Atomic Structure, Chemical Bonding, and Solutions

Posted on Mar 31, 2025 in Chemistry

Elements and Atoms

An element is a pure substance that cannot be broken down into simpler substances.

An atom is the smallest part of an element that has all the properties of that element.

Atomic Structure

Atoms have a very dense center called the nucleus. The nucleus is made up of protons and neutrons.

Protons are positively charged (+).
Neutrons are neutral (no charge).
Electrons are negatively charged (-).

You can tell how many protons an atom has by looking at the periodic table (the atomic number tells us the number of protons).

Atomic Definitions

Atomic Number: Tells us the number of protons.

Atomic Mass: The average mass of all naturally occurring isotopes of an element.

Mass Number: The total number of protons and neutrons in a specific atom’s nucleus. We get this number by rounding the atomic mass to the nearest whole number.

Particle Mass and Location

Protons and Neutrons have an atomic mass of about 1 atomic mass unit (amu) or approximately 1 g/mol.

They are located in the nucleus (which is tiny and in the center of the atom). [Image Placeholder: Diagram of an atom’s nucleus]

Electrons have so little mass we often approximate it as zero in mass number calculations.

They occupy a large volume of space around the nucleus, sometimes called the electron cloud.

Since electrons are negative and protons are positive, they attract each other. This electrostatic attraction holds electrons around the nucleus. Remember: opposites attract.

Atom Examples

Example: Carbon

Carbon has the atomic number 6 because it has 6 protons.

Example: Silicon (Si)

Look at Silicon on your periodic table.

How many protons does it have?
What is its atomic mass?
What is its mass number?
How many neutrons does it have? (Mass Number – Protons)
How many electrons does it have (in a neutral atom)?
What is its overall charge (in a neutral atom)?

Example: Calcium (Ca)

Now try Calcium.

Protons?
Atomic Mass?
Mass number?
Neutrons?
Electrons?
Charge (in a neutral atom)?

Sketching Atoms

Bohr Model

Niels Bohr developed the idea that electrons occupy specific energy levels around the atom’s nucleus.

Energy Level: A specific region of space around the nucleus where electrons are likely to be found. A maximum of 2 electrons can occupy the first energy level. A maximum of 8 electrons can occupy every level after the first (for the elements typically covered in introductory chemistry).

Example: Sketching Chlorine

Draw the Bohr diagram of a chlorine atom.

Protons?
Neutrons?
Electrons?

Lewis Dot Diagrams

Valence Electrons: The electrons in the outermost energy level. These are the electrons involved in chemical reactions and bonding.

A quicker way to represent an atom’s bonding potential is to draw only the outermost energy shell’s electrons. This is called an electron dot diagram or Lewis Dot Diagram.

Example: Lewis Dot Diagram for Oxygen

Draw the Bohr model and the Lewis Dot diagram for an oxygen atom.

Groups and the Periodic Table

The alkali metals (Group 1) all have 1 valence electron.
The halogens (Group 17 or 7A) all have 7 valence electrons.
The noble gases (Group 18 or 8A) typically have 8 valence electrons (a full outer shell, except for Helium which has 2).

Ionization and Ions

Atoms are electrically neutral because they have the same number of protons (+) and electrons (-).

[Example Placeholder: Diagram showing a neutral Oxygen atom’s protons and electrons]

Achieving Stability

Atoms tend to achieve a stable electron configuration, like that of the noble gases. They want to have full outer shells. Full energy levels typically have 2 electrons in the first shell and 8 electrons in every shell after that.

Atoms can get full outer shells by gaining or losing electrons.

An ion is an atom (or group of atoms) that has gained or lost electrons and therefore has an electrical charge.

An ion has a charge because the number of electrons is different from the number of protons.
Atoms will gain or lose their outermost (valence) electrons to end up with completely filled energy levels. [Example Placeholder: Ion formation diagram]

Types of Ions

An atom with a net electrical charge is called an ion.
A negatively charged ion (formed by gaining electrons) is called an anion.
A positively charged ion (formed by losing electrons) is called a cation.

Forces Between Charges

Electrons (-) are attracted to Protons (+). Opposites attract.
Electrons (-) repel each other.
Protons (+) repel each other.

Metals and Non-metals

Metals

Malleable (can be hammered into different shapes)
Ductile (can be stretched or drawn into wires)
Shiny/Lustrous
Very conductive (heat and electricity)
Tend to form positive ions (cations) by losing electrons.
Example: Aluminum forms Al³⁺

Non-metals

Often brittle (not flexible) in solid state
Poor conductors of heat and electricity
Tend to form negative ions (anions) by gaining electrons.
Example: Oxygen forms O²⁻

Compounds

A compound is a pure substance formed from two or more different elements chemically bonded together in fixed ratios.

Two main types relevant here:

Ionic Compounds
Molecular Compounds

Ionic Compounds (Salts)

An ionic compound is a pure substance typically formed from a metal and a non-metal. Example: NaCl (table salt) is made up of Na⁺ and Cl⁻ ions.
An ionic bond is the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). It involves the transfer of electrons from the metal to the non-metal.

Metals tend to lose electrons, giving them to non-metals.

This makes the metal atom a positive ion (cation) and the non-metal atom a negative ion (anion).
Opposites attract! Cations are attracted to anions, and this attraction holds the substance together, often forming a crystal lattice structure (like a sodium chloride crystal).

Molecular Compounds

Molecular compounds are pure substances formed when non-metal atoms bond together, usually by sharing electrons. They are held together by covalent bonds.
Example: Plastics (often made from Carbon and Hydrogen atoms).
A covalent bond is a chemical bond formed when two atoms share one or more pairs of electrons.
A molecule is the smallest electrically neutral unit of a molecular compound, held together by covalent bonds.
Example: Carbon (C) and Hydrogen (H) bonding in plastics. A plastic strip is made up of long strands (polymers) of carbon atoms sharing electrons with one another and with hydrogen atoms. Each long strand is a molecule.

There can be weaker attractions between these molecules, which contribute to the properties of substances like plastics.

Why Metals Conduct Heat and Electricity

In order for a substance to conduct electricity, there must be freely moving charged particles (usually electrons or ions).
Metallic bonding involves a ‘sea’ of delocalized electrons that are free to move throughout the metal structure.
Since these electrons are free, they can transfer electrical charge (current) and also kinetic energy (heat) much more effectively than the localized electrons in molecular compounds or the fixed ions in solid ionic compounds.

Chemical Change

[Section 1.3 in original] [Image Placeholder: Chemical reaction examples]

A chemical change is a process where one or more new substances are produced. Evidence of a chemical change may include:

A change in color
Formation of a gas (bubbles)
Formation of a solid (precipitate)
An odor is produced
Energy is produced or absorbed:
- Exothermic Reaction: Energy is released (often as heat or light).
- Endothermic Reaction: Energy is absorbed from the surroundings (if heat is absorbed, it may feel cold).

Solutions

A solution is a homogeneous mixture of two or more substances. They consist of a solute and a solvent.
Aqueous solution: A solution where water is the solvent.
Solute: The substance that is dissolved (present in a smaller amount).
Solvent: The substance that does the dissolving (present in a larger amount).

Why Water is a Good Solvent

Let’s look at the atomic structure of water (H₂O).

[Image Placeholder: Lewis Dot Diagram of Water H:O:H with dots]

Two Hydrogen atoms are covalently bonded to one Oxygen atom.

Water has a bent molecular shape.
Oxygen is more electronegative than hydrogen, meaning it attracts the shared electrons more strongly. This creates a partial negative charge (δ⁻) on the oxygen atom and partial positive charges (δ⁺) on the hydrogen atoms.

The uneven distribution of charge makes water a polar molecule.

Since water molecules are polar, they can be attracted to ions (particles with full positive or negative charges) and other polar molecules.

Example: Water and Static Charge

Why does a stream of water bend when a statically charged balloon is near it?

When a balloon is rubbed against hair, it often picks up electrons and gains a net negative charge.
The positive ends of the polar water molecules (the Hydrogen atoms, δ⁺) are attracted to the negatively charged balloon, causing the stream of water to bend towards the balloon.

What Can Water Dissolve?

Molecular compounds: Sometimes. If the molecule is polar (like sugar or ethanol), it will likely dissolve in water. The polar charges on water molecules attract and pull the polar solute molecules away from each other. If the molecule is nonpolar (like oil), it generally will not dissolve in water (“like dissolves like”).
Ionic Compounds: Usually. When an ionic compound (like NaCl) is placed in water and dissolves, it not only dissolves but also dissociates.

Dissociation means that the ionic compound breaks apart into its individual ions, which are then surrounded by water molecules. The positive ends of water molecules surround the anions (e.g., Cl⁻), and the negative ends of water molecules surround the cations (e.g., Na⁺).

Water as a Medium for Reactions

Why is water an excellent medium for many chemical reactions?

When ionic or polar molecular compounds dissolve in water, the bonds holding the solute particles together are overcome as water molecules pull the particles apart (dissolving/dissociation).
The dissolved particles (ions or molecules) are then free to move throughout the solution and collide with one another, allowing chemical reactions to occur.

Conductivity in Solutions

Why are some solutions able to conduct electricity, while others are not?

Remember: To conduct electricity, a substance needs freely moving charged particles.
Substances that dissociate in water to form mobile ions (positive and negative charges) will conduct electricity.
The only substances discussed here that do this effectively are ionic compounds when dissolved in water.
If a solution can conduct electricity, we call the solute an electrolyte (e.g., solutions of ionic compounds like NaCl).
If a substance dissolves in water but does not dissociate into ions (like sugar, a molecular compound), it forms a non-electrolyte solution, which does not conduct electricity well.

Solutions and Concentrations

[Section 1.4 in original]

Beverages, medicines, and household cleaners are common examples of solutions.

Manufacturers must ensure the ratio of solute to solvent (concentration) is appropriate and safe for the intended use.

Concentration: The ratio of the amount of solute to the amount of solvent or solution.

Concentrated solution: A solution containing a relatively high ratio of solute to solvent/solution.

Dilute solution: A solution containing a relatively low ratio of solute to solvent/solution.

Qualitative Properties of Solutions

Qualitative properties are characteristics you can observe using your senses.

The qualitative properties of a solution can include:

Color intensity
Taste
Odor
Transparency

Concentrated solutions generally have more dissolved particles per unit volume. This can result in higher conductivity (if it’s an electrolyte), stronger taste and smell, and darker color (if the solute is colored).
A dilute solution has fewer dissolved particles per unit volume, resulting in weaker conductivity, weaker taste and smell, and lighter color.

Calculating Concentrations

[Section 1.5 in original]

There are several methods for quantitatively expressing concentration. [Image Placeholder: Formula summary graphic]

Percent by Volume (% V/V)

Used when both solute and solvent are liquids.
Commonly used for consumer products like alcoholic beverages and some cleaners.
Formula: % V/V = (Volume of Solute / Volume of Solution) x 100%

Example 1: % V/V Calculation

A hair product requires combining 20.0 mL of hydrogen peroxide (solute) with enough water to produce a solution with a total volume of 120.0 mL. Determine the percent by volume concentration of the hydrogen peroxide solution.

Solution Steps:

Identify known values: Volume of solute = 20.0 mL, Volume of solution = 120.0 mL.
Identify the required formula: % V/V = (Volume of Solute / Volume of Solution) x 100%.
Check if rearrangement is needed: No.
Check units: Units (mL) are consistent and will cancel.
Plug in values and solve: % V/V = (20.0 mL / 120.0 mL) x 100% = 16.666…%
Round to the proper number of significant figures (3 sig figs based on input values): 16.7% V/V.

Example 2: Finding Solute Volume

A mosquito repellent label states that DEET makes up 45.0% V/V of the product. If you have a 75-mL sample of this repellent, determine the volume of DEET within the sample.

What is this question asking for? (Volume of Solute)
You would rearrange the % V/V formula to solve for Volume of Solute.

Parts Per Million (ppm)

Used for expressing the concentration of very dilute solutions.
Often used in environmental monitoring (e.g., contaminants in water) or food analysis where amounts are tiny.
1 part per million means one unit of solute for every one million units of solution.
For aqueous solutions, ppm is often approximated as mg of solute per L of solution (mg/L) because 1 L of water has a mass close to 1 kg (1,000,000 mg).
Formula (mass/mass): ppm = (Mass of Solute / Mass of Solution) x 1,000,000
Formula (mass/volume, dilute aqueous): ppm ≈ (Mass of Solute (mg) / Volume of Solution (L))

[Image Placeholder: ppm formula variations]