Atomic Structure & Chemical Bonds

A. The nucleus (containing protons & neutrons) is at the center of an atom and is surrounded by the electron cloud (an area of space where electrons travel)

1. Electrons – have a negative charge and do not travel in definite orbits

2. Protons – have a positive charge

3. Neutrons – have no charge

4. Each element has a different atomic structure consisting of a particular # of protons, neutrons and electrons

B. The # and arrangement of electrons in the electron cloud determines the physical and chemical properties of each element

1. Electrons are arranged in different energy levels at different distances from the nucleus

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

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3. Electrons in the cloud closest to the nucleus have the lowest amount of energy; electrons farthest from the cloud have the most energy

C. Data from the periodic table can be used to understand energy levels

1. The atomic# is the same as the # of protons (and electrons in an electrically neutral atom)

2. The # of electrons in an atom’s outermost energy level increases from left to right across a period

a) Group 1 elements have 1 electron in their outermost level; group 2 have 2, group 13 have 3, etc

3. Each column in the periodic table contains a family or group with similar chemical properties

a) The noble gases in group 8 do not combine easily with other elements because their energy levels are stable

b) The halogens in group 7 have electrons in their outer energy level so they combine easily with alkali metals (group 1 elements)

D. Electron Dot Diagram – made of the symbol for the element surrounded by as many dots as there are electrons in its outer energy level

1. Dots are written on 4 sides of the element symbol

a) One dot represents a single electron

b) Paired electrons are represented by 2 dots

Carbon in group 14

C. ..

.

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

a) Electron dot diagrams can be used to show how atoms bond with each other

b) Atoms bond with other atoms so each has a stable outer energy level

Na + Cl [Na]+ + [ Cl ]-. .. ..... .. ....

..

A.Atoms form bonds by losing electrons, gaining electrons, pooling electrons or sharing electrons

1. Compound – two or more atoms chemically bonded

2. Ion - an atom that is no longer electrically neutral because it has lost or gained electrons

B. Ionic Bond – forms when positive and negative ions attract each other

1. Sodium chloride (table salt) is formed when sodium ions and chlorine ions bond together

C. Covalent Bond – forms between atoms that share electrons

1. Atoms sharing electrons form a neutral particle called a molecule

2. Covalently bonded compounds are called molecular compounds

3. No electrons are gained or lost

D. Metallic Bond – form when metal atoms share their pooled electrons

1. This is responsible for metal’s conductive property because electrons in the outer levels of metal atoms can easily move from one to the next to the next

E. Polar (Covalent) Bonds – occurs when electrons are shared unevenly

1. Polar molecules such as water have 2 opposite ends or poles like a magnet

2. Nonpolar Bonds form between atoms of the same element

F. Symbols are used to represent atoms & compounds

1. Elements are represented by 1, 2 or 3 letters

2. Compounds are described using element symbols and numbers

3. In the formula H2 the small 2 is called a subscript and indicates the # of atoms of hydrogen in the molecule

G.Chemical formula - a combination of chemical symbols and numbers

1. It tells which elements are present and how many atoms of each element are present

2. No subscript means that only one atom of that element is present

3. Ex: H2O and NH3