The kinetic particle theory (particle theory) describes all matter as a collection of particles that are in constant, random motion, even if those movements are only small vibrations. The amount of movement a particle has is determined by the amount of energy it has. Matter generally exists in three states: solid, liquid, or gas (Figure 1.1). As energy is transferred away from, or transferred to, these molecules, the state of matter may change as the movement of the molecules within the matter changes.

Covalent bonds

A covalent bond forms when two atoms share a pair of electrons. Covalent bonding occurs in most non-metal elements, and in compounds formed between non-metals. Because the atoms are sharing the electrons, both atoms have complete outer shells. This sharing of electrons also makes this type of bond between atoms one of the strongest, requiring a large amount of energy to break. Compounds with covalent bonds are able to exist as a solid, liquid or gas at room temperature and normal atmospheric pressure. Therefore, it should come as no surprise that water is one of the most prevalent covalent compounds on our planet. Each water molecule contains two covalent bonds connecting the oxygen atom to each of the hydrogen atoms (Figure 1.3).

These bonds form when an oxygen atom, which only has six electrons in its outermost shell, reacts with two hydrogen atoms, with only one electron each. The hydrogen atoms share their individual electrons with the oxygen atom. The shared electrons orbit around the atoms connected in the bond, filling the outer shells of the oxygen and the hydrogen. Many compounds in seawater have covalent bonds (Figure 1.4).

As seen in Figure 1.4 (b) carbon dioxide has four covalent bonds as the single carbon molecule forms a double bond with each oxygen molecule. A double bond occurs when molecules share two pairs of electrons instead of just one.

Ionic bonds

An ion is an atom that has gained or lost an electron from its outer shell. This change in the number of electrons gives the atom an electrical charge. An electron will move from one atom, which results in either filled or empty outer electron shells. If an atom loses an electron, the ion created will have a positive charge because the protons in the nucleus (positive charge) now outnumber the electrons in the outer shell (negative charge). If an atom gains an electron, the ion created will be negatively charged due to an excess of electrons compared to protons. So, how do ionic bonds form? When an ion loses an electron, its positive charge is attracted to the newly formed negative ion that gained its electron. This electrostatic attraction causes an ionic bond to form. The process of forming an ionic bond is seen in Figure 1.5. In step (a) sodium and chloride both have incomplete outer shells, with sodium having a single electron and chloride having seven. In step (b) sodium’s single electron breaks away and moves to complete chloride’s outer shell, making both ions more stable in the process. In step (c) sodium has a positive charge, chloride has a negative charge. The electrostatic attraction between the positive sodium ions and negative chloride ions creates an ionic bond.

Salts are made from ions, which are very important compounds in our oceans, There are many types of salts found in the ocean, including sodium chloride (NaCl), calcium carbonate (CaCO3), and magnesium sulfate (MgSO4). These salts are all formed using ionic bonds

Hydrogen bonding

A hydrogen bond is a weaker bond that can occur between molecules containing a hydrogen atom bonded to an atom of oxygen, nitrogen or fluorine. Water is an example of such a molecule, as it has two hydrogen atoms bonded to an atom of oxygen. When creating this covalent bond, the oxygen and hydrogen atoms share electrons unequally. The oxygen atom has a much stronger attraction to the bonding pair of electrons between oxygen and hydrogen, resulting in these being pulled closer to the oxygen atom than the hydrogen atom. This unequal sharing causes a partial charge on the atoms involved in the bond: the hydrogen atoms are partially positive (δ+), and the oxygen atom is partially negative (δ−). When molecules have a partial charge on each end as water does, they are referred to as polar. Due to this polarity, the more positive hydrogen atoms of one water molecule will be attracted to the more negative oxygen atoms of a nearby water molecule (Figure 1.8) creating a dipole – a molecule with a separation of partial positive and negative charges.

While hydrogen bonds are easily broken, they have an incredible impact on the properties of water due to the sheer number of water molecules found within a single droplet of water. Therefore, many hydrogen bonds are continually forming between water molecules.