Bonding and Formulas

Bonding and Formulas

Theme: Based on their positions in the periodic table, atoms combine in predictable ways to form compounds with different properties.

• When atoms form compounds, each atom is more stable in the compound than it was by itself.

Vocabulary review

• Valence electrons: the electrons in the outermost energy level

• Element: substance made up of atoms that are all alike.

• Compound: substance formed from two or more elements that are chemically combined.

Multiple Select

An example(s) of an element is ____.

Choose all that apply.

• Water (H₂O)

• Table salt (NaCl)

• Hydrogen (H₂)

• Oxygen (O₂)

Multiple Select

An example(s) of a compound is _____.

(Choose all that apply)

• H₂O

• NaCl

• H₂

• O₂

Multiple Choice

Elements in group 14 (4A) have how many valence electrons?

• 4 valence electrons

• 7 valence electrons

• 2 valence electrons

Multiple Choice

Aluminum (Al) has _____ valence electrons.

• 7

• 1

• 3

Elements combine to form compounds, which often have new properties.

• Sodium (Na): reacts vigorously with water

• Chlorine (Cl): toxic, poisonous

• When combined, NaCl is essential to life, dissolves in water.

Chemical formula show what elements a compound contains and the exact number of atoms of each element in a unit of compound.

• Subscript means "written below"

• A subscript written after a symbol tells how many atoms of that element are in a single unit of that compound.

• Example: H₂O --> 2 hydrogens for every 1 oxygen atom

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Open Ended

Look at the chemical formula: SiO₂.

1. Which elements combine to form this compound?

2. How many atoms of each element are required to form this compound?

Open Ended

Look at the chemical formula: CaO.

1. Which elements combine to form this compound?

2. How many atoms of each element are required to form this compound?

Why do atoms form compounds?

• Atoms want to have a full outermost shell.

• Bonds are formed when valence electrons are gained, lost, or shared by atoms.

Multiple Choice

In which GROUP are the atoms most stable (full outermost energy shell)?

• Group 13 (3A)

• Group 17 (7A)

• Group 18 (8A)

• Group 1 (1A)

Bonding and valence electrons

• Noble gases rarely form compounds because they are unusually stable (they have a full outer energy level).

• An atom is chemically stable when its outer energy level is complete.

How do the electron dot diagrams represent elements, and how does that relate to their abilities to make compounds?

• It tells you how many valence electrons an atom has.

• When shells are not full, they become more stable when it is part of a compound.

• If the outer level is full, it is stable. Otherwise, it is likely to form bonds to gain stability.

How do elements change their outer electrons to become stable?

• Atoms with unstable outer energy levels can lose, gain, or share electrons with other atoms to obtain a stable outer energy level.

Chemical bonds

• When atoms gain, lose, or share electrons, an attraction forms between the atoms, pulling them together to form a compound.

• A chemical bond is the force that holds atoms together in a compound.

Metal + Nonmetal Bonding

Example: Na and Cl --> Na loses 1 electron and Cl gains 1 electron.

By doing this, they both form a stable outer level.

This is called an ionic bond (gain or lose electrons).

Nonmetal + Nonmetal Bonding

Example: H₂ and O share electrons.

By doing this, they are forming a stable outer level.

This is a covalent bond (sharing electrons).

Multiple Select

Which elements will form an IONIC BOND?

• Na + Cl

• H₂ + O

• C + O

• Li + Cl

Multiple Select

Which elements will form a COVALENT BOND?

• Na + Cl

• H₂ + O

• C + O

• Li + Cl

Open Ended

Explain why some elements are stable on their own while others are more stable in compounds.

• If an element has a full outer energy level, it has a tendency to not react with other atoms because it is already stable.

• If an element does not have a full outer energy level, then it will form a compound by gaining, losing, or sharing electrons with other atoms in order to achieve a full outer energy level.

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Multiple Select

What are two ways that bonds can form?

• By losing or gaining electrons (ionic bonds)

• By sharing electrons (covalent bonds)

• Bonds don't form between atoms.

Open Ended

The label on a box of cleanser states that it contains CH $_3$ COOH. What elements are in this compound? How many atoms of each element can be found in a unit of CH $_3$ COOH?

CH₃COOH

• Hydrogen, carbon, and oxygen are in the compound.

• Each unit contains: four hydrogen, two carbon, and two oxygen atoms.

Summary

• A chemical formula describes the numbers and types of atoms in a compound.

• The elements in group 18 (8A), the noble gases, rarely combine with other elements.

• Most atoms need 8 electrons to complete their outer energy levels (He needs 2).

• A chemical bond is the force that holds atoms together in a compound.