CHAPTER 16Atomic Structure and Chemical Bonds

ATOMIC STRUCTURE—EMPTY SPACE

• At the center of every atom is a nucleus containing protons and neutrons.

• The electrons travel in an area of space around the nucleus called the electron cloud.

• The nucleus of an atom has an positive charge and electrons have a negative charge.

• Each element has a different atomic structure consisting of a specific number of

protons, neutrons and electrons.

• The number of protons and electrons is always the same for a neutral atom of a

given element.

ELECTRON ARRANGEMENT

***The number and arrangement of electrons in the electron cloud of an atom are

responsible for many of the physical and chemical properties of that element.

The different areas for an electron in an atom are called energy levels.

Each level represents a different amount of energy.

• Each energy level can hold a maximum number of electrons.

• The farther an energy level is from the nucleus, the more electrons it can hold.

• 1st level=1 or 2 electrons

• 2nd level=up to 8 electrons

• 3rd level= up to 18 electrons

• 4th level= up to 32 electrons

• Electrons in the level closest to the nucleus have the lowest amount of energy and

said to be in energy level one.

• Electrons farthest from the nucleus have the highest amount of energy and are the

easiest to remove.

• The closer a negatively charged electron is to the positively charged nucleus, the

more strongly it is attracted to the nucleus.

• Removing electrons that are close to the nucleus takes more energy than removing

those that are farther away from the nucleus.

WHAT DETERMINES THE AMOUNT OF ENERGY AN ELECTRON HAS?

• The energy level it occupies; electrons in the lowest level has the

lowest energy, and electrons in the highest levels have the most

energy.

Atomic number = # of protons = # of electrons

This makes the atom electrically neutral.

You can determine the number of electrons in an atom by looking at the atomic

number.

The number of electrons in a neutral atom of the element increases by one from left to

right across a period.

Atoms with a complete outer energy level are stable.

Each period in the periodic table ends with a stable element.

ELEMENT FAMILIES

• Members of element families have similar chemical properties because they have

the same number of electrons in their outer energy levels.

• It was the repeating pattern of properties that gave Mendeleev the idea for his first

periodic table.

• Group 18 have 8 electrons in their outer energy levels.

• Their energy levels are stable, so they do not combine easily with other elements.

• Group 18 were once referred to as the inert gases because they were thought to be

completely unreactive.

• However it was discovered that they can react, just not easily so the name was

changed to the noble gases.

• Still the most stable element group.

• The stability of the element makes possible the use of noble gases to protect

filaments in light bulbs.

• They are also used to produce colored light in signs.

GROUP 17—THE HALOGENS

• The easier it is for a halogen to gain this electron for form a bond, the more reactive

it is.

• Fluorine is the most reactive of the halogens because its outer energy level is closest

to the nucleus.

• Bromine is less reactive than fluorine because the reactivity of the halogens

decreases down the group as the outer energy levels of each element's atoms get

farther apart.

• The alkali metals have one electron in their outer energy level. It is this electron

that is removed when alkali metals react.

• The easier it is to remove an electron, the more reactive the atom is.

• Unlike halogens, the reactivity of alkali metals increases down the group, that is,

elements in the higher numbered periods are more reactive than elements in the

lower numbered periods. Why?

• Because..their outer energy levels are farther from the nucleus.

• Cesium in period 6 loses an electron more readily and is more reactive than sodium

in period 3.

ELECTRON DOT DIAGRAM

• An electron dot diagram is the symbol for the element surrounded by as many dots

as there are electrons in its outer energy level.

• Only the outer energy level electrons are shown because these are what determine

how an element can react.

• A chemical bond is the force that holds two atoms together.

• The dots correspond to the number of electrons in the outer shell. You can tell this

by looking at the group number:

• Elements in group 1 have 1

• Elements in group 2 have 2

• Elements in group 13 have 3

• Etc….