HS Chemistry - Essentials

Unit Summary

Whenever we perform calculations in the lab, we convert the final answer to significant figures, keeping it as accurate as the least accurate number from the data set used to calculate it.

This is because the data readings are only accurate to a certain limit, so we can't have an answer that assumes the value of the third decimal place.

If the data readings differ in how many significant figures they each have, the entirety of each reading is still used when calculating the answer, but the final answer is rounded to the required number of significant figures at the end.

An atom is the smallest particle of an element that still retains that element's properties. There are 118 known elements.

Atoms are made of protons, neutrons, and electrons. Protons and neutrons are found in the nucleus, and electrons orbit the nucleus. Protons and neutrons are also called nucleons.

The atomic number $also known as the proton number$ is the number of protons in the nucleus.

The atom's charge is the difference between the number of protons and the number of electrons.

The atom's mass $in atomic mass units, or amu$ is equivalent the number of nucleons it has.

A molecule is the smallest particle of a compound that still retains that substance's properties. There are far more than 118 molecules.

Atoms bond together to form molecules. They also bond together to form formula units.

Isotopes are atoms of an element that contain the same number of protons but different numbers of neutrons. Therefore, they are still the same element, but they have different atomic masses.

Isotopes are also denoted using the following format:
- X is the element symbol, A is the atomic mass of the isotope, and Z is the atomic number of the element
Ions are atoms of an element that contain the same number of nucleons, but different numbers of electrons. Therefore, they have the same atomic number and atomic mass, but different charges.

In ionic bonding, non-metals turn into anions, and the metal into cations, and then they bond.

There are 7 electron shells surrounding the nucleus in which electrons are found. Not every atom has all 7 electron shells.

Furthermore, each electron shell is made up of subshells. Each subshell contains orbitals, and each orbital contains two electrons. However, not every shell contains all four subshells, and while every shell has its respective subshells, not every atom has electrons in those subshells.

The Aufbau principle is that electrons will fill lower-energy subshells before they fill higher-energy subshells:
- This does not mean that every shell is completely filled before moving on to the next shell.
The following diagram is used for the Aufbau principle:
The electron configuration of an atom shows how the electrons are distributed among the atom's subshells. Box notation shows how the electrons are distributed in the orbitals. Both have shorthand notations as well.

Noble gases do not have an electronegativity value, as they are unable to attract other atoms' electrons due to having a full valence shell.

The more valence electrons that the atom has, the higher its electronegativity. Because of this, non-metals have higher electronegativities than metals.

Fluorine is the most electronegative element, and Francium is the least electronegative element.

A more electronegative element in a period has a higher ionization energy than a less electronegative element in the same period.

However, an element with fewer valence shells will typically have a higher ionization energy than an element with more valence shells.

Due to the fact that electrons are all negatively charged particles, they will repel each other.

Electrons in the same orbital will repel each other while electrons in different orbitals won't. Therefore, if there any empty orbitals left in the same subshell, electrons will move into those first before filling up orbitals that already have electrons in them.

If only one more electron is needed to ensure that every orbital is occupied/filled, then one electron will leave the outermost s subshell and instead go to the lone empty orbital.

Electron shielding is when electrons in inner shells repel electrons in outer shells, shielding the outer electrons from some of the nucleus's attractive force.