Chapter 4

The Chemical Basis of Life

Matter

Matter = any material substance with Mass & Volume

Matter

Solid

Liquid

Gas

comes in 3 phases

Solid

Definite Shape

Definite Volume

Liquid

Indefinite Shape – takes the shape of the container

Definite Volume

Gas

Indefinite Shape – takes the shape of the container

Indefinite Volume – can expand and be compressed

Elements Pure substance that can not be broken down into other substances by chemical means

Examples of Elements

H = Hydrogen

C = Carbon

O = Oxygen

N = Nitrogen

S = Sulfur

Na = Sodium

Ca = Calcium

K = Potassium

I = Iodine

Cl = Chlorine

P = Phosphorus

• 4 elements make up 96% of all living matter

– Hydrogen (H)

– Oxygen (O)

– Nitrogen (N)

– Carbon (C)

• Most of remaining 4% is made of:– Calcium (Ca),

– phosphorus (P),

– potassium (K),

– sulfur (S)

• Trace elements Make up less than 0.01 % of body mass– Essential to life

Atom the smallest particle making up

elements

Sub-atomic Particles

Protons p+ - positive charge, in nucleus

Electrons - e- negative charge, orbiting nucleus

Neutrons n0 – no charge, in nucleus

Carbon Atom

C12.011

6

Atomic Mass

Atomic #

minus Atomic # = # of n0

= # of p+ and # of e-

Carbon has 6 p+ and 6 e-

Carbon has 6 n0

Drawing an Atom of Carbon

6 p+

6 n0

e-

e-

e- e-

e-

e-

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

HELIUM ATOM

Compounds

• Substance containing two or more elements combined in a fixed ratio

• Properties are usually much different than those of the elements they contain– Ex: H2O – liquid at room temperature, while

hydrogen and oxygen are both gases– Ex: NaClEx: NaCl – white crystal (table salt), while

sodium is a silver-gray metal and chlorine is a yellowish-green gas

Isotopes

Atoms with the same number of protons, but

different numbers of neutrons.

Atoms of the same element (same atomic number)

with different mass numbers

Most isotopes are stable—their nuclei do not

change over time

Radioactive Isotope

• Other isotopes contain nuclei that will change or decay over time

• Unstable isotopes• Nucleus changes giving off

radiation• May be harmful • Can be useful in research

and/or medical diagnosis and treatment

Let’s practice What We Just Learned

Bohr Diagram and Lewis Structure

Electrons are arranged in Energy Levels or Shells

around the nucleus of an atom.

• first shell a maximum of 2 electrons

• second shell a maximum of 8 electrons

• third shell a maximum of 8 electrons

Bigger energy level = higher energy

ATOMIC STRUCTURE

With the Bohr Diagram (Dot & Cross diagrams) elements

and compounds are represented by Dots or Crosses to show

electrons, and circles to show the shells. For example;

Nitrogen XX X

X

XX

X

N7

14

ATOMIC STRUCTURE: Bohr Diagram

7P+7n0

Bohr DiagramsDraw the Bohr Diagram for the following elements:

O Cl8 17

16 35a) b)

8p+8n0

X

X

X

X

X

X

X

X

17p+18n0

X

X

X

X X

X

X X

XX

XX

X

XX

X X

Draw the Bohr Diagram for the following elements:

Bohr Diagrams

B Ne5 10

11 20c) d)

5p+6n0

X

X

X

X

X

10p+10n0

X

X

X

X

X X

XX

X

X

Valence Electrons

• The electrons on the outermost energy level

• These electrons determine the element’s chemical properties and its ability to form chemical bonds.

Lewis Structure of Atom

• The chemical symbol for the atom is surrounded by a number of dots corresponding to the number of valence electrons.

• Examples

– Hydrogen atom has 1 valence electron H– Fluorine atom has 7 valence electrons F

Lewis Structure

• Draw the Lewis Structure for the following elements:

a) Li b) Cl

c) P d) Mg

Let’s practice What We Just Learned

Chemical Bonding

Ionic and Covalent

Chemical Bonds

• Atoms react with one another to fill their outer energy levels

• Transferring or sharing electrons creates an attraction (chemical bond) that holds atoms together

2p2n 10p10n 18p22n

He4

2 1020Ne

Ar 4018

2,82,8,8

Noble Gases• Helium, neon and argon are atoms which do not react

with other atoms.• We call them “Noble Gases” because of this.• Each of these gases has a full outer electron shell.

IONIC BONDS

• Ionic bond: one atom transfers an electron to another atom

• Oppositely charged ions are attracted to one another forming a chemical bond an ionic bond

Sodium

Na11

23

e.c. 2,8,1

11 protons

12 neutrons

11 electrons

11p

12n

17 protons

Chlorine

Cl17

35

17p

18n18 Neutrons

17 electrons

e.c. 2,8,7

Na

e.c. 2,8,1

(Na +)

Ion

Atom

e.c. (2,8)+

The Sodium loses 1 electron to leave a complete outer shell.

It is now a Sodium ion with a charge of 1 +

The Sodium atom has 1 Electron in it’s outer shell.

+

Cl

e.c. 2,8,7

(Cl - )

Ion

Atom

e.c. (2,8,8)-

The Chlorine gains 1 electron to gain a complete outer shell.

It is now a Chlorine ion with a charge of 1 -

The Chlorine atom has 7 electrons in it’s outer shell.

-

Sodium atom

Na

Sodium ion

(Na +)

Chlorine atom

Cl

Chlorine ion

(Cl -)

The Ionic Bond

• The sodium atom loses one electron to attain a complete outer shell and become a positive ion (Na +).

• The Chlorine atom gains one electron to attain a complete outer shell and become a negative ion (Cl –).

• Strong forces attract the sodium and chlorine ions.

+ -

Covalent Bond

• Formed when atoms share electrons

• Electrons may be shared equally or unequally

• Molecule: two or more atoms held together by covalent bonds

Chlorine atom 2,8,7

2 Chlorine atoms

Outer shells only

Chlorine molecule Cl2

Molecules have no overall electric charge

Forces (bonds) between atoms in the molecule are very strong

Each outer shell has 8 electrons

Electrons shared

Cl Cl

Chlorine Cl2

Cl Cl

Cl Cl

Covalent bonds can be represented in 3 ways:

Oxygen O168

Oxygen atom 2,6

2 Oxygen atoms (outer shells only)

Double covalent bond

O O

Let’s practice What We Just Learned

LIFE DEPENDS ON THE UNIQUE PROPERTIES OF

WATER

Water Molecule

• Water is a polar molecule• Electrons are not shared

equally between hydrogen and oxygen

• Electrons attracted to oxygen more strongly than to hydrogen Note: Hydrogen atoms in the water

molecule are slightly positive and the oxygen atom slightly negative

WATER’S LIFE-SUPPORTING PROPERTIES

• Polarity of water and effects of hydrogen bonding give water its unique properties:– Cohesion and adhesion– Temperature moderation– Low density of ice compared to water– Ability to dissolve substances

COHESION & ADHESION

• Cohesion: an attraction between like molecules• Adhesion: an attraction between unlike molecules

– Keep large molecules organized so they function properly in cells

– Help transport water through roots and leaves in plants

TEMPERATURE MODERATION

• Hydrogen bonds in water molecules allow water temperatures to change more slowly– Through evaporation (sweating), water

moderates temperature absorbs heat energy from skin cooling the body

LOW DENSITY OF ICE

• Density: amount of matter/given volume• For most substances, solids are more dense than

liquid state of matter• Due to hydrogen bonding, water is the opposite• Liquid water is more dense than solid water• Since less dense substances float in more dense

substances, ice floats

WATER DISSOLVES OTHER SUBSTANCES

• Solution: uniform mixture of two or more substances• Solvent: the substance that dissolves the other

substance; present in the greater amount• Solute: the substance that is dissolved; present in

the lesser amount• Aqueous solution: solution where water is the solvent

• Acid: – produce H+ ions in water– pH less than 7– sour taste

• Base: – removes H+ ions in water – pH greater than 7– bitter taste

• pH scale: range of numbers that describes how acidic or how basic a solution is – pH of 7 is neutral

• Buffers: substances that cause a solution to resist changes in pH

ACIDS, BASES, & pH