Elements• Elements are the primary types of matter

• Matter is anything that has mass and volume

• Elements can’t be broken down into more simple parts

(that retain the properties of the element)

• An element is made up of only one type of atom

• Each element has unique physical properties (color, odor,

density, melting point, etc.): no two are exactly alike

• Living things are made up mainly of CHON

• Each element is represented by a one or two letter symbol

(Table 2.2): learn these

The Periodic Table• Elements can be arranged into groups based on

similar physical and chemical properties

• These groups are the columns of the periodic table

• There are 8 main groups, some of which have special names:

- Group 1A = alkali metals

- Group 2A = alkaline earth metals

- Group 7A = halogens

- Group 8A = noble gases

Some Group 1A Elements

Some Group 7A Elements

Metals, Nonmetals and Metalloids• Elements are classified as metals, nonmetals or

metalloids (as shown on periodic table)

• Each type has different physical properties:

• Metals: shiny, ductile/malleable, conduct heat and electricity, high m.p. and density

• Nonmetals: dull, brittle, poor conductors, low m.p. and density

• Metalloids: have some properties of each, used as semiconductors

The Atom• Atoms are the building blocks of all matter

• An atom is the smallest particle that retains the

properties of the element

• Atoms of different elements can combine to

form compounds (discussed in Ch. 4)

• Chemical reactions involve changes in the

atomic make-up of reacting substances

(discussed in Ch. 6)

Structure of the Atom

• Atoms are mostly empty space• They have 3 main sub-atomic particles:• Protons: positively charged, in nucleus (mass = 1 amu)• Neutrons: neutral, in nucleus (mass = 1 amu)• Electrons: negatively charged, outside of nucleus

(mass is very small)• 1 amu is defined as 1/12 the mass of a carbon-12 atom

Nucleus (contains protons and neutrons)

Electron cloud (where electrons are most likely found)

Atoms and Charge

• Atoms are always neutral

- same number of protons as electrons

• Like charges repel and opposite charges attract

• Negative electrons are attracted to positive nucleus

(keeps them around it)

• Positive protons in two nuclei repel each other, but

negative electrons are attracted to positive nucleus of

neighboring nucleus, so atoms in a substance are kept

at an optimum distance from each other (bond length)

Atomic Number and Mass Number

• All atoms of an element have the same number of

protons (and electrons)

• Atomic number (Z) = number of protons

• Periodic table is arranged in order of increasing Z

• Mass number (A) = number of protons plus

number of neutrons

Isotopes and Atomic Mass• Not all atoms of an element have the same mass

number (why?)• Because they can have a different number of neutrons• Atoms of the same element with different mass

numbers are called isotopes• Each isotope has a specific isotopic mass (not the

same as atomic mass or mass number)• The atomic mass of an element, as reported on the

periodic table, is a weighted average of all the isotopic atomic masses

• Atomic symbols are used to show an elements atomic number (subscript) and mass number (superscript)

C

12

6

Calculating the Atomic Mass of an Element

Isotope % of Sample x Mass Number = Mass Contribution

C l

35

17 (75.8% = ) 75.8/100 x 35 = 26.5 amu

C l

37

17 (24.2% = ) 24.2/100 x 37 = 8.95 amu

Atomic Mass of Chlorine = 35.5 amu

Calculating the Atomic Mass of an Element

• Example:

If a sample of chlorine is found to contain 75.8% chlorine-35 and

24.2% chlorine-37, what is the approximate atomic mass of

chlorine?

% Isotope in Sample x Mass Number = Mass Contribution

(75.8% = ) 75.8/100 x 35 = 26.5 amu

(24.2% = ) 24.2/100 x 37 = 8.95 amu

Atomic Mass of Chlorine = 35.5 amu