Elements and Chemical Reactions Notes

More Elements

Chemical reactions occur to create chemically stable numbers of electrons around each atom.

Elements are defined by their atomic number, which is the number of protons they have.
In a neutral atom, the number of electrons is equal to the number of protons.
The number of neutrons can vary and can be found by subtracting the atomic number from the atomic mass.

The elements crucial for biology include Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), Phosphorus (P), and Sulfur (S).
These elements, excluding Hydrogen, are located adjacent to each other on the periodic table.
These elements are in periods 1, 2, and 3.

Every period of the periodic table has an inert element in its rightmost column.
Helium (He) is the inert element in Period 1 and has 2 electrons.
Neon (Ne) is the inert element in Period 2 and has 10 electrons.
Inert elements are stable and do not participate in chemical reactions.

Two Hydrogen (H) atoms combine to form H_2, allowing each atom to share 2 electrons, similar to Helium.
H_2 is a diatomic molecule: a gas made up of two atoms held together by covalent bonds.
Hydrogen atoms are never alone in nature; hydrogen gas is H_2.
In Period 2, elements share electrons to achieve 10 electrons, like Neon.
Nitrogen (N), Oxygen (O), and Fluorine (F) are found as diatomic molecules in nature: N2, O2, and F_2.
Each pair of like atoms forms a covalent bond, enabling them to share electrons, effectively giving each atom 10 electrons.
Energy is released when the bond forms and is required to break it.

Molecules can solve the problem of attaining 10 electrons, resulting in a more stable configuration.
Water (H_2O) is formed by combining two hydrogen atoms and one oxygen atom.
Each Oxygen (O) atom has 8 electrons, and each Hydrogen (H) atom has 1 electron; combining 1 O atom with 2 H atoms yields 10 electrons.
The chemical equation 2H + O \rightarrow H_2O represents the formation of water from individual hydrogen and oxygen atoms.
The more realistic equation is 2H2 + O2 \rightarrow 2H_2O.
The number of each type of atom remains constant throughout a reaction; atoms are rearranged, not created or destroyed.

Mixing hydrogen and oxygen gas in a container does not result in an immediate reaction because the bonds of the H2 and O2 molecules must be broken first.
Breaking the H2 and O2 molecular bonds requires energy, which can be supplied by a flame.
The flame initiates the reaction by breaking some bonds, and the resulting explosion breaks the remaining bonds.

The energy released in the explosion results in strong bonds holding the water molecule together.
The same amount of energy released in the explosion would be required to break apart the water molecule.
Water is very stable and represents the lowest energy state of H and O.

To reverse the reaction and split water into hydrogen and oxygen (2H2O \rightarrow 2H2 + O_2), energy equal to the energy released during the explosion must be added.
Plants perform this reaction to grow.