Question

An atom has 20 neutrons and 19 protons in its nucleus. How many electrons are in its outer (valence) electron shell? How many shells does it have? What is its atomic mass in g/mol? What is the element?

		1, 3, 39, K
		1, 4, 19, Mg
		2, 4, 19, Mg
		2, 4, 39, K
		1, 4, 39, K

Answer 1

• Nucleus of an atom comprises of two sub-atomic parts: protons and neutrons.
• The neutrons have no charge, while protons have positive charge.
• Electrons are negatively charged, located in the electron cloud surrounding the nucleus of an atom.

Potassium:

• Neutrons = 20
• Protons = 19
• Electrons = 19
• Atomic mass = Protons + Neutrons = 19 + 20 = 39
• Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
• Thus, number of electrons in outer shell = 1
• Number of shells = 4

• Number of electrons in outer (valence) shell = 1
• Number of shells = 4
• Atomic mass = 39
• Element = K
• Option: 1, 4, 39, K