Figures." Khan Academy. N.p., n.d. Web. 06 Jan. 2016.

"Prediction of Ionic Charges." Prediction of Ionic Charges. N.p., n.d. Web. 06 Jan. 2016.