Atoms, molecules and ions

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Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in the nucleus

2.3

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2.3

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6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

2.3

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2.4

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A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds

A diatomic molecule contains only two atoms

H2, N2, O2, Br2, HCl, CO

A polyatomic molecule contains more than two atoms

O3, H2O, NH3, CH4

2.5

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An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive charge

If a neutral atom loses one or more electrons

it becomes a cation.

anion – ion with a negative charge

If a neutral atom gains one or more electrons

it becomes an anion.

2.5

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A monatomic ion contains only one atom

A polyatomic ion contains more than one atom

2.5

Na+, Cl-, Ca2+, O2-, Al3+, N3-

OH-, CN-, NH4+, NO3-

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13 protons, 10 (13 – 3) electrons

34 protons, 36 (34 + 2) electrons

Do You Understand Ions?

2.5

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2.5

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2.6

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A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance

An empirical formula shows the simplest

whole-number ratio of the atoms in a substance

H2O

C6H12O6

CH2O

O3

O

N2H4

NH2

2.6

Slide 24

ionic compounds consist of a cation and an anion

the formula is always the same as the empirical formula

the sum of the charges on the cation and anion in each formula unit must equal zero

The ionic compound NaCl

2.6

Slide 25

Formula of Ionic Compounds