1. The structure of the atom
Chapter 4.3
How Atoms Differ

2. Atomic number
The number of protons in an atom that identifies it as an atom of a particular element (also equals the number of electrons)
Every element has a unique atomic number (unique number of protons that cannot be changed without change the identity of the element)
Atomic number = number of protons = number of electrons
In a neutral (no charge) atom, the number of electrons (- charge) always equals the number of protons (+ charge)
The periodic table is organized left to right and top to bottom by increasing atomic number.

3. Atomic Number Problem Complete the following table:
Element
Atomic Number
Protons
Electrons 19 35 Ne 10 K 19 19 Br 35 35 10 10

4. Mass of atoms Measured in atomic mass units (amu)
Based on carbon-12 as a standard
1 amu = 1/12 the mass of the carbon-12 atom
Masses of subatomic particles:
Electron = amu
Proton = amu
Neutron = amu

5. Mass number Mass number = number of protons + number of neutrons
How heavy the atom is
Nucleus is heaviest - Contains protons and neutrons
Mass number = number of protons + number of neutrons
A single element can have different mass numbers if there is more than one “version” of the element – called an isotope

6. Isotopes
Atoms of the same element with the same number of protons but different numbers of neutrons.
The mass of isotopes increases with increasing neutrons but chemical behavior stays the same
Chemical behavior is determined only by the the number of electrons that an atom has.

7. Isotope identification
Each isotope of an element is identified with the mass number
Copper-63 (Cu-63): mass number = 63 (29 protons + 34 neutrons)
Copper-65 (Cu-65): mass number = 65 (29 protons + 36 neutrons)
Atomic Mass: the weighted average mass of all of the isotopes of a particular element
This is the mass number listed on the periodic table with decimal places!!!

8. (Protons or electrons)
Isotope notation***
Isotopes are often written using a notation involving the chemical symbol, atomic number, and mass number
Mass Number
(Protons + Neutrons)
Copper-65 63 Cu 65 Cu 29 29
Copper-63
Atomic Number
(Protons or electrons)

9. Isotope problem: Determine the number of protons, electrons and neutrons for the isotopes in the table below; name each isotope and write its symbol
Element
Atomic Number
Mass Number a.
Neon 10 22 b.
Calcium 20 46 c.
Oxygen 8 17 d.
Iron 26 57 e.
Zinc 30 64 f.
Mercury 80
204

10. Calculating Atomic Mass
Atomic mass: the weighted average mass of all of the isotopes of that element
Mass of each isotope and isotope abundances are required to calculate this
Isotope abundances
Amount of element isotopes that readily exist (presented as percentages)

11. CALCULATING Atomic Mass
An element has 3 isotopes with the following mass numbers and abundances:
23A: mass 23 amu; abundance 7%
25A: mass 25 amu; abundance 76%
26A: mass 26 amu; abundance 17%
(23 x 0.07)+(25 x 0.76)+(26 x 0.17) = 25.03

12. Isotope Mass (amu) Percent Abundance
Atomic Mass Problem: Calculate the atomic mass of the element X and identify the element from the periodic table knowing that it has the isotopes listed in the table below
Isotope
Mass (amu)
Percent Abundance
54X
53.940
5.9%
56X
55.935
91.72%
57X
56.935
2.1%
58X
57.933
0.28%

13. Ion formation Ions are formed when atoms gain or lose electrons.
Positive Ion formation
When an atom LOSES one or more electrons
When an electron (negative charge) is lost, the ion ends up with a net positive charge (Ca – 2 e- = Ca2+)
Negative Ion Formation
When an atom GAINS one or more electrons
When an electron (negative charge) is gained, the ion ends up with a net negative charge (O + 2 e- = O2-)