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TEK 8.5C: Periodic Table
TEK 8.5C: Interpret the arrangement of the Periodic Table, including groups and periods, to explain how properties are used to classify elements.
Elements and the Periodic Table
An element is a substance that cannot be separated into simpler substances by physical or chemical means. An element is already in its simplest form.
The smallest piece of an element that still has the properties of that element is called an atom.
An element is a pure substance, containing only one kind of atom.
The Periodic Table of Elements is a list of all the elements that have been discovered and named, with each element listed in its own element square.
Elements are represented on the Periodic Table by a one or two letter symbol , and its name, atomic number and atomic mass.
The Periodic Table & Atomic Structure
The elements are listed on the Periodic Table in atomic number order , starting at the upper left corner and then moving from the left to right and top to bottom, just as the words of a paragraph are read.
The element’s atomic number is based on the number of protons in each atom of that element. In electrically neutral atoms, the atomic number also represents the number of electrons in each atom of that element.
For example, the atomic number for neon (Ne) is 10, which means that each atom of neon has 10 protons and 10 electrons. Magnesium (Mg) has an atomic number of 12, which means it has 12 protons and 12 electrons.
The element’s atomic mass as shown on the periodic table, is the average sum of protons and neutrons in each atom of that element. It is the numerically larger, non-whole number in the element square. (Note that the atomic mass for a single particular atom is a whole number, because it is not an average of many different atoms. There cannot be fractions of a proton or neutron in an actual atom.)
The number of neutrons in an atom of a particular element can vary. These are called isotopes of that element.
Because the atomic mass is an average number, it is not a whole number, and has to be rounded up or down to a whole number when used to calculate the number of neutrons in the most common isotope of that element.
To calculate the number of neutrons in the most common isotope of an element, subtract the atomic number from the rounded atomic mass. For example, the atomic mass of iron is 55.84, which rounds up to 56. Iron’s atomic number is 26. The number of neutrons is thus 56 – 26 = 30. 1
Periods
The horizontal (left-to-right) rows of the periodic table are called periods. There are seven periods in the periodic table.
Within each period, all elements have the same number of occupied Bohr Model electron “shells” (electron energy levels or orbitals).
The properties of the elements change gradually as you move across each period. For example, the atomic radius of each element gets smaller as you move from left to right within each period.
Period Numbers
57 La 89 Ac
(^1) H
(^2) Li
2 4 Be
Period
12 (^3) Na Mg
(^4) K
20 Ca
3 4 22 Sc Ti 40 (^5) Rb Sr Y Zr 56 (^6) Cs Ba Hf^72 88 (^7) Fr Ra
104 Rf
- The vertical, up-and-down, columns of elements are called
or.
- There are different groups.
- All of the elements in each column have similar
properties because they each have the same number of: .
