Homework & Reference: Trends In The Periodic Table (Page 2 of 2) in pdf

Periodic Trends

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Periodic trends are specific patterns that are present in the periodic table, which illustrate different aspects of a certain element, including its

size and its properties with electrons. The periodic trends that arise from the arrangement of the periodic table provide chemists with an

invaluable tool to quickly predict an element's properties. These trends exist because of the similar atomic structure of the elements within their

respective group families or period and the periodic nature of the elements.

Atomic Radius Trends

For atoms, the atomic radius is one-half the distance between the nuclei of two atoms is (just like a radius is half

the diameter of a circle). However, this idea is complicated by the fact that not all atoms are normally

bound together in the same way. Going through each of the elements of the periodic table, patterns of

the atomic radius can be seen.

Atomic size gradually decreases from left to right across a period of elements. This is because, within a

period or family of elements, all electrons are being added to the same shell. But, at the same time,

protons are being added to the nucleus, making it more positively charged. The effect of increasing

proton number is greater than that of the increasing electron number; therefore, there is a greater

nuclear attraction. This means that the nucleus attracts the electrons more strongly, having the atom's shell

pulled closer to the nucleus. The valence electrons are held closer towards the nucleus of the atom. As a

result, the atomic radius decreases.

Going down a group, it can be seen that atomic radius increases. The valence (outermost) electrons occupy

higher levels moving down. As a result, these electrons are further away from the nucleus. The inner electrons prevents these outer electrons from

being attracted to the nucleus (electron shielding); thus, they are loosely held and the resulting atomic radius is large.

• Atomic radius decreases from left to right within a period. This is caused by the increase in the number of protons and electrons across a

period. One proton has a greater effect than one electron; thus, a lot of electrons will get pulled towards the nucleus, resulting in a smaller

radius.

• Atomic radius increases from top to bottom within a group. This is caused by electron shielding.

Melting Point Trends

Melting points are the amount of energy required to break a bond(s) to change the

solid phase of a substance to a liquid. Generally, the stronger the bond between the

atoms of an element, the higher the energy requirement in breaking that bond. Melting

points are varied and don't generally form a distinguishable trend across the periodic

table. However, certain conclusions can be drawn from the following graph.

• Metals generally possess a high melting point.

• Most non-metals possess low melting points.

• The non-metal carbon possesses the highest boiling point of all the elements. The semi-

metal boron also possesses a high melting point.

Metallic Character Trends

The metallic character of an element can be defined as how readily an atom can lose an electron. As you move from right to left across a

period, metallic character increases because the attraction between valence electron and the nucleus is weaker, thus enabling an easier loss of

electrons. Metallic character increases as you move down a group because the

atomic size is increasing. When the atomic size increases, the outer shells are

farther away. The electrons of the valence (outermost) shell have less of an

attraction to the nucleus and, as a result, can lose electrons more readily, causing

an increase in metallic character.

• Metallic characteristics decrease from left to right across a period. This is caused

by the decrease in radius of the atom which allows the outer electrons to ionize

(escape) more readily.

• Metallic characteristics increase down a group. Electron shielding causes the

atomic radius to increase thus the outer electrons ionizes more • readily than

electrons in smaller atoms.

• Metallic character relates to the ability to lose electrons, and nonmetallic

character relates to the ability to gain electrons.

• Move left across period and down the group: increase metallic character (heading towards alkali and alkaline metals)

• Move right across period and up the group: decrease metallic character (heading towards nonmetals like noble gases)

Homework: Trends in the Periodic Table

9/17/14

Homework & Reference: Trends In The Periodic Table Page 2

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