Final answer:
In beta-plus decay, a proton inside the nucleus is transformed into a neutron, leading to the emission of a positron and a neutrino. The mass number stays constant, but the atomic number decreases by one, shifting the element's position on the nuclide chart diagonally: down one and right one.
Step-by-step explanation:
The process by which the nucleus changes during beta-plus decay involves the transformation of a proton into a neutron. This transformation is due to the weak nuclear force and is accompanied by the emission of a positron (or beta plus particle, ß+) and a neutrino which we usually ignore for simplicity. Notably, the mass number (A) of the nucleus remains unchanged, but the atomic number (Z), which represents the number of protons, decreases by one.
This is because the process results in one less proton and one additional neutron in the nucleus, as shown by the nuclear equation Na11 → Ne12+ß+ +ve. Consequently, the position of the nuclide on the chart of nuclides moves diagonally: down one and right one.
The nucleon number conservation is a paramount concept here, as the total number of nucleons remains the same after the beta-plus decay. Beta-plus decay is common among isotopes with low neutron-to-proton ratios that lie below the band of stability.