Answer:
A. Atomic number
B. Number of protons
Explanation:
Protons and electrons are what defines elements. If the number of protons or electrons was changed, you would have a completely different element.
The atomic number of an atom is the number of protons within its nucleus. Again, if you changed the atomic number, you would no longer have the same element.
Neutrons can be added or taken away from an element while keeping that elements properties. Atoms of the same element that differ in neutrons are called isotopes. Isotopes also differ in atomic mass from other atoms of that element, as the atomic mass of an atom is the amount of neutrons AND protons in its nucleus.
Electrons can also change in an atom. Atoms will lose and gain electrons when in contact with other atoms. An atom that loses an atom will have a positive charge (more protons than electrons), while atoms that gain an atom will have a negative charge (more electrons than protons). These are called positive and negative ions. You will likely learn about these later.
Important Vocabulary:
- Atomic number: the number of protons an atom has
- Atomic mass: the number of protons and neutrons an atom has (this defines the mass of the atom)
- Isotopes: atoms of the same element with different amounts of neutrons (these will also differ in atomic mass, see atomic mass definition)
