Bohr’s Atom

electrons in orbits

nucleus

What is an atom?

• Atom: the smallest unit of matter that retains the identity of the substance

Atomic Structure

• Atoms are composed of 2 regions:– Nucleus: the center of the atom that

contains the mass of the atom– Electron cloud: region that surrounds

the nucleus that contains most of the space in the atom

NucleusElectron

Cloud

What’s in the Nucleus?

• The nucleus contains 2 of the 3 subatomic particles:– Protons: positively charged subatomic

particles– Neutrons: neutrally charged subatomic

particles

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

ATOMIC STRUCTUREATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ charge

- charge

No charge

1

1

nil

Mass

What’s in the Electron Cloud?

• The 3rd subatomic particle resides outside of the nucleus in the electron cloud–Electron: the subatomic particle with a negative charge and relatively no mass

How do these particles interact?

• Protons and neutrons live compacted in the tiny positively charged nucleus

• They account most of the mass of the atom

• The negatively charged electrons are small and have a relatively small mass

• but occupy a large volume of space outside the nucleus

How do the subatomic particles balance each other?

• In an atom:– The protons = the electrons

•If 20 protons are present in an atom then 20 electrons are there to balance the overall charge of the atom—atoms are neutral

– The neutrons have no charge;

How do we know the number of subatomic particles in an

atom?• Atomic number: this number indicates

the number of protons in an atom– Ex: Hydrogen’s atomic number is 1

• So hydrogen has 1 proton– Ex: Carbon’s atomic number is 6

• So carbon has 6 protons

**The number of protons identifies the atom.

Ex. 2 protons = He, 29 protons = Cu

• Mass number: the number of protons and neutrons in the nucleus– Ex: oxygene can have a mass of 16. Since it has 8 proton it must have 8

neutrons

How do we know the number of subatomic particles in an

atom?

What about the electrons?• The electrons are equal to the

number of protons– Ex: O has a mass

of 16 and an atomic number of 8 and has 8 electrons.

What about the electrons?

• Model of the atom pictures the electrons moving around the nucleus in a region

called an electron cloud.

• The electron cloud is a cloud of varying density surrounding the nucleus.

What about the electrons?

• Electrons are found in specific circular paths (orbits) around the nucleus.

• The electrons have fixed energies called energy levels. (like rungs on a ladder)

• The amount of energy required to move an electron to another energy level is

called a quantum (shell).• The energy levels of electrons are

labeled by principal quantum numbers (n)

ATOMIC STRUCTUREATOMIC 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

How exactly are the particles arranged?

• Bohr Model of the atom:All of the protons and the neutrons

The 1st ring can hold up to 2 e-

The 2nd ring can hold up to 8 e-

The 3rd ring can hold up to 18 e-

The 4th ring and any after can hold up to 32 e-

What does carbon look like?

Mass number = 12 Atomic number = 6

p+ = 6 no = 6 e- = 6

6 p and 6 n live in the nucleus

ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION

With electronic configuration elements are

represented numerically by the number of

electrons in their shells and number of shells.

For example;

N

Nitrogen

7

14

2 in 1st shell

5 in 2nd shell

configuration = 2 , 5

2 + 5 = 7

ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION

Write the electronic configuration for the following elements;

Ca O

Cl Si

Na20

40

11

23

8

17

16

35

14

28B

11

5

a) b) c)

d) e) f)

1s2,8,8,2 2,8,1

2,8,7 2,8,4 2,3

2,6

DOT & CROSS DIAGRAMSDOT & CROSS DIAGRAMS

With Dot & Cross diagrams elements and

compounds are represented by Dots or Crosses to

show electrons, and circles to show the shells. For

example;

Nitrogen N XX X

X

XX

X

N7

14

DOT & CROSS DIAGRAMSDOT & CROSS DIAGRAMS

O Cl8 17

16 35a) b)

O

X

XX

X

X

X

X

X

Cl

X

X

X

X X

X

XX

X

X

X

X

X

XX

X

X

X

How is the dispertion of electrons in shell?

• The outer energy level or “shellshell” is considered full when it has the maximum number of electrons that can fit into that level.

How is the dispertion of electrons in shell?

• The outer energy level or “shellshell” is considered full when it has the maximum number of electrons that can fit into that level.

• For example, the 1st level is full when it has 2 electrons, but the 2nd and 3rd levels are full when it has 8 electrons.)

How is the dispertion of electrons in shell?

• If the outside energy level is full, it is considered non-reactive. It does not need to chemically react or bond with any other atom or molecule.

How is the dispertion of electrons in shell?

• If the outside energy level is full, it is considered non-reactive. It does not need to chemically react or bond with any other atom or molecule.

• If the outside energy level is incomplete (not full), it is considered reactive.

How is the dispertion of electrons in shell?

• If the outside energy level is full, it is considered non-reactive. It does not need to chemically react or bond with any other atom or molecule.

• If the outside energy level is incomplete (not full), it is considered reactive.

• It will either lose or gain electrons and chemically react or bond with another atom or molecule.

Orbitals

• The electrons of an atom are in constant motion .

Orbitals

• The electrons of an atom are in constant motion .

• The region of space that an electron occupies is known as an ORBİTAL.

Orbitals

• The electrons of an atom are in constant motion .

• The region of space that an electron occupies is known as an ORBİTAL.

• Orbital comprise sublevels of a principal energy level.

• The sublevels are named as s, p, d, f.

Orbitals

• Each sublevels containes a different number of electrons.

s = 1 orbital d = 5 orbitals

p = 3 orbitals f = 7 orbitals

Orbitals

• Each sublevels containes a different number of electrons.

s = 1 orbital d = 5 orbitals

p = 3 orbitals f = 7 orbitals• s and p orbitals most important in organic

and biological chemistry

Orbitals• Each sublevels containes a different number

of electrons.

s = 1 orbital d = 5 orbitals

p = 3 orbitals f = 7 orbitals• s and p orbitals most important in organic

and biological chemistry• s orbitals: spherical, nucleus at center• p orbitals: dumbbell-shaped, nucleus at

middle• d orbitals: elongated dumbbell-shaped,

nucleus at center

Sublevels continued

• The maximum number of electrons allowed in each sublevel are as follows:

• s = max 2 electrons• p = max 6 electrons• d = max 10 electrons• f = max 14 electrons

Orbitals

Orbitals• Orbital are grouped in shells of increasing

size and energy

Orbitals• Orbital are grouped in shells of increasing

size and energy• Different shells contain different numbers

and kinds of orbitals• Each orbital can be occupied by two

electrons.

Orbitals• Orbital are grouped in shells of increasing

size and energy• Different shells contain different numbers

and kinds of orbitals• Each orbital can be occupied by two

electrons.

SUMMARYSUMMARY

1. The Atomic Number of an atom = number of

protons in the nucleus.

2. The Atomic Mass of an atom = number of

Protons + Neutrons in the nucleus.

3. The number of Protons = Number of Electrons.

4. Electrons orbit the nucleus in shells.

5. Each shell can only carry a set number of electrons.