Elements, Compounds, and Molecules

Element: A pure substance that cannot be broken down into other substances by chemical means.

Molecule: Two or more atoms of the same element bond together

Compound: A substance made up of two or more different elements that have been chemically joined

• The ratio of atoms of different elements in a compound is fixed, meaning the number of atoms does not change.

• All compounds are molecules because a compound is a type of molecule

• But not all molecules are compounds

- This is because molecules can only be made by atoms of the same (one) element whereas compounds can be made by atoms of two different elements.

Ex: CH4 (Carbon and 4 hydrogen atoms; since there are more than 2 atoms of the same element, it’s a molecule; but it’s also a compound because it’s composed of two different types of elements)

Reactivity in Electrons

- If outer energy levels are not filled, elements are most reactive and likely to form bonds

• Most reactive elements= those with 1 or 2 electrons to gain or lose to fill outer shell

- If outer energy levels are completely filled, elements are least reactive

- An atom is only stable if it has 8 electrons in its outer shell

Covalent Bonds: When two atoms share one or more pairs of electrons

- The sharing of electrons

- Occurs between nonmetals

Ionic Bonds: When atoms are held together by the attraction between opposite charges

- When an atom transfers an electron to another atom

- Giving up electrons (positive or negative charge)

- Attraction between cations and anions

- Occurs between a metal and a nonmetal

How to identify a cation and anion formation?

- Cation atoms= Have more protons than electrons

- Anion atoms= Have more electrons than protons

Octet Rule; The tendency of atoms to bond with other atoms to have 8 electrons in their

outermost shell

Explanation: Sodium has one valence electron, which is why it tends to give that up, causing it to gain 1 positive charge. Chlorine has only 7 valence electrons, which is why it gains the electron from Sodium, causing it to be 1 negative charge.

Gaining electrons= negative charge

Giving up electrons= positive charge

Hydrogen Bonds: Attraction between a hydrogen atom with a positive charge and negative charge

- these type of molecules are called polar molecules because of their polarity

Ex: water molecules are polar because oxygen has a negative charge and hydrogen has a positive charge

• The oxygen atom attracts more electrons than the hydrogen’s single protons

• Since the oxygen atom gains electrons, it has negative charge

• And since the hydrogen atoms loses electrons, it has positive charge

Polarity: A molecule in which the charges are unevenly distributed (partial positive and negative)

- the unequal sharing of electrons

How does polarity apply to water molecules?

Ans: Water molecules have an uneven distribution of electrons, with the oxygen atom having a negative charge and the hydrogen atoms having a positive charge. This unequal sharing of electrons helps water molecules form multiple hydrogen bonds and is responsible for many of water’s special properties.

- Water molecules are an important solvent that dissolves many other molecules, such as polar molecules

- It’s important because many living organisms use this solvent for many of the processes that occur inside their bodies

Properties of Water

1. Cohesion: An attraction between molecules of the same substance

Ex: water molecules

- it causes water molecules to bond with each other

- leads to two important properties: surface tension and high specific heat

Surface Tension:  The attraction between water molecules at the surface of the liquid

- a thin film of “skin” is formed at the surface because the water molecules cling to each other

Importance: Allows the capillary action in plants and allows water striders to walk on water.

__High Specific Heat: __The amount of energy needed to gain or lose to change 1g of a substance to 1 degree celsius

- water takes up a lot of energy to warm up water/for warm water to cool down

- this is because water molecules are strongly attracted to each other and takes a lot of energy to separate in order to evaporate

• Evaporation: The faster the molecules are moving, the more likely they are able to change from a liquid to a gas. (heat causes this)

Importance: This property’s temperature moderation allows living organisms and Earth’s climate to maintain a certain temperature, which helps life on Earth to survive.

2. Adhesion: An attraction between molecules of different substances

Ex: water molecules and glass molecules

- The adhesion between water molecules and glass molecules is stronger than the cohesion of water molecules

- Capillary Action: When water rises in a narrow tube against the force of gravity

• Both adhesion and cohesion allows this to happen

Importance: The transport of water to plants by cohesion and adhesion allowing the water to travel from the roots all the way to the leaves, against the force of gravity.

3. Low Density of Ice

- Water molecules tend to expand to the maximum distance in their frozen state

- Water molecules in ice are less dense than in its liquid state

- This explains why ice floats on water

Importance to life: During winter, frozen waters will have a thin layer of ice on the surface which will simply float on the water instead of sinking. This allows aquatic life to survive during the cold.

4.Universal Solvent: Which is water, that is able to dissolve substances

Why is water called the universal solvent?

- Because water is a polar molecule, meaning it has negative and positive charges, which allows to help break things apart more than any other liquid.

Importance to life: It’s important because many living organisms use this solvent for many of the processes that occur inside their bodies