The pictorial model of how an atom might look is called the Bohr model, named for the scientist who described the arrangement of protons, neutrons and electrons for atoms. In the center of an atom is the nucleus. The nucleus is composed of equal numbers of protons and neutrons. The protons have a positive electrical charge, and the neutrons are electrically neutral. Circling the nucleus are the electrons. Electrons have a negative electrical charge. Electrons orbit the nucleus in a three dimensional orbit often described as an orbital or an electron cloud.
Each orbital is called an energy level or a shell. The first energy level or shell is the one closest to the nucleus. This is the lowest energy level. This lowest energy level is also known as the ground state. The last energy level is know as the highest energy level. The highest energy level is the last electron shell of an atom. The electrons which belong to the highest energy level in an atom are called the valence electrons. Atoms want to have their highest energy level (outer shell) full of electrons. If the outer shell of an atom is full, the atom is stable and will not participate in a chemical reaction. If the outer shell of an atom is not full, the atom is reactive and can take part in a chemical reaction.
The first element in the periodic table is hydrogen. Right above the symbol H is the number one. This is the atomic number for hydrogen. The atomic number is the number of protons. In an atom, the number of protons is equal to the number of electrons. Therefore, hydrogen has one electron. The first energy level (shell) of an atom can hold two electrons. So hydrogen is a reactive element. The second element in the periodic table is helium. Helium has two electrons. The first energy level (shell) can hold two electrons, so helium has a full shell. Helium is therefore a stable atom and does not react chemically with any other atom. The third element in the periodic table is lithium. Lithium has two electrons in its ground state and one electron in its highest energy level. The highest energy level (shell) of lithium could hold eight electrons. Since the highest energy level of lithium is not full, lithium is a reactive element.
The atom with an atomic number of 10 is neon. Neon has two electrons in its ground state and eight in its second (highest) energy level. Since the second energy level will only hold eight electrons, the highest energy level of neon is full. Therefore, neon is a stable element and does not react chemically with other atoms.
There are two primary ways that atoms react to form chemical bonds. Since all atoms want to have a full outer shell (highest energy level) they can share electrons with other atoms to achieve a full outer shell. Number 8, oxygen, has two electrons in the ground state and six in its highest energy level. This energy level could hold eight electrons. Hydrogen has one electron in its only energy level, which could hold two. Therefore, if two hydrogens shared electrons with one oxygen, the oxygen would have eight in its outer shell, and the two hydrogens would have two in their outer shell. All outer shells are now full and you have H2O (water), a stable molecule. When two or more atoms bond chemically by sharing electrons it is called a covalent chemical bond.
The other common way that chemicals bond is called the ionic bond. All atoms except for hydrogen have an equal number of protons, neutrons and electrons. Chlorine has an atomic number of 17. It has seven electrons in its outer shell. It would like to have eight. Sodium has an atomic number of 11. It has one electron in its outer shell. It would like to have eight. If chlorine stole an electron from sodium, both sodium and chloride would have full outer shells. If chlorine stole an electron from sodium, the chlorine would have an extra negative electrical charge and the sodium would now have less negative charges and be electrically positive. Positive and negatives attract each other and bond to form an ionic bond. When atom gain an electrical charge (either positive or negative), they are no longer called atoms. Instead, they are called ions.
1. Atoms have a number of electrons which is equal to the number of protons (atomic number).
2. Ions have a number of electrons which is not equal to the number of protons (atomic number). Therefore ions have an electrical charge. Oppositely charged ions attract to form an ionic bond.
The last column in the periodic table are the nobel gases: He, Ne, Ar, Kr, Xe, and Rn. The nobel gases have full outer shells and are therefore stable. These elements do not form compounds with any other elements.
The next to the last column in the periodic table are the halogens. Halite is the name for salt and the halogens are the salt forming elements. The halogens are F, Cl, Br, I and At. They form ionic bonds with metals such as Li, Na or K to form salts.
The halogens have outer shells (energy levels) which are one electron short of being full. Therefore, they often steal electrons from other elements and become negatively charged ions. When halogens become ions, their name changes:
Cl=chlorine Cl-=chloride *a negative ion has more electrons
F=fluorine F-=fluoride than protons. A positive ion has
Br=bromine Br-=bromide more protons than electrons.
I=iodine I-=iodide
The elements which compose living things are called the organic elements. The organic elements can be remembered by this pneumonic devise: C Hopkins CaFe Mg NaCl.
1. C=carbon 2. H=hydrogen 3. O=oxygen 4. P=phosphorus
5. K=potassium 6. I=iodine 7. N=nitrogen 8. S=sulfur
9. Ca=calcium 10. Fe=iron 11. Mg=magnesium 12. Na=sodium
13. Cl=chlorine
While these are the main elements which compose living things,
but not the only elements which compose living things.