Understanding HESI A2 Chemistry Topics

Elements and atoms and moles, oh my! Explaining HESI A2 Chemistry Topics

Need chemistry lessons? Don’t trust atoms. They make up everything!

If the joke above didn’t make sense to you, don’t worry. You’re in the right place. The rules of chemistry are vast and intense, so taking the leap into the world of chemistry can be intimidating. This article will provide organization and structure to all of these rules.

Let’s begin with the atom, and we’ll start here because the atom is considered the building block of chemistry.

The atom is the smallest unit of matter and has three main subatomic particles: the proton, the neutron, and the electron.
Protons and neutrons are found in the nucleus, which is in the center of the atom. Protons are positively charged and neutrons are neutrally charged.
Electrons are negatively charged and travel around the nucleus in distributions called orbitals.

Now…how do atoms relate to elements?

Elements are substances composed of the same type of atom. The periodic table is the organization system of elements.

Each element has its own atomic number which is equal to the number of protons in the atom.

In a neutral atom, the number of electrons will be equal to the number of protons. However, if the atom becomes an ion, the number of electrons will vary.

The number of neutrons in an atom can also vary; in this case, the element would be classified as an isotope.

An important piece of information that can be found on the periodic table is the atomic mass of an element. The mass number is equal to the number of protons and number of neutrons.

The atomic weight is listed under each element and is an average of the atomic masses of all isotopes of each element.

The periodic table is organized into four main groups: main group elements, transition metals, Lanthanides, and Actinides.

It’s important to know the differences between atomic weight, atomic mass, molecular weight, and molar mass. Let’s “weigh in.”

The sum of the weight of all the atoms in a given formula equals the molecular weight of that particular substance. To calculate the molecular weight:

Determine the number of atoms of each element

Multiply the atomic weight of each element by the respective number of atoms of each element

Add them

Using H20 as an example, there are two atoms of Hydrogen and 1 atom of Oxygen. The calculation would be (2 x 1.00794) + (1 x 15.999) = 18.01528 amu

So, you’ve heard all about stoichiometry and how difficult molar mass calculations can be, but it’s time to put a stop to those rumors. Let’s break it down.

The periodic table also provides information to solve for a compounds molar mass – the quantity of grams present in a mole of a substance.

The mole is a unit of measure. In baking, we say a dozen. In chemistry, we say a mole. Both simply describe a specific number of a quantity. For baking, a dozen is comprised of 12. For chemistry, a mole is comprised of 6.02 x 1023.

The molar mass is the standard unit in chemistry. It tells us how many grams of a substance are in 1 mole of the substance. To calculate molar mass:

Determine the number of atoms of each element

Multiply the number of atoms by the atomic weight

Add them

Using 02 as an example, there are 2 atoms of Oxygen. Therefore, the calculation is 2 x 16 = 32g/mol. This tells us there are 32 grams in one mole of oxygen.

Ready to step it up? Good! Because we are moving from simple building blocks to more complex compounds. Let’s see what happens when atoms are attracted to one another…

When atoms join together through chemical bonds, they create new substances with properties different than the starting elements. When bonds are formed, electrons are shared, gained, or lost. The electrons responsible for creating chemical bonds are the outermost electrons of an element – the valence electrons. Three types of chemical bonds are:

1. Hydrogen bonds – these are relatively weak bonds. They form when a hydrogen (which has a partial positive charge) interacts with a more electronegative atom (such as oxygen, nitrogen, or fluorine).

2. Ionic bonds – these are stronger bonds. They are formed when a metal & a non-metal interact and transfer electrons. Generally, one atom loses electrons (which results in a positive charge) and the other atom gains an electron (which results in a negative charge). These two charges typically cancel out and form a neutral compound.

3. Covalent bonds – these bonds are common in organic (carbon-containing) molecules. Covalent bonds form when atoms share electrons. Two nonmetals can bind through covalent bonding.

Like every relationship, attraction leads to reaction! Now that we’ve explored what happens when atoms are attracted, we can study what happens when chemicals become reactive.

Chemical reactions are vast and complex. Here we will present 7 categories of reactions:

1. Synthesis: Two or more substances combine to form 1 new compound. A+B → AB

2. Decomposition: A single compound is broken down into two or more substances. AB → A+B

3. Single replacement: One element takes the place of another element in a compound. AB+C → AC+B

4. Double replacement: Two elements take the place of another element in a compound. AB+CD → AC+BD

5. Neutralization: A special type of double replacement involving an acid and a base

6. Combustion: A reaction of a fuel with oxygen that produces energy (heat or light).

7. Redox: An oxidation-reduction wherein transfer of electrons must occur.


Congratulations! You made it through a very thorough and intense introduction to HESI A2 Chemistry topics. If you enjoyed and understood this article, you are ready for the next step – the study guide! For free HESI A2 chemistry practice questions check out www.ReadyForTheHESI.org for a free test!

Share: