I.1 The modern model of the atom has evolved over a long period of time through the work of many scientists.(3.1a)

I.2 Each atom has a nucleus, with an overall positive charge, surrounded by one or more negatively charged electrons. (3.1b)

I.3 Subatomic particles contained in the nucleus include protons and neutrons. (3.1c)

I.4 The proton is positively charged, and the neutron has no charge. The electron is negatively charged.(3.1d)

I.5 Protons and electrons have equal but opposite charges. The number of protons equals the number of electrons in an atom. (3.1e)

I.6 The mass of each proton and each neutron is approximately equal to one atomic mass unit. An electron is much less massive than a proton or a neutron. (3.1f)

I.7 In the wave-mechanical model (electron cloud model), the electrons are in orbitals, which are defined as the regions of the most probable electron location (ground state). (3.1h)

I.8 Each electron in an atom has its own distinct amount of energy. (3.1i)

I.9 When an electron in an atom gains a specific amount of energy, the electron is at a higher energy state (excited state). (3.1j)

I.10 When an electron returns from a higher energy state to a lower energy state, a specific amount of energy is emitted. This emitted energy can be used to identify an element. (3.1k)

I.11 The outermost electrons in an atom are called the valence electrons. In general, the number of valence electrons affects the chemical properties of an element. (3.1l)

I.12 Atoms of an element that contain the same number of protons but a different number of neutrons are called isotopes of that element. (3.1m)

I.13 The average atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes. (3.1n)