Notes for Standard 1 Objective 3 (Periodic Table)
The periodic table is used to organize elements by their structure. The chemical and physical properties of an element are related to its structure. The periodic table is organized by increasing atomic number (# of protons in the nucleus). The atomic mass is the number of protons plus neutrons. In a neutral atom, the number of protons equals the number of electrons. Flourine’s atomic number is 9 and its atomic mass is 19. Therefore flourine has 9 protons, 9 electrons, and 10 neutrons ( 9 protons + 10 neutrons = 19 atomic mass units). Atoms with the same number of protons but different numbers of neutrons are called isotopes. For example, carbon-12 has 6 protons and 6 neutrons, but carbon-13 has 6 protons and 7 neutrons. Carbon-12 and carbon-13 are isotopes.
On the periodic table, vertical columns are called groups and horizontal rows are called periods. Elements in the same group have similar chemical behavior because they have the same number of electrons in their outer main energy level. For example, group 1 (the alkali metals) are all very reactive, soft, metals. They each have only one electron in an s orbital of their outermost main energy level. Group 2 (the alkaline earth metals) are less reactive than the group one metals and are a little harder and more dense. Group two elements each have a filled s orbital (2 electrons) in their outermost main energy level.
Group 17 elements (the halogens) are very reactive non-metals. Group 17 elements each have 7 electrons in their outermost main energy level. Group 18 elements (the noble gases) are all unreactive gases. They are unreactive because they each have a completely filled outer main energy level of 8 electrons.
The stair-step line that begins between Al and Si divides the periodic table between metals on the left and non-metals on the right. The elements that border the stair-step line have properties of both metals and non-metals and are known as metalloids or semimetals or semiconductors.
The size of atoms generally increases as you move down a group. This is because as you move down the table, the electrons have to fill main energy levels that are farther from the nucleus. The size generally decreases as you move across a period from left to right.
This is because the nuclear charge (number of protons in the nucleus) is increasing but the electrons across a period are all in the same main energy level. The positively charged protons attract the negatively charged electrons and pull them in close so that the atom’s size becomes a little more compact as you move from left to right.
Francium is the most reactive metal because its single electron is in the 7 th main energy level far removed from the pull of the protons in the nucleus. This makes it very easy to give up that electron to another atom.
Flourine is the most reactive non-metal . Its seven outer electrons are in the 2 nd main energy level--very close to the pull of the nine protons in its nucleus. This makes it very easy for flourine to steal an extra electron from another atom so that it will have a total of eight electrons in its outer main energy level.