Basic Chemistry

I. Chemical Elements

A) Matter is everything around us that takes up space and has weight. It can be found in four main forms: solid (like ice), liquid (like water), gas (like air), and plasma (which is an energized gas). Everything is made up of tiny bits called elements.

B) An Element is a pure substance that can't be broken down into simpler substances. Each element is made of only one type of tiny particle called an atom. Six very important elements that make up most living things are Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur (easy to remember as CHNOPS).

C) An Atom is the smallest part of an element that still acts like that element. Atoms are made of even smaller particles:

• Protons: These have a positive electric charge, are found in the center (nucleus) of the atom, and have a weight of $1$ amu.
• Neutrons: These have no electric charge, are also in the nucleus, and weigh $1$ amu.
• Electrons: These have a negative electric charge, move around the nucleus in layers (shells), and are very light ($1/1000$ amu).

D) Atomic number and mass number help us identify atoms. The atomic number is simply the number of protons in an atom's nucleus. The mass number is the total number of protons and neutrons in the nucleus. Sometimes, an atom can have more neutrons than protons.

E) The Periodic table is a chart that organizes all known elements. Elements are placed in rows (periods) based on their atomic number, and in columns (groups) based on how they behave chemically. For example, elements in Group $1$ are called Alkaline Metals, and Group $18$ contains Noble Gases.

F) Isotopes are different versions of the same element; they have the same number of protons but different numbers of neutrons. This means they have different mass numbers. For example, carbon has isotopes like $^{12}\text{C}$, $^{13}\text{C}$, and $^{14}\text{C}$. The atomic mass shown on the periodic table is an average weight of all the natural isotopes of an element.

G) Electrons are arranged in specific energy shells (or levels) around the atom's nucleus, as explained by the Bohr model. For smaller atoms (under $18$ electrons):

• The first shell holds up to $2$ electrons.
• The second shell holds up to $8$ electrons.
• The third shell holds up to $8$ electrons.

The valence shell is the outermost electron shell. Atoms try to become stable by getting a full valence shell, either by gaining, losing, or sharing electrons with other atoms.

H) Molecules vs. Compounds: A molecule is formed when two or more atoms join together (like ${\text{H}}<em>{2}$ or sugar ${\text{C}}</em>{6}{\text{H}}<em>{12}{\text{O}}</em>{6}$). A compound is a special type of molecule made from two or more different elements joined together (like water ${\text{H}}<em>{2}{\text{O}}$ or methane ${\text{CH}}</em>{4}$). The chemical formula tells us which atoms and how many of each are in a molecule or compound.

I) Chemical bonds

When atoms join together, it's called a chemical reaction, and they form chemical bonds.

1. Types of Bonds:

a) Ionic bonds happen when one atom gives an electron to another, making both atoms charged (like tiny magnets). These oppositely charged atoms then attract each other. A charged atom is called an ion. A positive ion (cation) has lost an electron, and a negative ion (anion) has gained one. Salt (${\text{Na}}^{+}{\text{Cl}}^{-}$) is a good example.

b) Covalent bonds occur when atoms share electrons to fill their outer shells. If electrons are shared equally (e.g., in ${\text{H}}<em>{2}$ and ${\text{O}}</em>{2}$), it's a nonpolar covalent bond. If shared unequally (e.g., in water ${\text{H}}_{2}{\text{O}}$), it's a polar covalent bond. This unequal sharing happens because some atoms (like oxygen) pull electrons more strongly, a property called electronegativity. Water's polarity means it has slightly positive and slightly negative ends.

c) Hydrogen bonds are weak attractions between a slightly positive hydrogen atom in one molecule and a slightly negative atom (like oxygen) in another molecule. These bonds are very important for water's unique properties.

J) Properties of Water

Water has several special properties that are vital for life:

• High Heat Capacity: Water can absorb a lot of heat without getting much hotter itself. This means water bodies help keep Earth's temperature stable.
• Good Solvent: Because water is polar, it's great at dissolving many substances, especially other polar and ionic ones. Things that dissolve in water are hydrophilic ("water-loving"), while those that don't are hydrophobic ("water-fearing"). A solution is water with dissolved substances (solutes) in it.
• Cohesion: Water molecules like to stick to each other, creating surface tension (why some insects can walk on water).
• Adhesion: Water molecules also like to stick to other polar substances (e.g., how water moves up a plant stem, called capillary action).
• Ice is less dense: Unlike most substances, solid ice is lighter than liquid water, which is why ice floats.

K) pH scale

The pH scale measures how acidic or basic (alkaline) a substance is. The scale ranges from $0$ to $14$:

• pH < 7: Acidic (acids release ${\text{H}}^{+}$ ions).
• pH = 7: Neutral (like pure water).
• pH > 7: Basic (bases accept ${\text{H}}^{+}$ ions).

The pH scale actually measures the concentration of ${\text{H}}^{+}$ ions and is based on powers of $10$. This means a small change in pH represents a big change in ${\text{H}}^{+}$ concentration (e.g., pH $1$ is much, much more acidic than pH $4$).

A buffer is a substance that helps keep a solution's pH stable. For instance, buffers in our blood maintain its pH around $7.4$, using substances like carbonic acid and bicarbonate to neutralize acids and bases.