ELements, Atoms, & Ions
Electron (negative)
Proton (positive)
Neutron (neutral)
Sodium atom (Na)
The figure above on the left represents an atom. Known as the Bohr model, it is a simplified version of an actual atom. The atom consists of 3 subatomic particles: protons, neutrons, and electrons (see labeled). The nucleus of the atom is made of protons and neutrons, making the nucleus positively charged. Electrons surround the nucleus. When it comes to charges, opposites attract. Just as the gravitational pull of the sun keeps planets surrounding it in orbit, the positive charge of the nucleus pulls in the negatively charged electrons and keeps them in the atom.
The figure above on the right is the periodic table of elements. Elements are the purest substances that make up the world around us. The number in the top left of each element's box is called the atomic number, which indicates the number of protons in a given element. For a certain element to remain itself, the number of protons must stay the same, but the number of electrons and neutrons can change. In a neutral atom, the number of protons and electrons are the same because their charges cancel out. When atoms have not bonded to another atom, they are in their neutral state, like the neutral carbon atom given above. In order for atoms to bond together and create chemical bonds, electrons are either shared or transferred, as shown through the animation on the right. This is how the number of electrons changes for a given atom.
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Which of the two oxygen atoms is neutral?
(Hover over box for correct answer)
O
O
The atom in green is neutral because Oxygen always has 8 protons (see periodic table). To make it neutral and balance out the charge, it must have 8 electrons.
In general, atoms want to completely fill their shell (surrounding black rings on the Bohr model) with the maximum amount of electrons a shell can hold (8 electrons) because it makes them most stable. In the Bohr model, the first shell can hold up to 2 electrons, but the following rings can hold up to 8. The more stable an atom is, the less likely it is to react. The elements in column 18 of the periodic table on the far right are the most stable and unreactive because they hold the maximum number of electrons possible in their outer shell. These elements are called noble gases.
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For example, sodium (or Na) has 11 protons, which means that at neutral state it has 11 electrons. However, sodium is very inclined to get rid of that extra electron that is keeping it from a perfect 8 electrons in its outer shell (see first figure), so it will readily give up that electron to an element that needs it to get 8 electrons in its outer shell, like Chlorine (Cl). That results in NaCl, or table salt. This process is called bonding.
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When sodium gets rid of 1 electron, it becomes a positive ion (called a cation) because it lost a negative charge. The convention for writing this is Na+. When chlorine gains an electron, it is written as Cl-. If Sulfur (S) were to attain 8 electrons in its outer shell, it would be written as S2− because it is gaining 2 negative charges. A negative ion is called an anion.
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Keep in mind that ions occur only when compounds like NaCl have broken apart. For example, when salt is dissolved in water, it breaks into Na+ and Cl- ions. These ions are actually what makes saltwater such a good conductor of electricity. This is also why ions are needed to conduct nerve impulses throughout the body.
