Elements, Compounds, and Molecules

Elements, Compounds, and Molecules

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

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

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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)

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Reactivity in Electrons

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- 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

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

- The sharing of electrons

- Occurs between nonmetals

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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

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How to identify a cation and anion formation?

- Cation atoms= Have more protons than electrons

- Anion atoms= Have more electrons than protons

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

outermost shell

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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.

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Gaining electrons= negative charge

Giving up electrons= positive charge

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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

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

- the unequal sharing of electrons

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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.

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- 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

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Properties of Water

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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

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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.

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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.

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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.

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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

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